# What is Combinatorics? (A collection of quotes by Igor Pak)

This is a collection of quotes by various authors, trying, succeeding and occasionally failing to define it. They are listed in chronological order. Some of the early quotes are barely comprehensible, later quotes are somewhat defensive and most recent are rather upbeat. Read them all! - IP

P.S. If you are curious about my own views on this, read it in this blog post (2013), and the first answer in this interview (2021, errata here).

• Gottfried Leibniz, Dissertatio de arte combinatoria (Latin for Dissertation on the Art of Combinations), Leipzig, 1666. [English translation by L.E. Loemker, 1956. Note: tables ב and ג are missing, cf. the original.]

So there arise two kinds of variation: complexion and situs. [In modern terminology, "complexion" are combinations and "situs" are permutations. -- IP] And viewed in themselves, both complexion and situs belong to metaphysics, or to the science of wholeand parts. If we look at their variability, however, that is, at the quantity of variation, we must turn to numbers and to arithmetic. I am inclined to think that the science of complexions pertains more to pure arithmetic, and that of situs to an arithmetic of figure.

• Jakob Bernoulli, Ars Conjectandi (Latin for The Art of Conjecturing), Basel, 1713. [English translation by Fr. Matheres, London, 1795.]

It is easy to perceive that the prodigious variety which appears both in the works of nature and in the actions of men, and which constitutes the greatest part of the beauty of the universe, is owing to the multitude of different ways in which its several parts are mixed with, or placed near, each other. But, because the number of causes that concur in producing a given event, or effect, is oftentimes so immensely great, and the causes themselves are so different one from another, that it is extremely difficult to reckon up all the different ways in which they may be arranged, or combined together, it often happens that men, even of the best understandings and the greatest circumspection, ale guilty of that fault in reasoning which the writers on logick call the insufficient, or imperfect enumeration of parts, or cases: insomuch that I will venture to assert, that this is the chief, and almost the only, source of the vast number of erroneous opinions, and those too very often in matters of great importance, which we are apt to form on all the subjects we reflect upon, whether they relate to the knowledge of nature, or the merits and motives of human actions. It must therefore be acknowledged, that that art which affords a cure to this weakness, or defect, of our understandings, and teaches us so to enumerate all the possible ways in which a given number of things may be mixed and combined together, that we may be certain that we have not omitted any one arrangement of them that can lead to the object of our inquiry, deserves to be considered as most eminently ufeful and worthy of our highest esteem and attention. And this is the business of the art, or doctrine of combinations.

• I. Burmane, "Essai de caracteristique combinatoire", in Über combinatorische Analysis und Derivations-Calcul by Karl F. Hindenburg, Leipzig, 1803.

Si un art date de ses premiers essais, l'Analyse Combinatoire est ancienne: si, comme il est plus maturel, on fixe son origine à l'époque où elle a commencé de devenir un systême, on ne peut pas remonter plus haut qu'à LEIBNITZ. Ce génie transcendant, à qui nous devons tant de grandes vues, tenta le premier d' en faire une science et même une science universelle. Pour avoir trop étendu son plan, le philosophe alle mand occupé d'ailleurs d'une foule d'autres objets, ne fit que peu de progrès dans sa Combinatoire et ne vit même qu'un abyme dans une de ses principales opérations, la décomposition d'un nombre en toutes ses parties entières. Il défricha le champ et c'est beaucoup. Les heureuses idées qu'il séma fructifièrent sous de géomètres plus modérés. [..]

J'en ai assez dit pour faire sentir l'importance de l'Analyse Combinatoire. Que peut - on lui souhaiter encore? Une caractéristique commode qui la repande chez les géomètres de tous les pays, dont elle ne saurait manquer d' exciter l' émulation. Elle en est d' autant plus digne que ses usages sont innombrables, et qu'il n'est point de science, point d'Art, qui n'en puisse retirer de l' utilité. En Arithmétique (science toute combinatoire) elle enseigne la pratique la plus courte des grandes multiplications, divisions, éradications, compositions des périodes; elle facilite l'invention des facteurs, des nombres prémiers et figurés comme on veut, et surtout leur détermination sûre et rapide par des machines, dont on n'a pas encore tiré tout le parti possible.

• Peter Nicholson, Essays on the Combinatorial Analysis, London, 1818.

The Combinatorial Analysis is a branch of mathematics which teaches us to ascertain and exhibit all the possible ways in which a given number of things may be associated and mixed together; so that we may be certain that we have not missed any collection or arrangement of these things, that has not been enumerated. [..]

Besides its uses in mathematical investigations, it not only enables us to form our ideas of the elegant compositions of design, but to contemplate the prodigious variety which constitutes the beauties of nature, and which arises from the combinations of objects, by their number, forms, colour, and positions. It has a relation to every species of useful knowledge upon which the mind of man can be employed. [..]

It is to be regretted, that in this country, where genius abounds, and where the combinatorial analysis began to dawn (as well as other useful branches of the mathematical sciences), that it is only partially known. [..] To foreigners alone we owe the subsequent improvements and advancement of this branch of mathematical science.

• Rev. William A. Whitworth, Choice and Chance, Deighton, Bell and Co., Cambridge, 1870.

We have continually to make our choice among different courses of action open to us, and upon the discretion with which we make it, in little matters and in great, depends our prosperity and our happiness. Of this discretion a higher philosophy treats, and it is not to be supposed that Arithmetic has anything to do with it; but it is the province of Arithmetic, under given circumstances, to measure the choice which we have to exercise, or to determine precisely the number of courses open to us.

• Major Percy A. MacMahon, Combinatory Analysis: A Review of the Present State of Knowledge, in Proc. LMS (1896).

As your retiring President has the opportunity of making a few remarks upon some special topic, I will now pass on to that subject which may possibly be of some interest to those here to-night. It is the combinatory analysis, and my selection is influenced by the belief that this subject is not at present upon a proper footing. Its importance is not fully recognised, because much that properly belongs to it appears under other headings in all recent attempts to organize and arrange the various departments of mathematical science. [..]

The combinatory analysis in my opinion holds the ground between the theory of numbers and algebra, and is the proper passage between the realms of discontinuous and continuous quantity. It would appear advisable [..] to consider the theory of partitions an important part of combinatory analysis. [..]

The subject of combinatory analysis has for forty years been much neglected; may I urge that both the beauty of the methods of research available and the value of the results achieved indicate that it is well worthy the attention of mathematicians?

In conclusion, I will quote tho words of the greatest living English master of pure mathematics—Sylvester—words, which, I believe, are as applicable to tho combinatory analysis as a whole as to that particular branch—the theory of partitions — in connexion with which they were spoken:

“Partitions constitute the sphere in which analysis lives, moves, and has its being; and no power of language can exaggerate or paint too forcibly the importance of this till recently almost neglected, but vast, subtle, and universally permeating, element of algebraical thought and expression.”

• Eugen Netto, Lehrbuch der Combinatorik (in German), Teubner, Leipzig, 1901. [Translation by K. Adiprasito, August 2014; see here the original quote.]

Let us subjugate a collection of objects taking into account their qualities and differences each from another, then we are lead, in our mathematical perspective, to the study of integers and their connecting operations, that is, we are lead to Number Theory.

If we, however, disregard the qualities of each individual object and only account for the difference between two objects insofar that they are different, then we are lead to investigations which are concerned with the position, the order, the choosing of these objects. This branch of mathematics is called Combinatorics.

• Florian Cajori, Review of "Lehrbuch der Combinatorik", Science, Vol. 16, No. 403 (Sep. 19, 1902), pp. 469-470.

What are now considered the substantial parts of combinatoric have been developed outside of the German Combinatorial School. Associated with the early development are the great names of Pascal, Leibnitz, Wallis, James Bernoulli and De Moivre. [..]

While combinatoric is not now made the subject of lectures in our universities, it is nevertheless of importance. The student acquires much of it during the pursuit of other branches. It is touched upon in the study of ordinary algebra, of determinants, of substitution and group theory, of the theory of numbers, and of the theory of probability.

• Major Percy A. MacMahon, "Combinatorial Analysis" article in Encyclopædia Britannica (Eleventh Edition), 1911.

The Combinatorial Analysis, as it was understood up to the end of the 18th century, was of limited scope and restricted application. [..] Writers on the subject seemed to recognize fully that it was in need of cultivation, that it was of much service in facilitating algebraical operations of all kinds, and that it was the fundamental method of investigation in the theory of Probabilities.

• Major Percy A. MacMahon, Combinatory Analysis, vol. 1, Cambridge Univ. Press, 1915.

The combinatory analysis as considered in this work occupies the ground between algebra, properly so called, and the higher arithmetic. The methods employed are distinctly algebraical and not arithmetical. The essential connecting link between algebra and arithmetic is found in the circumstance that a particular case of algebraic multiplication involves arithmetical addition. [..] This link was forged by Euler for use in the theory of partitions of numbers. It is used here for the most general theory of combinations of which the partition of numbers is a particular case.

The theory of the partition of numbers belongs partly to algebra and partly to the higher arithmetic. The former aspect is treated here. It is remarkable that in the international organization of the subject-matter of mathematics "Partitions" is considered to be a part of the Theory of Numbers which is an alternative name for the Higher Arithmetic, whereas it is essentially a subdivision of Combinatory Analysis which is not considered to be within the purview of the Theory of Numbers. The fact is that up to the point of determining the real and enumerating Generating Functions the theory is essentially algebraical [..]

• Review on "Combinatory Analysis", Science Progress in the Twentieth Century, Vol. 10, No. 40 (April 1916), pp. 601-606.

