Abstract. (https://ucla.zoom.us/j/672060601)

The key step in the proof of the triangle removal lemma can be viewed as saying that we can identify a small number of edges in a graph as being the "exceptional" edges, and the remaining edges are sufficiently "representative of the neighborhood around them" that, if there are any triangles left, there must have been many triangles. This can be viewed as a amalgamation problem in the sense of model-theory: given types p(x,y), q(x,z), and r(y,z), each of which indicates that there is an edge between the vertices, when are the types p,q,r "large" in a way which guarantees that there are many (x,y,z) extending each of these types?

The exceptional types can be characterized as the non-Lebesgue points - that is, the points which fail to satisfy the Lebesgue density theorem in the right measure space. We give a way to generalize this to types of higher arity and use this to prove a new generalization, an "ordered hypergraph removal lemma", extending the recent ordered graph removal lemma of Alon, Ben-Eliezer, and Fischer.

Abstract. In his classical 1932 paper, von Neumann asked 3 questions: Can you classify the statistical behavior of differentiable systems? Are there systems where time-forward is not isomorphic to time-backward? Is every abstract statistical system isomorphic to a differentiable system? These questions can be addressed with some surprising consequences by embedding them in Polish Spaces. Indeed the tools answer other questions from the 60's and 70's such as the existence of diffeomorphisms with arbitrary Choquet simplexes of invariant measures. Moreover there are surprising analogues to Hilbert's 10th problem.

In a different category, building on work of Poincar\`{e}, Smale proposed classifying the \emph{qualitative} behavior of differentiable systems on compact manifolds. His 1967 paper explicitly argued that the equivalence relation of ``conjugacy up to homeomorphism" captures this notion and he proposes classifying it. Call this notion \emph{topological equivalence}. Very recent joint results with A. Gorodetski show:

- The equivalence relation $E_0$ is Borel reducible to topological equivalence of diffeomorphisms of any smooth 2-manifold.

- The equivalent relation of \emph{Graph Isomorphism} is Borel reducible to topological equivalence of diffeomorphisms of any smooth manifold of dimension 5 or above.

As corollaries, none of the classical numerical invariants such as entropy, rates of growth of periodic points and so forth, can classify diffeomorphisms of 2-manifolds, and there is no Borel classification at all of diffeomorphisms of 5-manifolds.

In the same 1967 paper Smale asks (in different language) whether there is a generic class that can be classified. This is still an open problem.

Link to the talk: https://zoom.us/j/8244003061

Abstract. There exist topological Ramsey spaces admitting metric projections where every Baire set has the Ramsey property. Jointly with N. Dobrinen, we gave a characterization of such spaces, answering a question of S. Todocervic. In this talk, we will discuss the characterization and present some examples (see https://drive.google.com/open?id=1l35j19JP9B6-fskoSc4I91VLnV-XABxw for the references).

Abstract. Introducing the notion of a stationary independence relation, Tent and Ziegler developed a framework for proving abstract simplicity of automorphism groups of a broad range of mathematical structures.

In current work with Calderoni and Kwiatkowska we are extending this to cover in particular order and tournament expansions of such structures and show that their automorphism groups are again simple groups.

Abstract. Strong coding trees were invented in order to solve the problem of whether or not the triangle-free Henson graph has analogues of Ramsey's theorem for colorings of finite triangle-free graphs. Since then, the method has been extended to handle all k-clique-free Henson graphs. Further, it has been useful for extending Ellentuck's infinite dimensional Ramsey theory to the rationals, and for extending the Galvin-Prikry theorem to the Rado graph. We will present some of the main ideas in these results, the history of the area, and some future directions.

Abstract. We will develop a framework for enriching various classical invariants from algebraic topology with descriptive set-theoretic information. Applying these ideas to Steenrod homology theory we get a new invariant for compact metrizable spaces up to homotopy equivalence which we call ``definable homology.'' Similarly we get a dual notion of ``definable cohomology'' for locally compact metrizable spaces by enriching the classical \v{C}ech cohomology theory. Our invariants are strictly finer than the original ones. In particular, we will show that $n$-dimensional (co)solenoids are completely classified up to homeomorphism by their definable (co)homology. In the process, we will generalize Veli\v{c}kovi\'c's rigidity theorem for definable automorphisms of $\mathcal{P}(\omega)/\mathrm{fin}$, to the arbitrary quotient $G/N$, where $G$ is a locally pro-finite abelian group and $N$ is a countable dense subgroup of $G$, and we will develop some basic definable homological algebra.

This is a joint work with J. Bergfalk and M. Lupini.

