I organize the Harvard "GTS" seminar, Fridays 3:30--4:30pm in Science Center 507.

__Spring 2019__

**February 1: **Zoltán Szabó (Princeton)

*Title: *Algebraic methods in knot Floer homology

*Abstract:* The aim of this talk is to present an algebraic description of knot Floer homology, discovered in a joint work with Peter Ozsváth.

**February 8:** no speaker

**February 15:** Cagatay Kutluhan (Buffalo)

*Title: *Can Floer-theoretic invariants detect overtwisted contact structures?

*Abstract:* In joint work with Matic, Van Horn-Morris, and Wand, we seek an answer to this question. We define a refinement of the contact invariant in Heegaard Floer homology that takes values in Z_{\ge 0} \cup {\infty}, called (spectral) order. Among other things, we prove that overtwisted contact structures have zero order, whereas Stein fillable contact structures have infinite order. Furthermore, we show that a strictly increasing sequence of positive integers is realized as the order of a family of contact structures with vanishing Heegaard Floor contact invariant. After defining our contact invariant and discussing some of its key properties, I will talk about its computability and some problems that are content of work in progress.

**February 22: **John Baldwin (Boston College)

*Title:* Instanton L-space knots

*Abstract:* A 3-manifold is said to be SU(2)-abelian if all homomorphisms from its fundamental group to SU(2) factor through the first homology of the manifold. Understanding which manifolds are SU(2)-abelian is a difficult and wide open problem, even in the case of 3-manifolds arising from surgery on a knot in the 3-sphere. Kronheimer and Mrowka showed, for instance, that n-surgery on a nontrivial knot is not SU(2)-abelian for n = 1 or 2, but it isn't even known whether the same is true for n = 3 or 4 (the same isn't true for n = 5 as 5-surgery on the right-handed trefoil is a lens space, which has abelian fundamental group). We approach this question by trying to understand which knots have instanton L-space surgeries. A rational homology sphere is said to be an instanton L-space if its framed instanton homology has the smallest rank possible, in analogy with Heegaard Floer homology; familiar examples include lens spaces and branched double covers of alternating knots. Moreover, it is generally the case that SU(2)-abelian manifolds are instanton L-spaces. We conjecture that if surgery on a knot K in the 3-sphere results in an instanton L-space then K is fibered and its Seifert genus equals its smooth slice genus, and we discuss a strategy for proving this.

**March 1: **Peter Ozsváth (Princeton)

*Title: *Computing knot Floer homology

*Abstract:* I will explain how to use bordered Floer homology to obtain a practical, algebraic computation of knot Floer homology. This is joint work with Zoltán Szabó.

**March 8: **Thomas Walpuski (Michigan State)

*Title: *Super-rigidity and Castelnuovo’s bound

*Abstract:* Castelnuovo’s bound is a very classical result in algebraic geometry. It asserts a sharp bound on the genus of a curve of degree d in n-dimensional projective space. It is an interesting question to ask whether analogues of Castelnuovo’s bound hold in almost complex geometry. There is a direct analogue in dimension four. In dimension at least eight genus bounds can be established for generic almost complex structures. These results leave open the case of dimension six.

Bryan and Panharipande introduced the notion of super-rigidity of an almost complex structure. They also speculated that this condition might hold for a generic almost complex structure (compatible with a fixed symplectic structure). It had been believed for a long time that super-rigidity will play an important role in the proof of the Gopakumar–Vafa conjecture. However, it turned that Ionel and Parker’s recent proof of this conjecture did not make use of it. Nevertheless, super-rigidity has important consequences. I will present one of these consequences, namely, a genus bound for index zero pseudo-holomorphic curves. This is joint work with Aleksander Doan and, heavily, relies on work by De Lellis, Spadaro, and Spolaor and ideas of Taubes.

There has been a lot of progress towards establishing Bryan and Pandharipande’s super-rigidity conjecture in the work of Wendl. In fact, based on his ideas, Aleksander Doan and I have developed an abstract framework for equivariant transversality/Brill–Noether type questions. Wendl’s work shows that the super-rigidity conjecture holds provided generic real Cauchy-Riemann operators satisfy an easy to state analytic condition. I will explain what this condition means and discuss a few cases in which this condition (or versions of it hold).

**March 15: **Emmy Murphy (Northwestern)

*Title: *Inductively collapsing Fukaya categories and flexibility

*Abstract: *A Weinstein manifold is a symplectic manifold which admits a Lagrangian skeleton, and associated to any Weinstein manifold is its wrapped Fukaya category, a powerful algebraic invariant. One important case is that the wrapped category of C^n is trivial. The talk will discuss a partial converse: if X is any Weinstein 6-manifold which is contractible, admitting an arboreal skeleton so that the wrapped category is inductively collapsing, then X is symplectomorphic to C^3. A large portion of the talk will be defining the notion of inductively collapsing: this is a purely algebraic condition, but it depends on a presentation of the wrapped category, which itself comes from a chosen Lagrangian skeleton.