Combinatory Analysis is well known even to students of elementary algebra, where it figures under the title of Permutations and Combinations; while in theory of equation the subject is intimately connected with symmetric functions. The formal statement of the objects of this branch of mathematics is that it includes the formation, enumeration, and other properties of the different groups of a finite number of elements which are arranged according to prescribed laws. By its subject-matter combinatory analysis is related to some of the most ancient problems which have exercised human ingenuity.

• James B. Shaw, Review on "Combinatory Analysis", Science, Vol. 45, No. 1165 (Apr. 27, 1917), pp. 409-411.

One of the four grand divisions of what may be called properly static mathematics is the theory of configurations. It includes the construction out of given elements of compound forms under certain given conditions or restrictions [..] These constructions vary from the mere permutation of a linear series of elements up to the complicated trees of chemical combinations studied by Cayley, and in general to all sorts of problems in what has been happily denominated tactics by Cayley, or syntactics by Cournot. [..]

The study of configurations usually begins with combinatory analysis. By this is usually meant the study of arrangements along a line of a collection of objects, either as individuals or in groups; arrangements at the nodes of a lattice, combinations of arrangements. Such problems arise not only as matters of tactic, curious problems or puzzles, but in the determination of the number of such arrangements needed in solving problems in the theory of probabilities.

• Dorothy Wrinch, Book Review of "An Introduction to Combinatory Analysis" by P.A. MacMahon, Nature, Vol. 107 (May 1921), pp. 357-358.

For some reason which is not at present clear, the theory of the combinations of different sets of similar possibilities (which can conveniently be represented as the distributions of balls in boxes) is of utmost importance in many branches of science. [..] The subject is therefore of great importance in applied as well as in pure mathematics, and might very well prove another example of the extraordinary way in which abstract mathematics leads the way in applied science.

• G. B. Mathews, Tactic, Science Progress in the Twentieth Century, Vol. 16, No. 61 (July 1921), pp. 139-142.

The term "combinatorial analysis" hardly admits of exact definition, and is not used in the International Schedule of pure mathematics. Broadly speaking, it has come to mean the discussion of problems which involve selections from, or arrangements of, a finite number of objects; or combinations of these two operations. For the purpose of this article it will be convenient to use Sylvester's term "tactic" as a synonym for "combinatorial analysis."

• John Riordan, An Introduction to Combinatorial Analysis, John Wiley, New York, 1958.

Combinatorial Analysis, though a well-recognized part of mathematics, seems to have a poorly defined range and position. Leibniz, in his "ars combinatoria", was the originator of the subject, apparently in the sense of Netto (Lehrbuch der Combinatorik, Leipzig, 1901) as the consideration of the placing, ordering, or choice of a number of things together. This sense appears also in the title, Choice and Chance (W.A. Whitworth, fifth edition, London, 1901), of one of the few books in English on the subject. This superb title also suggests the close relation of the subject to the theory of probability. P.A. MacMahon, in the most ambitious treatise on the subject (Combinatory Analysis, London, vol. I, 1915, vol. II, 1916), says merely that it occupies the ground between algebra and the higher arithmetic, meaning by the latter, as he later explains, what is now called the Theory of Numbers.

A current American dictionary (Funk and Wagnalls New Standard, 1943) defines "combinatoric" – a convenient single word which appears now and then in the present text-as "a department of mathematics treating of the formation, enumeration, and properties of partitions, variations, combinations, and permutations of a finite number of elements under various conditions".

The term "combinatorial analysis" itself, seems best explained by the following quotation from Augustus DeMorgan (Differential and Integral Calculus, London, 1842, p. 335): "the combinatorial analysis mainly consists in the analysis of complicated developments by means of a priori consideration and collection of the different combinations of terms which can enter the coefficients".

No one of these statements is satisfactory in providing a safe and sure guide to what is and what is not combinatorial. The authors of the three textbooks could be properly vague because their texts showed what they meant. The dictionary, in describing the contents of such texts, allows no room for new applications of combinatorial technique (such as appear in the last half of Chapter 6 of the present text in the enumeration of trees, networks, and linear graphs). DeMorgan's statement is admirable but half-hearted; in present language, it recognizes that coefficients of generating functions may be determined by solution of combinatorial problems, but ignores the reverse possibility that combinatorial problems may be solved by determining coefficients of generating functions.

Since the subject seems to have new growing ends, and definition is apt to be restrictive, this lack of conceptual precision may be all for the best. So far as the present book is concerned, anything enumerative is combinatorial; that is, the main emphasis throughout is on finding the number of ways there are of doing some well-defined operation.

• Leo Moser, Math. Reviews 20 #3077 (AMS, 1958), on "An Introduction to Combinatorial Analysis" by J. Riordan.

This is the only modern book on combinatorial analysis. It will be extremely useful to anyone who wishes to make a systematic study of the subject, as well as to the many mathematicians and mathematical practitioners who are sometimes faced with particular problems involving a count of the number of ways in which some well defined operation can be carried out. Techniques developed in this book are of increasing importance in the study of modern statistics, computing devices, communications engineering and in the recent applications of graph theory to chemistry and problems in the social sciences. [..]

In the two-volume treatise by MacMahon [Combinatory analysis, University Press, Cambridge, 1915–16] many enumeration problems were claimed to be solved when, in fact, they were merely reformulated. In this book such an approach is avoided. Explicit formulae are the order of the day and in many cases the formulae are accompanied by short tables. Nevertheless, the author is usually satisfied with an explicit formula, even though it may be comparatively unmanageable for large values of the arguments. Very little is said about asymptotic developments for such cases. There are many problems usually considered combinatorial in nature in which the question is not primarily "in how many ways can this be done'' but rather "can this be done in at least one way?'' These include important problems of finite geometries, difference sets and related combinatorial designs. Such problems are not treated here.

These omissions are indications of the size of the subject rather than of the shortcomings of the book. In fact, the author has admirably succeeded in marshalling many interesting facts that were not nearly so accessible before. He has infused pattern and order into his subject and will thereby, we hope, attract more mathematicians to this fascinating field.

• Frank Harary, Book Review on "An Introduction to Combinatorial Analysis" by J. Riordan, Bulletin AMS (1959).

Some mathematicians feel that combinatorial analysis is not a branch of mathematics but rather a collection of clever but unrelated tricks. This book successfully refutes that viewpoint.

The subject is, without doubt, one of the hardest in which to write an effective exposition. The reason for this is the fact that so much of the material occurs in an isolated fashion in so many different applications both to pure and applied mathematics and to other fields. Combinatorial analysis is a subject in which many of the fundamental results are frequently rediscovered by people in different fields, from first principles. [..]

• Solomon Kullback, Book Review on "An Introduction to Combinatorial Analysis" by J. Riordan, SIAM Review (1960).

This book is an excellent introduction to a field of study which so far as the author is concerned covers anything enumerative. The author’s main emphasis throughout is on finding the number of ways there are of doing some well defined operation. [original emphasis]

• A.W. Tucker, Combinatorial Problems, Foreword to special issue of the IBM Journal, Nov. 1960.

Combinatorial Mathematics, or “Combinatorics”, regarded as originating in the Ars Combinatoria of Leibniz, has to do with problems of arrangement, operation, and selection within a finite or discrete system-such as the aggregate of all possible states of a digital computer. Until recently, preoccupation with continuous mathematics has inhibited the growth of discrete mathematics. But now it is realized that combinatorial methods can be developed to attack profitably, in modern science and technology, a vast variety of “problems of organized comp1exity” – an apt designation of Warren Weaver. In 1947 Hermann Weyl wrote as follows (rearranged slightly for quotation here):

“Perhaps the philosophically most relevant feature of modern science is the emergence of abstract symbolic structures as the hard core of objectivity behind – as Eddington puts it – the colorful tale of the subjective storyteller mind. The combinatorics of aggregates and complexes deals with some of the simplest such structures imaginable. It is gratifying that combinatorial mathematics is so closely related to the philosophically important problems of individuation and probability, and that it accounts for some of the most fundamental phenomena in inorganic and organic nature. This structural viewpoint occurs in the foundations of quantum mechanics. In a widely different field John von Neumann’s and Oskar Morgenstern’s attempt to found economics on a theory of games is characteristic of the same trend. The network of nerves joining the brain with the sense organs is a subject that by its very nature invites combinatorial investigation. Modern computing machines translate our insight into the combinatorial structure of mathematics into practice by mechanical and electronic devices.”

• Herbert J. Ryser, Combinatorial Mathematics, John Wiley, New York, 1963.

The modern era has uncovered for combinatorics a wide range of fascinating new problems. These have arisen in abstract algebra, topology, the foundations of mathematics, graph theory, game theory, linear programming, and in many other areas. Combinatorics has always been diversified. During our day this diversification has increased manyfold. Nor are its many and varied problems successfully attacked in terms of a unified theory. Much of what we have said up to now applies with equal force to the theory of numbers. In fact, combinatorics and number theory are sister disciplines. They share a certain intersection of common knowledge, and each genuinely enriches the other.

Combinatorial mathematics cuts across the many subdivisions of mathematics, and this makes a formal definition difficult. But by and large it is concerned with the study of the arrangement of elements into sets. The elements are usually finite in number, and the arrangement is restricted by certain boundary conditions imposed by the particular problem under investigation. Two general types of problems appear throughout the literature. In the first the existence of the prescribed configuration is in doubt, and the study attempts to settle this issue. These we call existence problems. In the second the existence of the configuration is known, and the study attempts to determine the number of configurations or the classification of these configurations according to types. These we call enumeration problems. This monograph stresses existence problems, but many enumeration problems appear from time to time.