Abstract. In this talk I will show that only finite fields are definable in the theory of nonabelian free groups. This is joint work with Ayala Byron.

Abstract. In this talk, we show that the class of finite metric spaces (of diameter at most 1) has an almost sure theory in the following sense: for each sentence $\sigma$ in the language of metric spaces, there is a real number $r$ such that, for any $\epsilon>0$, for large enough finite metric spaces $X$, with high probability, the value of $\sigma$ in $X$ differs from $r$ by at most $\epsilon$. This almost-sure theory is in fact the theory of a particular metric space, which we call the almost-sure metric space AS, and which can be defined as the Fraisse limit of the class of all finite metric spaces with nontrivial distances in the interval [1/2,1].

This is joint work with Bradd Hart.

Abstract. We are interested in structural properties of countable ergodic, but non-strongly ergodic, equivalence relations. Firstly, we will discuss a question posed by Jones and Schmidt in the 80s. In particular, we will provide examples and non-examples of equivalence relations all of whose almost invariant sets come from a single hyperfinite quotient. Secondly, we will study stable equivalence relations, i.e. those that can be written as a direct product of some equivalence relation with the hyperfinite one. We will then show that the stabilization of any strongly ergodic equivalence relation admits a unique stable decomposition in a precise sense.

Abstract. In his famous paper, "An Unsolvable Problem of Elementary Number Theory," Alonzo Church (1936) identified the intuitive notion of effective calculability with the mathematically precise notion of recursiveness. This proposal, known as Church's Thesis, has been widely accepted. In this talk I consider two early arguments against it. Pepis criticized Church's Thesis already in 1937 in his dissertation and in a letter written to Church. I will display recently found documents showing Pepis's stance. The main focus of the talk will be László Kalmár's famous "An Argument Against the Plausibility of Church's Thesis" from 1959. As this paper is quite short, my aim will be to present Kalmár's argument and to fill in missing details based on his general philosophical thoughts on mathematics and his writings published on these issues in Hungarian.

Abstract. I will survey recent work on first order expansions of $(\mathbb{R},<,+)$. Time permitting, I will discuss connections to automata theory and $\mathrm{NIP}$.

Abstract. It became clear through work of Borovik and Cherlin in 2008 that the classification theory for groups of finite Morley rank (fMr) is sufficiently developed that general questions about permutation groups of fMr can be settled even though the classification itself remains incomplete. This is a particularly salient direction since the study of uncountably categorical theories is intertwined with binding groups of fMr (acting on realizations of types).

Borovik and Cherlin posed several motivating problems, many of which are centered around the notion of ``generic $n$-transitivity''. In this talk, we will discuss the status and implications of their conjecture that the only transitive and generically $(n+2)$-transitive group of fMr acting on a set of rank $n$ is $operatorname{PGL}_{n+1}$ acting naturally on projective $n$-space. Among other things, we will highlight a general approach to constructing a projective geometry in this context, and we will also illustrate how this conjecture is intertwined with minimal fMr representations of the finite symmetric groups. The talk will begin with a brief overview of the fMr landscape---knowledge of the advanced theory of groups of fMr will be not be required.

Abstract. We will talk about some partial results and methods which point in the direction of more definable counterexamples of the continuum hypothesis. The goal is to try to identify a canonical generic extension of L(R) in which Theta^L(R)>aleph_3, the axiom of choice holds and whose theory is set-forcing invariant in the presence of large cardinals. From the point of view of Woodin's Omega-logic, the idea behind this work is to identify axioms which mirror forcing axioms and which settle Sigma_2 truth of initial segments of V but under which there are definable counterexamples to CH. We will first review the context and motivation of this investigation and previous results.

Abstract. The orbit equivalence relation of any continuous action of a Polish group $G$ has trivial Borel complexity if $G$ is compact. Similarly it is a theorem of A.S. Kechris that whenever a locally--compact Polish group acts continuously on a Polish space, the orbit equivalence relation of the action is essentially countable ---that is, Borel reducible to the orbit equivalence relation of an action of a countable group. A.S. Kechris asked for ``inverses'' to these results: (1) does every non--compact Polish group admit a continuous action that is not concretely classifiable? (2) does every non--locally--compact Polish group admit a continuous action that is not essentially countable?

Question (1) was positively answered by S. Solecki in a paper where he also answered question (2) for the special case where $G$ is the additive group of a separable Banach space. It is also resolved for the case where $G$ is an Abelian pro--countable group, by results of M. Malicki. In this talk I will present recent work on this problem, including a new dynamical proof of question (1) and a positive solution of question (2) in the case of non--Archimedean Polish groups.

