# Geometry & Topology Seminar

## Location

HH 312
Thursdays, 15:30 - 16:30

## Upcoming talks

### December 7, 2017 - Tyrone Ghaswala (University of Waterloo):

Coverings, groupoids, and embedding braids in mapping class groups

Given a finite-sheeted branched cover between surfaces with boundary, it is natural to ask whether every mapping class of the covering surface projects to the base surface, and whether every mapping class of the base surface lifts to the covering surface.
In this talk we will address these questions, with automorphisms of fundamental groupoids playing an integral role. As a consequence of the main result, we construct new embeddings of the braid group in the mapping class group of a surface.
This is joint work with Alan McLeay.

## Past talks

### November 30, 2017 - David Duncan (McMaster University):

Some open problems in gauge theory, symplectic geometry, and low-dimensional topology

I will discuss a variety of open problems and partial results (due to myself and others), each relating to at least two of the fields appearing in the title. Some of the more famous open problems will include Witten's conjecture and the Atiyah-Floer conjecture; however, I will spend most of the time discussing more tractable variants of these, and how they relate.

### November 23, 2017 - Christopher Davis (University of Wisconsin at Eau Claire):

Concordance of knots in homology spheres and the solvable filtration

The classical topological knot concordance group $\mathcal{C}$ consists knots in $S^3$ modulo those which bound flat disks in the 4-ball. There is a natural generalization, $\widehat{\mathcal{C}}$, the setting of knots in homology spheres. Recently, Adam Levine showed that in the smooth (as opposed to topological) setting the map $\mathcal{C} \rightarrow \widehat{\mathcal{C}}$ is not a surjection, producing knots in homology spheres not smoothly concordant to any knot in $S^3$. During this talk we will produce strong evidence that the opposite is true topologically. Namely we show that modulo any term of the solvable filtration of knot concordance introduced by Cochran-Orr-Teichner, the map $\mathcal{C} \rightarrow \widehat{\mathcal{C}}$ an isomorphism. Since every known invariant of topological concordance vanishes at some level of this filtration, our result implies that any possible difference between $\mathcal{C}$ and $\widehat{\mathcal{C}}$ not be detected by any currently existent technology.

### November 17, 2017 - Nikolai Saveliev (University of Miami):

On gauge theoretic invariants of homology $S^1 \times S^3$

I will survey some recent joint work on invariants of smooth 4-manifolds with homology of $S^1 \times S^3$. In our older work with Tom Mrowka and Daniel Ruberman we defined a Seiberg-Witten invariant of such manifolds. I will present a new formula (from our recent work with Jianfeng Lin and Daniel Ruberman) which expresses this invariant in terms of the monopole Floer homology and leads to some interesting topological applications.

### November 9, 2017 - Katherine Raoux (Michigan State University):

$\tau$-invariants for knots in rational homology spheres

Using the knot filtration on the Heegaard Floer chain complex, Ozsváth and Szabó defined an invariant of knots in the three sphere called $\tau(K)$, which they also showed is a lower bound for the 4-ball genus. Generalizing their construction, I will show that for a (not necessarily null-homologous) knot, $K$, in a rational homology sphere, $Y$, we obtain a collection of $\tau$-invariants, one for each spin-c structure on $Y$. In addition, these invariants can be used to obtain a lower bound on the genus of a surface with boundary $K$ properly embedded in a negative definite 4-manifold with boundary $Y$.

### November 2, 2017 - Anja Randecker (University of Toronto):

On Loch Ness monsters and wild singularities - a Halloween-inspired introduction to infinite translation surfaces

Finite translation surfaces are obtained by gluing finitely many polygons along parallel edges of the same length. I will introduce a generalization of this concept and explain some issues that make the study more difficult. This involves more complicated kinds of singularities, infinite topological type, and flows that get trapped. Along the talk, there will be lots of examples and pictures.