It may be remarked that the second category of problems is nothing more than a refinement or obvious extension of the first. This is the case. But in practice if the existence of a configuration requires intensive study, then almost nothing can be said about the corresponding enumeration problem. On the other hand, if the enumeration problem is tractable, the corresponding existence problem is usually trivial.

• John Riordan, Math. Reviews 27 #51 (AMS, 1963), on Combinatorial mathematics, by H.J. Ryser.

Though the author equates combinatorial mathematics with combinatorial analysis, his book is addressed to the "new'' combinatorial analysis, whose emphasis lies in the existence rather than the enumerative side of the subject. Indeed, this is the area of his own work.

• Gian-Carlo Rota, Review on Combinatorial mathematics, by H.J. Ryser, MAA Monthly (1965).

The field of combinatorial analysis has been shockingly and incredibly neglected of late; too many mathematicians, blinded by their own overspecialized work, regard it with an air of snobbish condescension which betrays a lurking fear of anything that may "rock the boat." Yet, the need for more systematic understanding of combinatorial phenomena is now making itself clearly felt in all branches of mathematics, and even more so in the natural sciences. From the physics of elementary particles to genetics, from communication theory to computing, the need is increasing for the study of discrete structures. In view of this situation, one may predict with fair probability that the next decades will witness an explosion of combinatorial activity not unlike the development of topology since the beginning of this century. In the words of the author, "we believe that the greatest truths of combinatorial analysis are yet to be revealed."

• George Pólya, A Note of Welcome, first issue of Journal of Combinatorial Theory, 1966.

The first issue of this Journal may afford an opportunity to meditate upon the scope, the methods, and the applications of Combinatorial Theory.

This first issue could also be regarded as celebrating the tercentenary of the theory the name of which first appeared in the title of Leibniz's Dissertatio de Arte Combinatoria printed in 1666. Leibniz had great projects and entertained high hopes about the future of his Combinatorial Art. This Art should deal, according to one of his notes posthumously printed, with "the same and the different, the similar and the dissimilar, the absolute and the relative." These words may leave to combinatorial theory a wide enough scope, but they scarcely describe it too clearly. I am afraid that it is impossible to give a satisfactory definition of "Combinatorial Theory" and to trace out a sharp boundary of its subject matter, but mathematicians readily recognize what is of "combinatorial character."

To delimit the methods for solving combinatorial problems seems not only impossible, but may be also undesirable. Yet we have to mention here a favorite tool of combinatorial mathematics: the generating functions. Their name is due to Laplace, but they were already extensively and efficiently used by Euler, and their use was foreshadowed by certain passages of De Moivre.

Leibniz claimed "applications to the whole sphere of sciences" of his Combinatorial Art. We find very little concrete achievement in his Dissertatio on which such a claim could be based, yet it is probably more equitable to regard his words as a prophecy and not as a statement of facts. At any rate, there is a practically unlimited variety of subjects in which combinatorial considerations play a role. Without any pretension to system, I wish to mention a few with which l came into contact through my own work: probability, graph theory, organic chemistry, crystallography, statistical mechanics, propositional logic, switching circuits.

I have failed, I am afraid, in defining the scope or the methods or the applications of Combinatorial Theory, but there is little doubt that this theory was greatly developed in all respects in the last decades and it is to be hoped that this Journal will be a stepping stone to further progress.

• John Riordan, Combinatorial Identities, John Wiley, New York, 1968.

Combinatorialists use recurrence, generating functions, and such transformations as the Vandermonde convolution; others, to my horror, use contour integrals, differential equations, and other resources of mathematical analysis.

• Murray Edelberg, Book review of "Combinatorial Identities" by J. Riordan, Bulletin LMS (1969).

Combinatorial analysis is a rather fragmentary subject: it has few general techniques and few unifying principles. From time to time, mathematicians (for example, MacMahon) have sought to bring some sort of order and structure to the subject by writing textbooks. This is what Mr. Riordan, one of the world's leading combinatorialists, now sets out to do with combinatorial identities. [..]

• Marshall Hall, Combinatorial Theory, Blaisdell, Waltham, MA, 1969.

Combinatorial theory is the name now given to a subject formerly called "combinatorial analysis" or "combinatorics", though these terms are still used by many people. Like many branches of Mathematics, its boundaries are not clearly defined, but the central problem may be considered that of arranging objects according to specified rules and finding out in how many ways this may be done. If the specified rules are very simple, then the chief emphasis is on the enumeration of the number of ways in which the arrangement may be made. If the rules are subtle or complicated, the chief problem is whether or not such arrangements exist, and to find methods for constructing the arrangements. An intermediate area is the relationship between related choices, and a typical theorem will assert that the maximum for one kind of choice is equal to the minimum for another kind.

• Gian-Carlo Rota, Discrete Thoughts, Birkhäuser, Boston, 1969.

Combinatorial analysis, or – as it coming to be called, combinatorial theory – is both the oldest and one of the least developed branches of mathematics. [..] Combinatorial problems are found nowadays in increasing numbers in every branch of science, even in those where mathematics is rarely used.

Combinatorial theory has been slowed in its theoretical development by the very success of the few men who have solved some of the outstanding combinatorial problems of their day, for, just as the man of action feel little need to philosophize, so the successful problem-solver in mathematics feels little need for designing theories, that would unify, ant therefore enable the less-talented worker to solve, problems of comparable and similar difficulty. But the sheer number and the rapidly increasing complexity of combinatorial problems has made the situation no longer tolerable. It is doubtful that one man alone can solve any of the major combinatorial problems of our day.

• Paul Erdős, On the application of combinatorial analysis, in Proc. ICM Nice, 1970.

Though combinatorics has been successfully applied to many branches of mathematics these can not be compared neither in importance nor in depth to the applications of analysis in number theory or algebra to topology, but I hope that time and the ingenuity of the younger generation will change this.

• Gian-Carlo Rota, Combinatorial theory, old and new, in Proc. ICM Nice, 1970.

Combinatorial analysis, or combinatorial theory, as it has come to be called, is currently enjoying an outburst of activity. This can be partly attributed to the abundance of new and highly relevant problems brought to the fore by advances in discrete applied mathematics, and partly to the fact that only lately has the field ceased to be the private preserve of mathematical acrobats, and attempts have been made to develop coherent theories, thereby bringing it closer to the mainstream of mathematics.

• Claude Berge, Principles of Combinatorics, Dunod, Paris, 1968 (English translation: Academic Press, New York, 1971)

Nowadays, combinatorial analysis (or “combinatorics”) is the focus of much attention; yet, nowhere in the literature does there seem to be a satisfactory definition of this science and its many ramifications.

We wish to offer here a definition of combinatorics, which depends on the very precise concept of ‘‘configuration.” A configuration arises every time objects are distributed according to certain predetermined constraints.

Just as arithmetic deals with integers (with the standard operations), algebra deals with operations in general, analysis deals with functions, geometry deals with rigid shapes, and topology deals with continuity, so does combinatorics deal with configurations. Combinatorics counts, enumerates, examines, and investigates the existence of configurations with certain specified properties. With combinatorics, one looks for their intrinsic properties, and studies transformations of one configuration into another, as well as “subconfigurations” of a given configuration.

The preoccupations of combinatorics are exactly the same as those of the other branches of modern mathematics. Nevertheless, it is surprising that this particular discipline has developed on the edge of, or away from, the mainstream of modern mathematics. The elementary theorems of the subject have been forgotten and rediscovered several times. [..] It is important to rectify this.

• John Riordan, Math. Reviews 38 #5635 (AMS, 1968), on Principles of Combinatorics, by Claude Berge.

[Berge's book] begins boldly by proposing a definition of combinatoire (combinatorial mathematics): it is the study of configurations (for which a precise definition is possible). The definition given for configuration is: an application of a set of objects in an abstract finite set provided with a known structure; the term application (which needs no translation from French to English) is taken as understood, it seems to mean a functional association of the objects with the elements of the abstract set, that is, a mapping of the object set into the abstract set. The precision is a little illusory since, as the author recognizes, many combinatorial problems are concerned with configurations subject to given constraints, and the character and range of constraints are too varied for exact description.

• W.T. Tutte, Recent progress in combinatorics, in Proceedings of the Third Waterloo Conference on Combinatorics, Academic Press, 1969.

It is difficult to find a definition of combinatorics that is both concise and complete, unless we are satisfied with the statement “Combinatorics is what combinatorialists do.”

• Louis Comtet, Advanced Combinatorics, Reidel, Dordrecht, 1974 (translated from French original, Presses Universitaires de France, Paris, 1970).

For a long time the aim of combinatorial analysis was to count the different ways of arranging objects under given circumstances. Hence, many of the traditional problems of analysis or geometry which are concerned at a certain moment with finite structures, have a combinatorial character. Today, combinatorial analysis is also relevant to problems of existence, estimation and structuration, like all other parts of mathematics, but exclusively for finite sets.

• Gian-Carlo Rota, Foreword to the English translation of Principles of Combinatorics, by Claude Berge, 1971.

Most mathematicians of this day, confronted with an argument requiring combinatorial thinking, react with one of two stock phrases: (a) “This is a purely combinatorial argument,” (b) “This is a difficult combinatorial argument.” Hypnotic repetition of either of these slogans is likely to have the same balming effect on the speaker: freed from all scruples, he will pass the buck and unload the work onto someone else’s shoulders.

While the end result of this oft-repeated vignette is an overwhelming variety of problems for specialists in the art, the impression grows that among mathematicians, especially “pure” mathematicians, total ignorance of combinatorial theory is as proudly flaunted as-in bygone days-an aristocrat’s ignorance of his country’s vernacular.