This is in joint work with J. Zielinski.

Abstract. The main object of study of ergodic theory are the measure-preserving actions of (countable) groups on probability spaces. I will discuss a formalization of this setup in the framework of continuous logic and explain how some important notions studied in ergodic theory have a natural model-theoretic interpretation. This allows for some quick proofs of known results as well as a new rigidity theorem for strongly ergodic, distal actions. This is joint work with Tomas Ibarlucia.

Abstract. NIP theories are the first class of the hierarchy of n-dependent structures. The random n-hypergraph is the canonical object which is n-dependent but not (n-1)-dependent. Thus the hierarchy is strict. But one might ask if there are any algebraic objects (groups, rings, fields) which are strictly n-dependent for every n? We will start by introducing the n-dependent hierarchy and present all known results on n-dependent groups and fields.

Abstract. The property of "flatness" of a pregeometry (matroid) is best known in model theory as the device with which Hrushovski showed that his example refuting Zilber's conjecture does not interpret an infinite group. I will dedicate the first part of this talk to explaining what flatness is, how it should be thought of, and how closely it relates to hypergraphs and Hrushovski's construction method. In the second part, I will conjecture that the family of flat pregeometries associated to strongly minimal sets is model theoretically nice, and share some intermediate results.

Abstract. In this talk I will review some fundamental concepts in computability theory and show how they can be applied to analyze the relative complexity of real-valued functions. I will review some old results that come from this analysis, in particular, the continuous degrees of Miller. Then I will introduce some new hierarchies that results from joint work with Downey and Westrick. One of these new hierarchies refines the Bourgain rank for Baire class 1 functions and allows a new way to extend this rank to all Baire measurable functions.

Abstract. For $L$-structures $B$, $C$ we use the notation ${C choose B}$ to denote the set of all substructures of $C$ isomorphic to $B$. We say that a countable, locally finite structure $I$ ordered by a relation $<$ has RP (the Ramsey property) if for all $A_0, B_0 in textrm{age}(I)$ and integers $k geq 1$ there is some $C_0 in textrm{age}(I)$ such that

$C_0 rightarrow (B_0)^{A_0}_k$. In other words, for all functions $c: {C_0 choose A_0} rightarrow k$ there is some $B' subseteq C_0$, $B' cong B_0$ such that $c$ restricted to ${B' choose A_0}$ is a constant function.

We will approach the question of when RP transfers from one countable structure to another, where these structures are in possibly different languages. We will look at universal algebraic and model theoretic criteria.

Abstract. I will present a few basic applications of model theory in theoretical computer science, e.g. in verification, databases, and algorithms. I will also discuss some initial ideas employing (ideas from) stability theory to solve algorithmic problems concerning graphs.

Abstract. Shelah's work on saturation spectra, Hrushovski on PAC structures, and Cherlin-Hrushovski on quasi-finite structures gave the initial impetus for the development of simple theories. A general theory, which unified and explained these different lines of research, was developed by Kim and Pillay using the notion of non-forking independence, which in turn spawned a remarkably rich line of model-theoretic research. In my talk, we will describe a parallel theory for the broader class of NSOP1 theories centered around the notion of Kim-independence and the applications that this theory made possible. We will survey results in a series of papers joint with Artem Chernikov, Itay Kaplan, Alex Kruckman, and Saharon Shelah (though not all at once).

Abstract. For $L$-structures $B$, $C$ we use the notation ${C choose B}$ to denote the set of all substructures of $C$ isomorphic to $B$. We say that a countable, locally finite structure $I$ ordered by a relation $<$ has RP (the Ramsey property) if for all $A_0, B_0$ in age$(I)$ and integers $k geq 1$ there is some $C_0$ in age$(I)$ such that $C_0 rightarrow (B_0)^{A_0}_k$. In other words, for all functions $c: {C_0 choose A_0} rightarrow k$ there is some $B' subseteq C_0$, $B' cong B_0$ such that $c$ restricted to ${B' choose A_0}$ is a constant function.

We will approach the question of when RP transfers from one countable structure to another, where these structures are in possibly different languages. We will look at universal algebraic and model theoretic criteria.

Abstract. Let $T_{k+1, k}$ denote the theory of the k-ary, k+1-clique free random hypergraph, for k >= 3. Malliaris and Shelah have famously proven that $T_{k+1, k}$ is not below $T_{k'+1, k'}$ in Keisler's order, whenever k+1 < k'; hence, Keisler's order has infinitely many classes. I have since improved the combinatorics to obtain the same result whenever k < k', and I obtain model-theoretic upper and lower bounds for the relevant dividing lines detected by Keisler's order. These bounds correspond to various kinds of k-dimensional amalgamation properties.