**March 22: **Harvard's** **spring break

**March 29 (Room 222): **Chris Scaduto (Simons Center)

*Title: *Instantons and lattices of smooth 4-manifolds with boundary

*Abstract:* Given a 3-manifold Y, what are the possible definite intersection forms of smooth 4-manifolds with boundary Y? Donaldson's theorem says that if Y is the 3-sphere, then all such intersection forms are standard integer Euclidean lattices. I will survey some new progress on this problem, for other 3-manifolds, that comes from instanton Floer theory.

**April 5: **Melissa Liu (Columbia)

*Title: *The Yang-Mills Equations over Klein Surfaces

*Abstract:* In "The Yang-Mills equations over Riemann surfaces", Atiyah and Bott studied Yang-Mills functional over a Riemann surface from the point of view of Morse theory, and derived results on topology of the moduli space of algebraic bundles over a complex algebraic curve. In this talk, I will discuss Yang-Mills functional over a Klein surface (a 2-manifold equipped with a dianalytic structure) from the point of view of Morse theory, and derive results on topology of the moduli space of real or quaternionic vector bundles over a real algebraic curve. This is based on joint work with Florent Schaffhauser.

**April ****12: **Joel Fish (UMass Boston)

*Title: *Feral curves and minimal sets

*Abstract:* TBA

**April ****19: **Aliakbar Daemi (Simons Center)

*Title/Abstract:* TBA

**April ****26:** Jennifer Hom (Georgia Tech)

*Title: *Heegaard Floer and homology cobordism

*Abstract:* We show that the three-dimensional homology cobordism group admits an infinite-rank summand. It was previously known that the homology cobordism group contains an infinite-rank subgroup and a Z-summand. The proof relies on the involutive Heegaard Floer homology package of Hendricks-Manolescu and Hendricks-Manolescu-Zemke. This is joint work with I. Dai, M. Stoffregen, and L. Truong.

**May 3: **Dan Cristofaro-Gardiner (Santa Cruz)

*Title/Abstract:* TBA

**May 10 (maybe postponed until September): **Jon Bloom (Broad Institute)

*Title/Abstract:* TBA

__Fall 2018__

**September 14:** Denis Auroux (Harvard)

*Title: *An invitation to homological mirror symmetry

*Abstract:* We will give a gentle introduction to some recent developments in the area of homological mirror symmetry. We will use simple examples to illustrate Kontsevich's conjecture and its extension beyond the Calabi-Yau setting in which it was first formulated. We will mostly focus on a one-dimensional example, the pair of pants, to give a flavor of the geometric concepts involved in a general formulation of homological mirror symmetry. If time permits, we will then describe a program to prove homological mirror symmetry for essentially arbitrary complete intersections in toric varieties (joint work in progress with Mohammed Abouzaid).

**September 21: **Weiyi Zhang (Warwick)

*Title: *From smooth to almost complex

*Abstract:* An almost complex manifold is a smooth manifold equipped with a smooth linear complex structure on each tangent space. We will discuss differential topology of almost complex manifolds, explain how to use transversality statements for smooth manifolds to formulate and prove corresponding results for an arbitrary almost complex manifold. The examples include intersection of almost complex manifolds, structure of pseudoholomorphic maps and zero locus of certain harmonic forms.

One of the main technical tools is Taubes' notion of "positive cohomology assignment", which plays the role of local intersection number. I will begin with explaining its motivation through multiplicities of zeros of a smooth function.

Our results would lead to a notion of birational morphism between almost complex manifolds. Various birational invariants, including Kodaira dimension, for almost complex manifolds will be introduced and discussed (this part is joint with Haojie Chen).

**September 28: **no speaker

**October 5: **Anar Akhmedov (University of Minnesota, visiting scholar @Harvard)

*Title: *Construction of symplectic 4-manifolds and Lefschetz fibrations via Luttinger surgery

*Abstract:* Luttinger surgery is a certain type of Dehn surgery along a Lagrangian torus in a symplectic 4-manifold. The surgery was introduced by Karl Luttinger in 1995, who used it to study Lagrangian tori in R^4. Luttinger's surgery has been very effective tool recently for constructing exotic smooth and symplectic structures on 4-manifolds. In this talk, using Luttinger surgery, I will present the constructions of smallest known closed exotic simply connected symplectic 4-manifolds and exotic Lefschetz fibrations over 2-sphere whose total spaces have arbitrary finitely presented group G as the fundamental group. If time permits, we will also discuss some applications to the geography of symplectic 4-manifolds and Lefschetz fibrations. Part of these are (separate) joint works with B. Doug Park, Burak Ozbagci, and Naoyuki Monden.