It is tempting to react to this rejection, which in the past has succeeded in finessing combinatorialists into the mathematical proletariat, by a ringing ça ira. This might well take the form of a concerted attack on one of the currently fashionable branches of mathematics. The barrage of definitions and the superstructure of grammatical gamesmanship removed, little more than a few puny combinatorial facts would be left, which would then be dealt an embarrassingly easy coup de grace by the application of standard combinatorial techniques. Fortunately, this course will not be followed, for a sound reason: combinatorialists have better fish to fry.

• Richard K. Guy, Book Review of Principles of Combinatorics, by Claude Berge, Zentralblatt MATH (1971).

The author starts with the question: what is combinatorics? He succeeds in answering it no better than others, but a more sincere attempt is made. Six aspects are listed: study of a known configuration; search for an unknown one; exact enumeration of configurations; taking a census of configurations; and optimization.

• Richard P. Stanley, Book Review of Principles of Combinatorics, by Claude Berge, Bull. AMS, 1971.

The current resurgence of combinatorics (also known as combinatorial analysis and combinatorial theory) is by now recognized by all mathematicians. Scoffers regard combinatorics as a chaotic realm of binomial coefficients, graphs, and lattices, with a mixed bag of ad hoc tricks and techniques for investigating them. In reality, there has been a tremendous unifying drive to combinatorics in recent years. We now have a broad and sophisticated understanding of such standard combinatorial concepts as inversion, composition, generating functions, finite differences, and incidence relations.

Another criticism of combinatorics is that it "lacks abstraction." The implication is that combinatorics is lacking in depth and all its results follow from trivial, though possible elaborate, manipulations. This argument is extremely misleading and unfair. It is precisely the "lack of abstraction," i.e., the concrete visualization of the concepts involved, which helps to make combinatorics so appealing to its adherents. On the other hand, the "depth" of the subject is rapidly increasing as it increasingly draws upon more and more techniques and concepts from other branches of mathematics, such as group representation theory, statistical mechanics, harmonic analysis, homological algebra, and algebraic topology, to say nothing of the increasing sophistication of various new purely combinatorial techniques.

• George B. Dantzig, On the relation of operations research to mathematics, panel talk before AMS, in Atlantic City (Jan 23, 1971).

Broadly speaking combinatorial analysis is now taught in two parts which I will label: The first classical, the second important. [emphasis added -- IP] Classical combinatorics is concerned with counting problems. [..] As a mathematician, I like classical combinatorics. It is full of interesting devices: permutations, combinations, generating functions, amusing identities, etc. Relevant, it is not, except as a possible supplement to a basic course in probability. [..] Classical combinatorics is sometimes useful in preventing people from using an exhaustive procedure on the computer such as listing all combinations or examining all the cases. [..] The part of combinatorial analysis which I have labeled "important" is concerned with selecting the best combination out of all the combinations. This is what linear programming is all about.

• H. Halberstam, Combinatorial Analysis, Nature (May 21, 1971).

Workers in combinatorial analysis approach their subject with an endearing exuberance, born of the excitement of a relatively new and rapidly evolving field, with many applications within as well as outside mathematics.

• Gina Bari Kolata, Combinatorics: Steps toward a Unified Theory, Science (March 1, 1974).

According to Gian-Carlo Rota of the Massachusetts Institute of Technology, combinatorics is one of the least developed fields of mathematics. He attributes this lack of development to the extreme difficulty of combinatorial problems and to the lack of a unified theory that would give direction to those who attempt to solve such problems. Now some mathematicians are optimistic that a unified approach to combinatorics may be forthcoming. [..]

Ronald Graham of Bell Laboratories in Murray Hill, New Jersey, believes that the lack of combinatorial theories is due in part to the fact that combinatorics has traditionally not been considered a subject unto itself. Only recently, he says, have researchers emphasized the unity of combinatorics. Moreover, the large number of as yet unsolved problems makes it difficult to build theories. However, the recent rapid development of combinatorics is encouraging to those who, like Rota, believe that combinatorial theories are necessary to the future of the mathematical, physical and life sciences.

• Lennart Carleson (c. 1974), communicated by John B. Garnett.

Every hard problem in mathematics has something to do with combinatorics.

• Herbert S. Wilf, Book Review, in Bull. AMS (1975).

In combinatorial mathematics [..] there is now such vigorous activity on so many fronts that an overview of the whole battlefield would show only smoke: is the permanent function really fundamental or just a gadget? same for the Möbius function; is the four color problem a dead end or is its fallout stimulating the whole subject? in what ways will the blossoming study of computer algorithms reshape combinatorics itself? The issues are, of course, not posed in these stark terms, but instead, all areas are being advanced simultaneously, to the despair of those who seek unifying threads.

• Jean A. Dieudonné, A panorama of pure mathematics. As seen by N. Bourbaki, translated from the French by I. G. Macdonald, 1977.

The history of mathematics shows that a theory almost always originates in efforts to solve a specific problem (for example, the duplication of the cube in Greek mathematics). It may happen that these efforts are fruitless, and we have our first category of problems:
(I) Stillborn problems (examples: the determination of Fermat primes, or the irrationality of Euler's constant).

A second possibility is that the problem is solved but does not lead to progress on any other problem. This gives a second class:
(II) Problems without issue (this class includes many problems arising from "combinatorics").

A more favorable situation is one in which an examination of the techniques used to solve the original problem enables one to apply them (perhaps by making them considerably more complicated) to other similar or more difficult problems, without necessarily feeling that one really understands why they work. We may call these
(III) Problems that beget a method (analytic number theory and the theory of finite groups provide many examples).

In a few rather rare cases the study of the problem ultimately (and perhaps only after a long time) reveals the existence of unsuspected underlying structures that not only illuminate the original question but also provide powerful general methods for elucidating a host of other problems in other areas; thus we have
(IV) Problems that belong to an active and fertile general theory (the theory of Lie groups and algebraic topology are typical examples at the present time).

• Marc Kac, John Riordan, in JCTA (1978).

It is difficult to imagine that Combinatorial Theory, which today is one of the most rapidly developing and highly popular parts of Mathematics, was only a short thirty years ago a nearly forgotten throwback to the last century. Relegated to a misty corner of the mathematical edifice the once proud Ars Combinatoria was practiced almost clandestinely and only by a vanishingly small group of mathematicians, who appeared to their contemporaries as dedicated though somewhat misguided eccentrics.

• L. Mirsky, Book Review, in Bull. AMS (1979).

Even a sleepy observer of the contemporary mathematical scene cannot but be struck by the spectacular growth of combinatorial studies in the very recent past. As little as, say, twenty years ago enthusiasm for work in this field was still regarded as a sign of mild eccentricity, and the problems investigated by combinatorialists were nowhere near the centre of the mathematical stage. Thus the late J. H. C. Whitehead probably did no more than express a tacit consensus when he described the theory of graphs as 'the slums of topology'. Nous avons changé tout cela-decisively and rapidly. [..]

[Combinatorics] is obviously no longer to be shrugged off as being merely of marginal significance-its presence is too strongly felt and its influence too pervasive for the continuation of Whitehead's dismissive stance. Indeed, every one of a score of outstanding mathematicians who have contributed crucial ideas to the recent development of combinatorics could assert with rightful pride: exegi monumentum. Voices have even been heard insisting on the fundamental role of combinatorics for the entire body of mathematical knowledge. Claims as far-reaching as these seem to me excessive; but it is symptomatic of the current prestige of the subject that they should be made at all. [..]

As I see it, combinatorics is a range of linked studies which have something in common and yet diverge widely in their objectives, their methods, and the degree of coherence they have attained. Most are concerned with criteria for the existence of certain 'patterns' or 'arrangements' or 'configurations', where these terms need to be interpreted in a very broad sense. Further, in many problems, the emphasis is on quantitative aspects: we seek to determine or estimate the number of objects of a specified kind or to characterize the patterns which are, in some way, extremal. Now the expert can live a happy and useful life without the aid of my fumbling attempts at elucidation while the mathematician unfamiliar with combinatorial problems will not derive from them any perceptible enlightenment. By the shades of Euclid and Aristotle! I have evidently not supplied much of a definition-but is there a better one?

• Norman L. Biggs, The roots of combinatorics, Historia Mathematica, 1979.

Combinatorics has often been associated with unusual or unconventional aspects of scholarship; the motley combinations of the Hindus and the mystical magic squares of the Chinese and the Arabs are good examples of this. In contrast, most of mathematics has been stimulated by problems of unchallenged depth and seriousness. However, in recent times the Combinatorial Art has found a variety of uses, in both pure and applied science, and consequently it has become part of the mainstream of modern mathematics.

• Martin Aigner, Combinatorial Theory, Springer, 1979.

It is now generally recognized that the field of combinatorics has, over the past years, evolved into a fully-fledged branch of discrete mathematics whose potential with respect to computers and the natural sciences is only beginning to be realized. Still, two points seem to bother most authors: The apparent difficulty in defining the scope of combinatorics and the fact that combinatorics seems to consist of a vast variety of more or less unrelated methods and results. As to the scope of the field, there appears to be a growing consensus that combinatorics should be divided into three large parts:

• Enumeration, including generating functions, inversion, and calculus of finite differences;
• Order Theory, including finite po sets and lattices, matroids, and existence results such as Hall's and Ramsey's;
• Configurations, including designs, permutation groups, and coding theory.

• László Lovász, Combinatorial Problems and Exercises, North-Holland, Amsterdam, 1979.