### October 26, 2017 - Jonathan Hillman (University of Sydney):

Complements of closed hypersurfaces in $S^4$

We show that if a 3-manifold $M$ has an embedding in $S^4$ such that both complementary regions have abelian fundamental groups then the latter groups are cyclic, $\mathbb{Z} \oplus{\mathbb{Z}/n\mathbb{Z}}$ for some $n>1$, $\mathbb{Z}^2$ or $\mathbb{Z}^3$. If an homology handle $M$ with fundamental group $\pi$ embeds in $S^4$ then the Blanchfield pairing is neutral. We show that an homology handle has an embedding with both complementary fundamental groups abelian if and only if the commutator subgroup $\pi'$ is perfect, and the embedding is then essentially unique.

### October 19, 2017 - George Dragomir (McMaster University):

Metric transforms yielding Gromov hyperbolic spaces

A real valued function of one variable $\varphi$ is called a metric transform if for every metric space $(X,d)$ the composition $d_\varphi = \varphi\circ d$ is also a metric on $X$. In this talk we present a complete characterization of the class of approximately nondecreasing, unbounded metric transforms $\varphi$ such that the half line $[0,\infty)$ with the transformed Euclidean metric $\varphi(|x-y|)$ is Gromov hyperbolic. As an application we obtain a type of rigidity with respect to metric transformations of roughly geodesic Gromov hyperbolic spaces. This is based on joint work with Andrew Nicas.

### October 19, 2017 - Matthias Nagel (McMaster University):

Concordance and the Gordon-Litherland lattice

### October 2, 2017; in HH 207 - Klaus Kröncke (University of Hamburg):

Stability of ALE Ricci-flat manifolds under Ricci-flow

We prove that if an ALE Ricci-flat manifold $(M,g)$ is linearly stable and integrable, it is dynamically stable under Ricci flow, i.e. any Ricci flow starting close to g exists for all time and converges modulo diffeomorphism to an ALE Ricci-flat metric close to g. By adapting Tian's approach in the closed case, we show that integrability holds for ALE Calabi-Yau manifolds which implies that they are dynamically stable. This is joint work with Alix Deruelle.

### September 21, 2017 - Hari Kunduri (Memorial University of Newfoundland):

Geometric inequalities for black hole initial data sets

The standard picture of gravitational collapse leads (using heuristic arguments) to an inequality relating the ADM mass and angular momentum of axisymmetric spacetimes. Such geometric inequalities have been rigorously proved for axisymmetric, asymptotically flat maximal initial data for the vacuum Einstein equations in dimension d=4. After a broad review, I will discuss recent work on extending this class of inequalities to d>4, where a number of qualitative differences arise (e.g. black holes can have non-spherical horizon topology). In particular, I will discuss how a lower bound for the mass, in terms of a (regularized) harmonic energy functional is obtained. The unique minimizer of this energy is then shown to correspond to extreme black hole initial data for fixed angular momenta. This provides a variational characterization of extreme black hole.

### September 14, 2017 - Steven Boyer (UQAM):

Branched Covers of Quasipositive Links and L-Spaces.

We show that if $L$ is an oriented strongly quasipositive link other than the trivial knot or a link whose Alexander polynomial is a positive power of $(t-1)$, or a quasipositive link with non-zero smooth 4-ball genus, then the Alexander polynomial and signature function of $L$ determine an integer $n(L) \geq 1$ such that $\Sigma_n(L)$, the n-fold cyclic cover of $S^3$ branched over $L$, is not an L-space for $n \geq n(L)$. If $K$ is a strongly quasipositive knot with monic Alexander polynomial such as an L-space knot, we show that $\Sigma_n(K)$ is not an L-space for $n \geq 6$, and that the Alexander polynomial of K is a non-trivial product of cyclotomic polynomials if $\Sigma_n(K)$ is an L-space for some $n = 2,3,4,5$. Our results allow us to calculate the smooth and topological 4-ball genera of, for instance, quasi- alternating quasipositive links. They also allow us to classify strongly quasipositive alternating links and strongly quasipositive alternating 3-strand pretzel links. This is joint work with Michel Boileau and Cameron Gordon.