The combinatorics involved is rather technical; however, the model-theoretic upper and lower bounds are not. I aim to introduce and motivate them; in particular, we will explore a connection between generalized amalgamation properties and the chromatic numbers of hypergraphs of partial types. It is open if the various k-dimensional amalgamation properties we introduce are equivalent.

Abstract. This talk discusses a new combinatorial proof of the existence of expander graphs, which can be carried out in the bounded arithmetic theory VNC$^1$ corresponding to alternating linear time. As an application, we prove that the monotone propositional sequent calculus polynomially simulates the full propositional sequent calculus. Prior to this, only a quasipolynomial simulation was known. Joint work with Valentine Kabanets, Antonina Kolokolova, and Michal Koucky.

Abstract. Lachlan's problem is asking whether any countable theory $T$ with $1<I(omega, T)$=the no. of countable models of $T<omega$ must have the strict order property. It is not fully answered yet, but Lachlan himself in 70s proved that such $T$ is non-superstable. The result is extended by me in 90s to non-supersimple $T$. Now I extend it further in NSOP$_1$ theory context.

Theorem: If $T$ is NSOP$_1$ with nonforking existence, then $1<I(omega, T)<omega$ implies that there must be a finite tuple whose own preweight is $omega$.

The proof of the theorem relies on recent exciting developments on NSOP$_1$ theories initiated by I. Kaplan and N. Ramsey for models, and continued for sets in a joint work with J. Dobrowolski and N. Ramsey.

Abstract. The extent to which ordinal definable sets can capture essential information about the universe V has been extensively studied in the last few years.

One main line of study in this vein has been initiated by Shelah, who proved that for every singular strong limit cardinal k of uncountable cofinality, there exists a single subset x of k such that the ordinal definable class HOD(x) (consisting of sets which are hereditarily definable in ordinals and x) contains all other subsets of k. The theorem does not address cardinals k of countable cofinality and it is natural to ask whether Shelah's approximation result can be extended to all strong limit singular cardinals.

In 2016, James Cummings, Sy Friedman, Menachem Magidor, Assaf Rinot, and Dima Sinapova, have established the failure of a version of Shelah's theorem for cardinals k of countable cofinality. The purpose of the talk is to discuss the consistency strength of this result and described new consistency bounds. This is a joint work with Moti Gitik, Itay Neeman, and Spencer Unger.

Abstract. TBA

Abstract. (This is joint work with Martin Pizarro). We prove the equationality of the theory of pairs of algbraically closed fields by exhibiting a nice set of equations. We do no know if these equations have the DCC outside characteristic 0.

Abstract. I will give an overview of the many developments in the descriptive set theory of maximal almost disjoint ("mad") families (and their cousins, maximal eventually different families and maximal cofinitary groups) that have happened in the last 5 years.

Abstract. We explore how generalisations of Kleene's theory of recursion in type 2 objects (which can be used to characterise complete $Pi^1_1$ sets and open determinacy) can be lifted to $Sigma^0_3$-Determinacy. The generalisation requires the use of so-called infinite time Turing machines, and the levels of the Gödel constructible hierarchy needed to see that such machines models produce an output are, perhaps surprisingly, intimately connected with those needed to prove the existence of such strategies: the generalised halting problem has the same informational content as a listing of the $Sigma^0_3 $-games won by Player I. The subsystem of analysis needed for this work is $Pi^1_3$-$mathsf{CA_0}$.

Abstract. In 1932 von Neumann proposed classifying the statistical behavior of diffeomorphisms of compact manifolds. In modern language this means classifying up to conjugacy by measure preserving transformations. Von Neumann's question resulted in an enormous literature, with partial solutions involving invariants such as entropy, spectrum and several other properties.

In this talk we show that the general problem is impossible in the sense that no countable protocol, using arbitrary countable resources can solve it. Thus concrete methods such as spectral theory or entropy have no chance of succeeding-any classification must involve a some tool such as the uncountable axiom of choice.

Precisely, we show: {(S,T): S and T are conjugate ergodic diffeomorphisms of the 2-torus} is NOT a Borel set in the C-infinity topology on pairs of diffeomorphism.

For dessert, we give an example of a recursive diffeomorphism T such that the statement "T is isomorphic to its inverse" is independent of ZFC (and an S such that S being isomorphic to its inverse is independent of "ZFC + there is a supercompact cardinal" and so forth).