Having vegetated on the fringes of mathematical science for centuries, combinatorics has now burgeoned into one of the fastest growing branches of mathematics – undoubtedly so if we consider the number of publications in this field, its applications in other branches of mathematics and in other sciences, and also the interest of scientists, economists and engineers in combinatorial structures. The mathematical world was attracted by the successes of algebra and analysis and only in recent years has it become clear, due largely to problems arising from economics, statistics, electrical engineering and other applied sciences, that combinatorics, the study of finite sets and finite structures, has its own problems and principles. These are independent of those in algebra and analysis but match them in difficulty, practical and theoretical interest and beauty.

Yet the opinion of many first-class mathematicians about combinatorics is still in the pejorative. While accepting its interest and difficulty, they deny its depth. It is often forcefully stated that combinatorics is a collection of problems which may be interesting in themselves but are not linked and do not constitute a theory. It is easy to obtain new results in combinatorics or graph theory because there are few techniques to learn, and this results in a fast-growing number of publications.

The above accusations are clearly characteristic of any field of science at an early stage of its development – at the stage of collecting data. As long as the main questions have not been formulated and the abstractions to a general level have not been carried through, there is no way to distinguish between interesting and less interesting results – except on an aesthetic basis, which is, of course, too subjective. Those techniques whose absence has been disapproved of above await their discoverers. So underdevelopment is not a case against, but rather for, directing young scientists toward a given field.

In my opinion, combinatorics is now growing out of this early stage. There are techniques to learn: enumeration techniques, matroid theory, the probabilistic method, linear programming, block design constructions, etc. There are branches which consist of theorems forming a hierarchy and which contain central structure theorems forming the backbone of study: connectivity of graphs (network flows) or factors of graphs, just to pick two examples from graph theory. There are notions abstracted from many nontrivial results, which unify large parts of the theory, such as matroids or the concept of good characterization. My feeling is that it is no longer possible to obtain significant results without the knowledge of these facts, concepts and techniques. (Of course, exceptions may occur, since the field is destined to cover such a large part of the world of mathematics that entirely new problems may still arise.)

• Gina Bari Kolata, Gian-Carlos Rota and Combinatorial Math: One strength of combinatorics is its interaction with other areas of mathematics and science, Science (Apr 6, 1979).

Combinatorics, Rota explains, began as a separate discipline about 30 years ago [..], and it began partly because so many combinatorial problems has arisen in other disciplines. These problems had been ignored or concerted to other forms, but it as becoming increasingly obvious that techniques were needed to solve them. The time was right for the birth of combinatorics as a separate field, especially because the advent of the computer made possible experimental work, thereby opening new adventures in solving problems.

The field gained momentum in the past decade because of a movement in mathematics towards greater concreteness. Combinatorics, which deals with finite sets of objects and how to count or arrange them, is the epitome of concreteness. [..] As combinatorics gained followers it also gained methodology. Now the field is at a point where there are some standard techniques for solving problems and there are a great many important problems to be solved. [..]

"Early combinatorial mathematicians used very crude methods," Rota says, "and so they had to be very bright people. Now even people who are not too bright can do research." [..]

Combinatorics research does have a distinctive flavor and requires a specific talent, which is often quite different from the talents required by other fields of mathematics. Those who enjoy combinatorics, Rota says, like to count, enjoy puzzles and finite structures and like symmetries. [..] Combinatorial mathematicians tend to be extremely constructivist, meaning they like to demonstrate how to find particular mathematical objects as opposed to showing only that the objects exist.

• Béla Bollobás, Book Review of Combinatorial Problems and Exercises by László Lovász, Bull. AMS (1981).

Perhaps combinatorics is no longer deemed to be the slum of topology but it still has a remarkable polarising effect on mathematicians. The practitioners of combinatorics tend to idolise it as the only truly interesting branch of mathematics, while people not active in combinatorics are likely to have no respect for it and dismiss it as a collection of scattered results and trivial artificial problems. This highly unsatisfactory situation cannot be blamed entirely on the youth of the subject, though it is certainly one of the reasons. Those of us who work in combinatorics are also at e efault, for most of our journals do publish more than their fair share of below par papers. Furthermore, as combinatorics fails to command the respect of the majority of the mathematical community, some combinatorialists feel entitled to disregard the huge developments in the main branches of mathematics.

There are signs that these lean years for combinatorics will soon be over. This is the hope expressed by Lovász in the Preface of Combinatorial problems and exercises. "Having vegetated [..]" [This book] will certainly help to establish the respectability and worth of the subject.

• Ulrich Faigle, The Greedy Algorithm as a Combinatorial Principle, Sém. Lothar. Combin. B05k (1981), 4 pp.

It seems to be an open problem to give a definition of “combinatorics” which everyone agrees upon. For our purposes, the following standpoint is appropriate: combinatorics is the study of the processes involved in building up a combinatorial object step by step so that certain requirements are met.

• Pierre Cartier, in Séminaire Bourbaki (1982)

La combinatoire est en gros l’étude des géométries finies. Une partie est consacrée à la construction et à l’étude qualitative des configurations finies (graphes, plans projectifs, matroides, et plus récemment immeubles....). D’un autre côté, on s’occupe à compter les objets d’une certaine espèce, le plus souvent au moyen de séries génératrices; inversement, et cette préoccupation est plus récente, on s’efforce d’interpréter des identités entre polynômes et séries de puissances en introduisant les structures finies que comptent les coefficients.

La partie la plus vénérable de la combinatoire s’occupe des propriétés des permutations (avec ou sans répétitions) et des fonctions symétriques; les tableaux de Young y jouent un rôle prédominant. Il s’agit d’un retour aux traditions de l’Algèbre du siècle dernier, mais enrichi par tout l’arsenal des notions modernes. On met l’accent sur les méthodes constructives et algorithmiques; les points de contact avec le reste des Mathématiques sont nombreux, et en particulier avec la théorie des groupes, la géométrie algébrique, la topologie des classes caractéristiques, et aussi la physique mathématique. De plus, les problèmes et les méthodes de l’informatique théorique jouent un rôle croissant.

• I.M. Gelfand (1980s), quoted in V.V. Prasolov and V.M. Tikhomirov, Geometry, MCCMO, Moscow, 1997 (English translation: AMS, 2001)

Once, Israil Gelfand said that mathematics has three parts: analysis, geometry, and combinatorics. "What is combinatorics?" the listeners asked. The answer was: "This is a science not yet created ..."

• N.G. de Bruijn (recorded by J.W. Nienhuys, 1980s), in de Bruijn’s Combinatorics, Classroom notes (Ling-Ju Hung and Ton Kloks, Eds.)

What is combinatorics? You could say that combinatorics is that part of mathematics that concerns itself with finite systems. But this is not entirely true: the theory of finite groups, rings and fields does not belong to combinatorics. It is with combinatorics as it is with asymptotics. You would like to define asymptotics as the theory of limits, but large parts of the differential– and integration-calculus do not belong to asymptotics.

For a long time combinatorics consisted only of puzzles and games. Before the 1960s there was probably no course in combinatorics.

Combinatorics is mainly about counting. The nice thing about counting is that you learn something about the thing that you are counting. While counting, you notice that your knowledge about the subject is not sufficiently precise, and so counting becomes an educational activity. When two sets have the same number of elements, you try to understand why that is the case by establishing some natural bijection between the two.

• Gian-Carlo Rota, Preface to Combinatorial Enumeration by I.P. Goulden and D.M. Jackson, 1983.

The progress of mathematics can be viewed as a movement from the infinite to the finite. At the start, the possibilities of a theory, for example, the theory of enumeration appear to be boundless. Rules for the enumeration of sets subject to various conditions, or combinatorial objects as they are often called, appear to obey an indefinite variety of and seem to lead to a welter of generating functions. We are at first led to suspect that the class of objects with a common property that may be enumerated is indeed infinite and unclassifiable.

As cases file upon cases, however, patterns begin to emerge. Freakish instances are quietly discarded; impossible problems are recognized as such, and what is left organizes itself along a few general criteria. We would like these criteria to eventually boil down to one, but by and large we must be content with a small finite number.

• G. Pólya, R.E. Tarjan and D.R. Woods, Notes on Introductory Combinatorics, 1983.

Combinatorics can be classified into three types: enumerative, existential, and constructive. Enumerative combinatorics deals with the counting of combinatorial objects. Existential combinatorics studies the existence or nonexistence of combinatorial configurations. Constructive combinatorics deals with methods for actually finding specific configurations (as opposed to merely demonstrating their existence theoretically). [..] In constructive combinatorics, the problem is usually one of finding a solution efficiently, [..] using a reasonable length of time.

• Gian-Carlo Rota, Mathematics, Philosophy and Artificial Intelligence, Los Alamos Science, No. 12, 1985.

Combinatorics is an honest subject. No adèles, no sigma-algebras. You count balls in a box, and you either have the right number or you haven’t. You get the feeling that the result you have discovered is forever, because it’s concrete. Other branches of mathematics are not so clear-cut. Functional analysis of infinite-dimensional spaces is never fully convincing: you don’t get a feeling of having done an honest day’s work. Don’t get the wrong idea—combinatorics is not just putting balls into boxes. Counting finite sets can be a highbrow undertaking, with sophisticated techniques.

Much combinatorics of our day came out of an extraordinary coincidence. Disparate problems in combinatorics. ranging from problems in statistical mechanics to the problem of coloring a map, seem to bear no common features. However, they do have at least one common feature: their solution can be reduced to the problem of finding the roots of some polynomial or analytic function. The minimum number of colors required to properly color a map is given by the roots of a polynomial, called the chromatic polynomial; its value at N tells you in how many ways you can color the map with N colors. Similarly, the singularities of some complicated analytic function tell you the temperature at which a phase transition occurs in matter. The great insight, which is a long way from being understood, was to realize that the roots of the polynomials and analytic functions arising in a lot of combinatorial problems are the Betti numbers of certain surfaces related to the problem, Roughly speaking, the Betti numbers of a surface describe the number of different ways you can go around it. We arc now trying to understand how this extraordinary coincidence comes about, If we do, we will have found a notable unification in mathematics.