**October 12: **Paul Feehan (Rutgers)

*Title: *Lojasiewicz inequalities and Morse-Bott functions

*Abstract:* The Lojasiewicz gradient and distance inequalities for real analytic functions on Euclidean space were first proved by Stanislaw Lojasiewicz (1965) using methods of semianalytic and subanalytic sets, arguments later simplified by Bierstone and Milman. We shall first describe a more direct proof of the Lojasiewicz gradient inequality that uses resolution of singularities for real analytic varieties to reduce to the case of functions with simple normal crossings, where the Lojasiewicz exponent may be computed explicitly — thus giving insight into its geometric meaning. It is well-known and easy to prove that if a function on a Banach space is Morse-Bott, then its Lojasiewicz exponent is 1/2. We show that the less obvious converse is also true: if the Lojasiewicz exponent of an analytic function on a Banach space is 1/2 at a critical point, then the function is Morse-Bott on a neighborhood of that point. We illustrate these phenomena with applications of Lojasiewicz inequalities to the Yang-Mills energy function near the critical set of flat connections on a principal G-bundle over a closed Riemannian manifold.

**October 19: **Aleksander Doan (Stony Brook)

*Title: *Harmonic Z/2 spinors and wall-crossing in Seiberg-Witten theory

*Abstract:* The notion of a harmonic Z/2 spinor was introduced by Taubes as an abstraction of various limiting objects appearing in compactifications of gauge-theoretic moduli spaces. I will explain this notion and discuss an existence result for harmonic Z/2 spinors on three-manifolds. The proof uses a wall-crossing formula for solutions of generalized Seiberg-Witten equations in dimension three, a result itself motivated by Yang-Mills theory on Riemannian manifolds with special holonomy G_2. The talk is based on joint work with Thomas Walpuski.

**October 26: **Michael Hutchings (Berkeley)

*Title: *Equivariant symplectic capacities

*Abstract:* We define a sequence of symplectic capacities using positive S^1-equivariant symplectic homology. These capacities are conjecturally equal to the Ekeland-Hofer capacities. However they satisfy axioms which allow them to be computed for many examples, such as convex toric domains. They also give sharp obstructions to symplectic embeddings of cubes into convex toric domains. The asymptotics of these capacities are conjecturally related to Lagrangian embeddings. This is a joint work with Jean Gutt.

**November 2: **Laure Flapan (Northeastern)

*Title: *Monodromy of Kodaira fibrations

*Abstract: *A long-standing question in studying the topology of complex algebraic varieties is the question of what groups can occur as the fundamental group of a smooth projective variety. One approximation of this question in the case of fibered varieties is to ask what groups can occur as the monodromy group of such a fibration. We use Hodge theory to investigate this question in the case of fibered algebraic surfaces, called Kodaira fibrations, whose fibers are all smooth and draw connections with questions about Shimura varieties and the moduli space of smooth algebraic curves.

**November 9:** Mohammed Abouzaid (Columbia)

*Title: *A formalism for Floer theory with coefficients

*Abstract:* I will discuss a formalism for defining (Lagrangian) Floer homology with coefficients which is a little more abstract than has been used so far in the literature, and is designed to generalise a recent construction of Rezchikov, in the exact setting, to the case of Lagrangians which may bound holomorphic discs. Various parts of this talk will report on joint work with Pardon, and with Blumberg and Kragh.

**November 16:** Harvard's "Current Developments in Mathematics" conference

**November 23:** Thanksgiving break

**November 30:** John Pardon (Princeton)

*Title: *Structural results in wrapped Floer theory

*Abstract:* I will discuss results relating different partially wrapped Fukaya categories. These include a Kunneth formula, a 'stop removal' result relating partially wrapped Fukaya categories relative to different stops, and a gluing formula for wrapped Fukaya categories. The techniques also lead to generation results for Weinstein manifolds and for Lefschetz fibrations. The methods are mainly geometric, and the key underlying Floer theoretic fact is an exact triangle in the Fukaya category associated to Lagrangian surgery along a short Reeb chord at infinity. This is joint work with Sheel Ganatra and Vivek Shende.

**December 7:** Raphael Zentner (Regensburg)

*Title: *Irreducible SL(2,C)-representations of homology-3-spheres

*Abstract:* We prove that the splicing of any two non-trivial knots in the 3-sphere admits an irreducible SU(2)-representation of its fundamental group. This uses instanton gauge theory, and in particular a non-vanishing result of Kronheimer-Mrowka and some new results that we establish for holonomy perturbations of the ASD equation. Using a result of Boileau, Rubinstein, and Wang (which builds on the geometrization theorem of 3-manifolds), it follows that the fundamental group of any integer homology 3-sphere different from the 3-sphere admits irreducible representations of its fundamental group in SL(2,C).