This is joint work with Benjy Weiss and Hans Gaebler.

Abstract. Fons van Engelen used the description of Wadge degrees of Borel sets to analyze Borel homogeneous spaces. I will explain the first steps we have made with Andrea Medini and Sandra Müller towards the generalization of van Engelen's results in the projective hierarchy.

Abstract. This talk looks at the relationship between three foundational systems: Goedel's Constructible Universe of Sets, the naive conception of set found in consistent fragments of Frege's Grundgesetze, and the intensional logic of Church's Logic of Sense and Denotation. One basic result shows how to use the constructible sets to build models of fragments of Frege's Grundgesetze from which one can recover these very constructible sets using Frege's definition of membership. This result also allows us to solve the related consistency problem and joint consistency problems for abstraction principles with limited amounts of comprehension. Another basic aim of this talk is to show how to "factor" this result via a consistent fragment of Church's Logic of Sense and Denotation: so one may use the constructible sets to build models of Church's Logic of Sense and Denotation, from which one may then define models of the consistent fragments of Frege's Grundgesetze. This talk is based on [1]-[3].

[1] S. Walsh. Fragments of Frege's Grundgesetze and Goedel's constructible universe. The Journal of Symbolic Logic, 81(2):605-628, 2016.

[2] S. Walsh. Predicativity, the Russell-Myhill paradox, and Church's intensional logic. The Journal of Philosophical Logic, 45(3):277-326, 2016.

[3] S. Walsh. The strength of abstraction with predicative comprehension. The Bulletin of Symbolic Logic, 22(1):105-120, 2016.

Abstract. Given a countable family G of analytic hypergraphs on a Polish space X, one can form the sigma-ideal generated by Borel sets which are anticliques in at least one hypergraph in the family G. One can also form the quotient forcing P(G) of Borel sets modulo the sigma ideal. It turns out that in large classes of cases, the quotient forcing is proper, and simple combinatorial properties of the hypergraph family G translate into forcing properties of the quotient poset P(G). Various dichotomies lead to partial classification of posets obtained from restrictive classes of hypergraphs. The machinery allows for simple computation of the iteration and product ideal in terms of the hypergraphs, which in turn leads to overwhelmingly simple proofs of various preservation theorems.

Abstract. $H$-fields are ordered differential fields which serve as an abstract generalization of both Hardy fields (ordered differential fields of germs of real-valued functions at $+\infty$) and transseries (ordered valued differential fields such as $\mathbb{T}$ and $\mathbb{T}_{\log}$). A Liouville closure of an $H$-field $K$ is a minimal real-closed $H$-field extension of $K$ that is closed under integration and exponential integration. In 2002, Lou van den Dries and Matthias Aschenbrenner proved that every $H$-field $K$ has exactly one, or exactly two, Liouville closures, up to isomorphism over $K$. Recently (in arxiv.org/abs/1608.00997), I was able to determine the precise dividing line of this dichotomy. It involves a technical property of $H$-fields called $\lambda$-freeness. In this talk, I will review the 2002 result of van den Dries and Aschenbrenner and discuss my recent contribution.

Abstract. We consider a 1903 theorem of G. H. Hardy and a more recent question of Peter Nyikos regarding its converse. We answer that question by producing a forcing model of (ZF + there exists an uncountable wellorderable set of reals + omega_1 is regular + there does not exist a ladder system).

Abstract. Classification problems occur in all areas of mathematics. Descriptive set theory provides methods to assign complexity to such problems. Using a technique developed by Hjorth, Kechris and Sofronidis proved, for example, that the problem of classifying all unitary operators $\mathcal{U}(\mathcal{H})$ of an infinite dimensional Hilbert space up to unitary equivalence $\simeq_U$ is strictly more difficult than classifying graph structures with domain $\mathbb{N}$ up to isomorphism. We present a game--theoretic approach to anti--classification results for orbit equivalence relations and use this development to reorganize conceptually the proof of Hjorth's turbulence theorem. We also introduce a dynamical criterion for showing that an orbit equivalence relation is not Borel reducible to the orbit equivalence relation induced by a CLI group action; that is, a group which admits a complete left invariant metric (recall that, by a result of Hjorth and Solecki, solvable groups are CLI). We deduce that $\simeq_U$ is not classifiable by CLI group actions.

This is a joint work with Martino Lupini.

Abstract. I will present results on expansions of the additive group of integers which satisfy certain model theoretic notions of tameness, including stability, simplicity, and dp-minimality. The focus will be on how these model theoretic notions control the combinatorial and number theoretic properties of definable sets.