• Paul Erdős, Notes of Welcome, Graphs and Combinatorics, 1985.

There are several reasons for the rapid growth of graph theory and combinatorics. There is a purely mathematical reason: More and more mathematicians have realized that there are many interesting problems and applications to other branches of mathematics in combinatorics.

• Gian-Carlo Rota, Foreword to Enumerative Combinatorics by R.P. Stanley, vol. 1, 1986.

It has been said that combinatorics has too many theorems, matched with very few theories; Stanley's book belies this assertion.

• I.M. Gelfand, Kyoto Inamori Foundation Lecture, 1989.

What new things will come from mathematics? I want to mention two of them. The first new thing is a very old one and has been on the backburner of mathematics — it is combinatorics.

• Noga Alon, Non-Constructive Proofs in Combinatorics, in Proc. ICM Kyoto, 1990.

One of the main reasons for the fast development of Combinatorics during the recent years is certainly the widely used application of combinatorial methods in the study and the development of efficient algorithms. It is therefore somewhat surprising that many results proved by applying some of the modern combinatorial techniques, including Topological methods, Algebraic methods, and Probabilistic methods, merely supply existence proofs and do not yield efficient (deterministic or randomized) algorithms for the corresponding problems.

• Chris Godsil, Algebraic Combinatorics, Chapman & Hall, 1993.

There are people who feel that a combinatorial result should be given a "purely combinatorial" proof, but I am not one of them. For me the most interesting parts of combinatorics have always been those overlapping other areas of mathematics.

• Ronald Graham, Martin Grötschel and László Lovász, Preface, in Handbook of Combinatorics, vol. 1, Elsevier, Amsterdam, 1995.

Combinatorics belongs to those areas of mathematics having experienced a most impressive growth in recent years. This growth has been fuelled in large part by the increasing importance of computers, the needs of computer science and demands from applications where discrete models play more and more important roles. But also more classical branches of mathematics have come to recognize that combinatorial structures are essential components of many mathematical theories.

• L. Lovász, D.B. Shmoys and É. Tardos, Combinatorics in computer science , in Handbook of Combinatorics, vol. 2, Elsevier, Amsterdam, 1995.

It is not surprising that computer science is perhaps the most important field of applications of combinatorial ideas. Modern computers operate in a discrete fashion both in time and space, and much of classical mathematics must be "discretized" before it can be implemented on computers as, for example, in the case of numerical analysis. The connection between combinatorics and computer science might be even stronger than suggested by this observation; each field has profited from the other. Combinatorics was the first field of mathematics where the ideas and concepts of computer science, in particular complexity theory, had a profound impact.

• Joel Spencer, Probabilistic methods, in Handbook of Combinatorics, vol. 2, Elsevier, Amsterdam, 1995.

Does the heart of mathematics lie in the building of structures or in the solving of individual problems? Not an either – or question, to be sure, but one that is particularly effective in splitting the ranks of combinatorialists. Use of algebraic structure to explain discrete phenomena will be central to some, to others grotesque. A clever argument is beautiful to the problem-solver, a curiosity to a structuralist. The very term "combinatorial methods", has to this author, an oxymoronic character. It is the brilliant proofs, those that expand and/or transcend known technologies, which express the soul of the subject.

• Peter J. Cameron, Combinatorics: topics, techniques, algorithms, Cambridge University Press, 1995.

Combinatorics is special. Most mathematical topics which can be covered in a lecture course build towards a single, well-defined goal, such as the Prime Number Theorem. Even if such a clear goal doesn’t exist, there is a sharp focus (e.g. finite groups). By contrast, combinatorics appears to be a collection of unrelated puzzles chosen at random. Two factors contribute to this. First, combinatorics is broad rather than deep. Second, it is about techniques rather than results.

Combinatorics could be described as the art of arranging objects according to specified rules. We want to know, first, whether a particular arrangement is possible at all, and if so, in how many different ways it can be done. If the rules are simple, the existence of an arrangement is clear, and we concentrate on the counting problem. But for more involved rules, it may not be clear whether the arrangement is possible at all.

• Richard K. Guy, The unity of combinatorics, in C.J. Colbourn and E.S. Mahmoodian (Editors), Combinatorics Advances, Kluwer, 1995, 129-159.

One reason why Combinatorics has been slow to become accepted as part of mainstream Mathematics is the common belief that it consists of a bag of isolated tricks, a number of areas [long list -- IP] with little or no connexion between them. We shall see that they have numerous threads weaving them together into a beautifully patterned tapestry.

• Gian-Carlo Rota, Combinatorics, Representation Theory and Invariant Theory, in Indiscrete Thoughts, Birkhäuser, Boston, 1997.

The field normally classified as algebra really consists of two quite separate fields. Let us call them Algebra One and Algebra Two for want of a better language. Algebra One is the algebra whose bottom lines are algebraic geometry or algebraic number theory. Algebra One has by far a better pedigree than Algebra Two, and has reached a high degree of sophistication and breadth. Commutative algebra, homological algebra, and the more recent speculations with categories and topoi are exquisite products of Algebra One.

Algebra Two has had a more accidented history. [..] In the beginning Algebra Two was largely cultivated by invariant theorists. Their objective was to develop a notation suitable to describe geometric phenomena which is independent of the choice of a coordinate system. In pursuing this objective, the invariant theorists of the nineteenth century were led to develop explicit algorithms and combinatorial methods. The first combinatorialists, MacMahon, Hammond, Brioschi, Trudi, Sylvester, were invariant theorists. One of the first papers in graph theory, in which the Petersen graph is introduced, was motivated by a problem in invariant theory. Clifford's ideal for invariant theory was to reduce the computation of invariants to the theory of graphs. [..]

Algebra Two has recently come of age. In the last twenty years or so, it has blossomed and acquired a name of its own: algebraic combinatorics. Algebraic combinatorics, after a tortuous history, has at last found its own bottom line, together with a firm place in the mathematics of our time.

• Anders Björner and Richard P. Stanley, A Combinatorial Miscellany, L’Enseignement mathématique, Monograph no. 42, 2010 (an early version with the same quote has appeared in 1997).

A recent newcomer to the the center stage of modern mathematics is the area called combinatorics. Although combinatorial mathematics has been pursued since time immemorial, and at a reasonable scientific level at least since Leonhard Euler (1707–1783), the subject has come into its own only in the last few decades. The reasons for the spectacular growth of combinatorics come both from within mathematics itself and from the outside.

Beginning with the outside influences, it can be said that the recent development of combinatorics is somewhat of a cinderella story. It used to be looked down on by “mainstream” mathematicians as being somehow less respectable than other areas, in spite of many services rendered to both pure and applied mathematics. Then along came the prince of computer science with its many mathematical problems and needs — and it was combinatorics that best fitted the glass slipper held out.

The developments within mathematics that have contributed to the current strong standing of combinatorics are more difficult to pinpoint. One is that, after an era where the fashion in mathematics was to seek generality and abstraction, there is now much appreciation of and emphasis on the concrete and “hard” problems. Another is that it has been gradually more and more realized that combinatorics has all sorts of deep connections with the mainstream areas of mathematics, such as (to name the most important ones) algebra, geometry, probability and topology.

[..] Fortunately, the problems and results of combinatorics are usually quite easy to state and explain, even to the layman. Its accessibility is one of its many appealing aspects.

• Daniel J. Kleitman, On the future of combinatorics, in Essays on the future, Birkhäuser, Boston, MA, 2000.

First, what is combinatorics? I have never been able to find a satisfactory answer to this question. The term "combinatorics" seems to refer to discrete or finite aspects of mathematics, rather than those involving continuity. This definition won't do. There are combinatorial aspects of all mathematical subjects, especially analysis, and continuous aspects are perfectly acceptable in combinatorics. Furthermore, there fields that can be and are called combinatorial topology, combinatorial geometry, and algebraic combinatorics; logic and probability are highly combinatorial subjects.

Yet, there is a perfectly good definition of the subject, or rather of the concept of a combinatorial argument. Combinatorics is the area of mathematics that is concerned with, relates to, employs, or studies combinatorial arguments. So what is a combinatorial argument? [..] A combinatorial argument is the one that consists predominately of ingenuity or detailed reasoning rather than knowledge of existing mathematics. This is in contrast with knowledge-based argument, which relies heavily on piecing together known results. [..]

Combinatorics will survive as long as mathematics does. Some of it, particularly Algebraic Combinatorics, has already developed into an ordinary legitimate branch of mathematics with a close relationship to the study of group representations. Others will be (and have been) incorporated into applied fields like Operations Research or Theoretical Computer Science. But the real world will still create new variations of problems, which will still require new methods and new ideas, and these will qualify as combinatorics in all respects. [..]

There is one other direction in which combinatorics can develop in importance both within mathematics and without, and that is in expanding the notion of mathematics as a kind of aesthetics, as an art form. [..] Mathematics, and, more specifically, combinatorics, is full of intellectually beautiful arguments and structures. These constructs are an important part of the appeal of mathematics to mathematicians. They represent an important aesthetic resource of mankind, yet they lie hidden to almost everyone outside mathematics, and to many within it. That mathematics is a realm of magnificent aesthetic joys was one of the best kept secrets of the 20th century.

• Gilbert Labelle, Manipulating Combinatorial Structures, Canadian Mathematics Education Study Group (May 2000).

A combinatorial structure is a finite construction made on a finite set of elements. In real-life situations, combinatorial structures often arise as "skeletons" or "schematic descriptions" of concrete objects. For example, on a road map, the elements can be cities and the finite construction can be the various roads joining these cities. Similarly, a diamond can be considered as a combinatorial structure: the plane facets of the diamond are connected together according to certain rules. Combinatorics can be defined as the mathematical analysis, classification and enumeration of combinatorial structures.

• Ian Macdonald, talk at the Isaac Newton Institute (Fall 2000); reported by Lauren Williams (May 2018).

One starts out in life trying to do mathematics, and winds up doing combinatorics.

• W.T. Gowers, The two cultures of mathematics, in Mathematics: frontiers and perspectives, AMS, 2000, 65–78.

Combinatorics appears to many to consist of a large number of isolated problems and results, and therefore to be at a disadvantage in this respect. Each result individually may well require enormous ingenuity, but ingenious people exist, especially in Hungary, and future generations of combinatorialists will not have the time or inclination to read and admire more than a tiny fraction of their output.

Let me attempt to answer this criticism. It is certainly rare in combinatorics for somebody to and a very general statement which suddenly places a large number of existing results in their proper context. It is also true that many of the results proved by combinatorialists are somewhat isolated and will be completely forgotten (but this does not distinguish combinatorics from any other branch of mathematics). However, it is not true that there is no structure at all to the subject. The reason it appears to many mathematicians as though combinatorics is just a miscellaneous collection of individual problems and results is that the organizing principles are less explicit.

• Peter J. Cameron, Combinatorics entering the third millennium, 2001 (third draft; the final version is less charming).

Henry Whitehead reportedly said, “Combinatorics is the slums of topology”. This prejudice, the view that combinatorics is quite different from ‘real mathematics’, was not uncommon in the twentieth century, among popular expositors as well as professionals. In his biography of Srinivasa Ramanujan, Robert Kanigel describes Percy MacMahon in these terms:

“ [MacMahon’s] expertise lay in combinatorics, a sort of glorified dicethrowing, and in it he had made contributions original enough to be named a Fellow of the Royal Society.”
In the later part of the century, attitudes changed. [..] I have told the story mostly through examples. If the views and examples are somewhat personal, perhaps this is better than having no views at all. [..] What is clear, though, is that combinatorics will continue to elude attempts at formal specification.

• Noga Alon, Discrete Mathematics: Methods and Challenges, in Proc. ICM Beijing, 2002.

While many of the basic combinatorial results were obtained mainly by ingenuity and detailed reasoning, without relying on many deep, well developed tools, the modern theory has already grown out of this early stage. [..] Most of the new significant results obtained in the area are inevitably based on the knowledge of these well developed concepts and techniques, and while there is, of course, still a lot of room for pure ingenuity in Discrete Mathematics, much of the progress is obtained by relying on the fast growing accumulated body of knowledge. [..]

It seems safe to predict that in the future there will be additional incorporation of methods from other mathematical areas in Combinatorics. However, such methods often provide non-constructive proof techniques, and the conversion of these to algorithmic ones may well be one of the main future challenges of the area. Another interesting recent development is the increased appearance of Computer aided proofs in Combinatorics, starting with the proof of the Four Color Theorem, and including automatic methods for the discovery and proof of hypergeometric identities. A successful incorporation of such proofs in the area, without losing its special beauty and appeal, is another challenge. These challenges, the fundamental nature of the area, its tight connection to other disciplines, and the many fascinating specific open problems studied in it, ensure that Discrete Mathematics will keep playing an essential role in the general development of science in the future as well.

• Ivan Cherednik, Double Hecke Algebras and Applications, NSF Award Abstract #0200276 (2002).

Combinatorics is the science of counting the possible arrangements and ways of organizing collections of anything (e.g., atoms, pebbles, star clusters).

• Mark Haiman, lecture, c. 2003; reported by Lauren K. Williams (May 2018). Attributed to "Anon" in Combinatorial Reciprocity Theorems by Matthias Beck and Raman Sanyal, 2018.

Combinatorics isn't a field. It's an attitude.

• Cristian Lenart, The Many Faces Of Modern Combinatorics, survey, 2003.

Combinatorics could be described as the study of arrangements of objects according to specified rules. We want to know, first, whether a particular arrangement is possible at all, and, if so, in how many different ways it can be done. Algebraic and even probabilistic methods play an increasingly important role in answering these questions. If we have two sets of arrangements with the same cardinality, we might want to construct a natural bijection between them. We might also want to have an algorithm for constructing a particular arrangement or all arrangements, as well as for computing numerical characteristics of them; in particular, we can consider optimization problems related to such arrangements. Finally, we might be interested in an even deeper study, by investigating the structural properties of the arrangements. Methods from areas such as group theory and topology are useful here, by enabling us to study symmetries of the arrangements, as well as topological properties of certain spaces associated with them, which translate into combinatorial properties.

• Mark Haiman, Combinatorial aspects of geometry and representation theory, NSF Award Abstract #0301072 (2003).

Combinatorics is the mathematical study of things that can be described, counted and manipulated in simple and explicit terms: things like trees, Young diagrams, or strings of symbols, that we can write on paper or encode in a computer. In my view, what makes combinatorics interesting is that deeply abstract mathematical concepts--such as geometric spaces in many dimensions, or algebras of symmetries--tend to have an underlying combinatorial structure.

• Sara Billey, Schubert Varieties Under a Microscope, talk at the Summer Institute in Algebraic Geometry, University of Washington, Seattle, Aug. 4, 2005 (this was an early variation on the quote, the exact quote is here).

Combinatorics is the nanotechnology of mathematics.

• Doron Zeilberger, Opinion 69, Dec. 7, 2005.

EVERYTHING IS COMBINATORICS. Classify Lie Algebras? It is just root systems and Dynkin Diagrams. Finite Groups? The Monster is a Combinatorial Design. Even when it is not obviously combinatorics, it could be made so. If it is too hard for us, then we need a computer! But computer science is all Discrete Math, alias combinatorics. In a way Logic is too. But Logic is so low-level, like machine language. It is much more fun and gratifying to work in Maple, and do higher-level combinatorics.

• Mireille Bousquet-Mélou, Rational and algebraic series in combinatorial enumeration, in Proc. ICM Madrid, 2006.

Enumerative problems arise spontaneously in various fields of mathematics, computer science, and physics. Among the most generous suppliers of such problems, let us cite discrete probability theory, the analysis of the complexity of algorithms, and the discrete models of statistical physics, like the famous Ising model. More generally, counting the objects that occur in one’s work seems to answer a natural curiosity. It helps to understand the objects, for instance to appreciate how restrictive are the conditions that define them. It also forces us to get some understanding of the structure of the objects: an enumerative result never comes for free, but only after one has elucidated, at least partly, what the objects really are.

• Irit Dinur and Omer Reingold, Assignment testers: Towards a combinatorial proof of the PCP-theorem, SIAM Journal on Computing, 2006.

Therefore, of particular interest is coming up with proofs of “combinatorial” nature [...] In that we were also influenced by the view that such combinatorial proofs, albeit more messy, are sometimes more intuitive (or at least may shed new intuition). We also note that such new proofs and constructions have the potential to imply new results, unknown with previous techniques.

• Peter J. Cameron, Notes on Combinatorics, 2007.

It is impossible to define combinatorics, but an approximate description would go like this. We are given the job of arranging certain objects or items according to a specified pattern. Some of the questions that arise include:

• Is the arrangement possible?
• In how many ways can the arrangement be made?
• How do we go about finding such an arrangement?

• Nati Linial, What is high-dimensional combinatorics?, Random-Approx, 2008.

Combinatorics must always have been fun. But when and how did it become a serious subject? I see several main steps in this development:

• The asymptotic perspective.
• Extremal combinatorics (in particular extremal graph theory).
• The emergence of the probabilistic method.
• The computational perspective.

• Richard P. Stanley, interview with The Tech, by Hyun Soo Kim (MIT, 2008).

HSK: In your opinion, how is the state of combinatorics?

RPS: Right now, I think it’s a very good, extremely active subject. But I think to do really high-level research now, particularly in algebraic and enumerative combinatorics, which I worked in, you have to know more than you used to. Back when I was a graduate student, you could use the simplest results from other areas like topology. Take the Euler characteristic. You could interpret that combinatorially and come up with all kinds of interesting results. Now, the topological combinatorics gets into some of the deeper more recent aspects of topology. Combinatorial representation theory is a huge subject now, probably one of the main areas of combinatorics. At the beginning, the very simplest representation theory – groups acting on sets – was enough to get all kinds of neat things. Now you have to be into all the latest algebras, like affine Hecke algebras, quivers, and very sophisticated, mainstream stuff that people are working on. You have to know more now than you used to.

• Timothy Gowers, June Barrow-Green and Imre Leader, The Princeton Companion to Mathematics, Princeton University Press, Princeton, NJ, 2008.

There are various ways in which one can try to define combinatorics. None is satisfactory on its own, but together they give some idea of what the subject is like. A first definition is that combinatorics is about counting things. [..]

Combinatorics is sometimes called “discrete mathematics” because it is concerned with “discrete” as opposed to “continuous” structures. Roughly speaking, an object is discrete if it consists of points that are isolated from each other and continuous if you can move from one point to another without making sudden jumps. [..] There is a close affinity between combinatorics and theoretical computer science (which deals with the quintessentially discrete structure of sequences of 0s and 1s), and combinatorics is sometimes contrasted with analysis, though in fact there are several connections between the two.

A third definition is that combinatorics is concerned with mathematical structures that have “few constraints.” This idea helps to explain why number theory, despite the fact that it studies (among other things) the distinctly discrete set of all positive integers, is not considered a branch of combinatorics.

• Noga Alon and Michael Krivelevich, Extremal and Probabilistic Combinatorics, in The Princeton Companion to Mathematics, Princeton University Press, Princeton, NJ, 2008.

Although combinatorics is probably as old as the human ability to count, the field has experienced tremendous growth during the last fifty years and has matured into a thriving area with its own set of problems, approaches, and methodology.

It is hard to give a rigorous definition of combinatorics. Instead, let us start with a few examples to illustrate what the area is about. [...] The examples above suggest that combinatorics is a basic mathematical discipline that plays a crucial role in the development of many other mathematical areas.

• Imprints newspaper interview with Béla Bollobás, in Yu Kiang Leong, Creative minds, charmed lives, World Sci., 2010.

YKL: How do you see the future of combinatorics, especially random graphs?

BB: Hilbert, I think, said that a subject is alive only if it has an abundance of problems. It is exactly this that makes combinatorics very much alive. I have no doubt that combinatorics will be around in a hundred years from now. It will be a completely different subject but it will still flourish simply because it still has many, many problems.

YKL: You mentioned that there is an abundance of problems in combinatorics. It seems that combinatorial problems are very easy to formulate but very hard to solve.

BB: For me, the difference between combinatorics and the rest of mathematics is that in combinatorics we are terribly keen to solve one particular problem by whatever means we can find. So if you can point us in the direction of a tool that may be used to attack a problem, we shall be delighted and grateful, and we’ll try to use your tool. However, if there are no tools in sight then we don’t give up but we’ll try to use whatever we have access to: bare hands, ingenuity, and even the kitchen sink. Nevertheless, it is a big mistake to believe that in combinatorics we are against using tools — not at all. We much prefer to get help from “mainstream” mathematics rather than use “combinatorial” methods only, but this help is rarely forthcoming. However, I am happy to say that the landscape is changing. [..]

A totally ignorant and unfair way of judging a result in combinatorics is to ask the author: “What have you used to prove your theorem?” Then, upon being told that such and such a theorem was used, comes the retort: “Oh, that’s very easy. I could have done it.” What nonsense. Yes, of course it’s easy once you are told what to do. The achievement is in finding the tool that can crack the problem after a series of clever manipulations that make the problem amenable to the application of the tool. [..]

The trouble with the combinatorial problems is that they do not fit into the existing mathematical theories. [..] More often than not, we simply do not have the machinery to attack our problems. This is certainly not the situation in other branches of mathematics. In fact, it may happen that first a wonderful machine is built and then the search starts for a worthwhile problem that this machine can be applied to. This attitude is totally foreign to combinatorics. In combinatorics we have our problem which at the beginning looks like a Chinese box: there seems to be no way in, there is no indication as to how to start it. Here’s the problem: we want to solve it and we don’t care in what way we solve it.

• John Stillwell, Mathematics and Its History, Springer, Berlin, 2010.

Like number theory before the 19th century, combinatorics before the 20th century was thought to be an elementary topic without much unity or depth. We now realize that, like number theory, combinatorics is infinitely deep and linked to all parts of mathematics.

• Lorenz J. Halbeisen, Combinatorial Set Theory: With a Gentle Introduction to Forcing, Springer, November 2011.

Combinatorics with all its various aspects is a broad field of Mathematics which has many applications in areas like Topology, Group Theory and even Analysis. A reason for its wide range of applications might be that Combinatorics is rather a way of thinking than a homogeneous theory, and consequently Combinatorics is quite difficult to define. Nevertheless, let us start with a definition of Combinatorics which will be suitable for our purpose:

Combinatorics is the branch of Mathematics which studies collections of objects that satisfy certain criteria, and is in particular concerned with deciding how large or how small such collections might be.

• Peter J. Cameron, interview with Dimitri Leemans, in Newsletter of New Zealand Math. Society, December 2014.

DL: What would you say to people who say that combinatorics is something you can do while waiting for your bus?

PJC: I believe any subject of mathematics is something you can do while waiting for your bus. At a certain point you have to sit down and write the calculations down on paper, check them and check all the arguments carefully but mathematics is a subject that we do in our heads and particularly, in the back of our heads while we’re doing something else, waiting for a bus or going for a walk or sleeping or whatever. We are constantly processing and pushing forward mathematics. Combinatorics has taken a lot of flak because it is a subject that is broad rather than deep. Nobody could prove something like the Riemann hypothesis in combinatorics. There are no comparable questions but the best results in combinatorics produce unexpected links between entirely different-looking parts of the subject and the insight to do that could happen to you just as easily while you are waiting for the bus as anywhere else. I think that anyone who says that combinatorics is not really mathematics ignores the fact that almost every great mathematician has used combinatorial arguments in at least part of their work, has sometimes even explicitly acknowledged that. So, for example, Roger Lyndon, a group theorist, was asked in an interview what he thought was the common feature in his best work and he said “combinatorics.”

• Josephine Yu, Tropical Combinatorics and Applications, NSF Award Abstract #1600569 (2016).

Combinatorics is the art and science of distilling a complex mathematical structure into simple attributes and developing from this a deeper understanding of the original structure.

• Robin J. Wilson, Combinatorics: A Very Short Introduction, Oxford University Press, 2016. [See Chapter 1, What is combinatorics?]

We may loosely describe [combinatorics] as the branch of mathematics concerned with selecting, arranging, constructing, classifying, and counting or listing things. [..]

Much of combinatorics originated in recreational pastimes [..] But in recent years the subject has developed in depth and variety and has increasingly become a part of mainstream mathematics. Prestigious mathematical awards such as the Fields Medal and the Abel Prize have been given for groundbreaking contributions to the subject, while a number of spectacular combinatorial advances have been reported in the national and international media.

Undoubtedly part of the reason for the subject's recent importance has arisen from the growth of computer science and the increasing use of algorithmic methods for solving real-world practical problems. These have led to combinatorial applications in a wide range of subject areas, both within and outside mathematics, including network analysis, coding theory, probability, virology, experimental design, scheduling, and operations research.

• William Gasarch, My Answers to My P vs NP Poll, SIGACT News Open Problem Column, (2019).

It has been said that combinatorics is both the easiest and hardest field of mathematics. Easy since a lot of it requires no prerequisite knowledge. Hence a High School Student can do work in it. Hard because a lot of it requires no prerequisite knowledge. Hence you can’t easily apply continuous techniques.

• Noga Alon, interview with Enumerative Combinatorics and Applications, by Toufik Mansour (October 2020).

TM: Would you tell us broadly what combinatorics is?

NA: Combinatorics is primarily the mathematics of finite objects, investigating the properties of combinatorial structures. Its areas of study include exact and asymptotic enumeration, graph theory, probabilistic and extremal combinatorics, designs and finite geometries. The subject has applications in other areas including geometry, number theory, optimization and theoretical computer science.

• Richard P. Stanley, interview with Enumerative Combinatorics and Applications, by Toufik Mansour (October 2020).

TM: Would you tell us broadly what combinatorics is?

RPS: Not so easy! No definition will be entirely satisfactory, but I think of combinatorics as the study of discrete objects with only simple additional structure. A symmetric binary relation R, not allowing xRx but with no further conditions, is quite simple, and this defines the combinatorial concept of simple graphs. On the other hand, a group has too much “additional structure” to be considered a pure combinatorial object. If you combine in a suitable sense simple and more complicated structures, then you get subjects like algebraic, geometric, and topological combinatorics.

• Doron Zeilberger, interview with Enumerative Combinatorics and Applications, by Toufik Mansour (November 2020).

TM: Would you tell us broadly what combinatorics is?

DZ: Combinatorics is everything. All our worlds, the physical, mathematical, and even spiritual, are inherently finite and discrete, and so-called infinities, be their actual or potential, as well as the ‘continuum’, are ‘optical illusions’. [original emphasis -- IP]

• Stephen Melczer, An Invitation to Analytic Combinatorics, Springer, 2021.

If you ask a mathematician what they love most about mathematics, certain answers invariably arise: beauty, abstraction, creativity, logical structure, connection (between disciplines and between people), elegance, applicability, and fun. [..] Combinatorics is the branch of mathematics best situated to embody and illustrate all of these virtues.

• Douglas West, Combinatorial Mathematics, Cambridge Univ. Press, 2021.

Combinatorics is now a mature discipline. Although some see it as a maelstrom of isolated problems, it has central themes, techniques, and results that make it a surprisingly coherent subject.

• Xavier Viennot, interview with Enumerative Combinatorics and Applications, by Toufik Mansour (April 2021).

TM: Would you tell us broadly what combinatorics is?

XV: Let us say that combinatorics is the study of finite mathematical objects having a poor underlying structure. In other words, some objects where a kid can give examples once you explain to him the definition (permutations, Young tableaux, movements of a tower on a chessboard, tilings the same chessboard with dimers, etc). Enumerating finite structures is not necessarily combinatorics (just think of the number of simple finite groups!). Nowadays combinatorics, especially bijective combinatorics, is rather an attitude that is transversal to all mathematics (and also physics, computer science, theoretical biology, etc) where classical (and non-classical) theories are studied with a “combinatorial” point of view. [..] G.C. Rota said that mathematicians were first studying continuum as a first step before going to the study of the finite. With this (broad) point of view, the beauty of Nature is a reflection of the underlying beauty of combinatorics. Even if it is a belief, I love this point of view.