Columbia Geometric Topology Seminar

Spring 2024


Organizers: Francesco LinSiddhi Krishna, Ross Akhmechet

The GT seminar typically meets on Fridays at 2:00pm Eastern time in Room 307, Mathematics Department, Columbia University. 

Other area seminars. Our e-mail list. Archive of previous semesters

Spring 2023

Date Time (Eastern) Speaker Title

January 26

3:30pm Eastern (note unusual time!)

Bojun Zhao

Co-orientable taut foliations in Dehn fillings of pseudo-Anosov mapping tori with co-orientation-reversing monodromy

February 2

3:30pm Eastern (note unusual time!) Sebastian Hurtado-Salazar

Length functions on Lie groups and lattices

February 9

2pm Eastern

Mike Willis

Spectral Jones-Wenzl projectors and differences between Khovanov homology and stable homotopy

February 16

3:30pm Eastern (note unusual time!)

Alex Zupan

The square knot bounds infinitely many non-isotopic ribbon disks

February 23

2pm Eastern

Sally Collins

Homology cobordism & smooth knot concordance

March 1

2pm Eastern

Eugen Rogozinnikov

Parametrizing spaces of positive representations

March 7 (Thursday)

3pm Eastern (note unusual day and time!)

Francesco Costantino

Stated skein modules of 3-manifolds and TQFTs

March 8

2pm Eastern

Onkar Gujral


March 15

2pm Eastern

no seminar

Spring Break

March 22

2pm Eastern

Emmanuel Wagner


March 29

2pm Eastern

no seminar

Simons Annual Meeting

April 5

2pm Eastern

Emily Stark  

April 12

2pm Eastern

Ian Zemke


April 19

2pm Eastern Henry Segerman


April 26

2pm Eastern Patrick Orson


April 30 (Tuesday)

TBA Bruno Martelli




Bojun Zhao

Date:  January 26

Title: Co-orientable taut foliations in Dehn fillings of pseudo-Anosov mapping tori with co-orientation-reversing monodromy

Abstract: Let f be a pseudo-Anosov homeomorphism on a compact orientable surface with nonempty boundary, such that f has a co-orientable stable foliation and reverses the co-orientation on it. Let M denote the mapping torus of f. In this talk, I will discuss some constructions of co-orientable taut foliations in those Dehn fillings of M with filling multislopes constrained by a bound from the degeneracy loci on the boundary components of M. In certain cases where M has connected boundary and is Floer simple, we can construct co-orientable taut foliations in all non-L-space Dehn fillings of M.



Sebastian Hurtado-Salazar

Date:  February 2

TitleLength functions on Lie groups and lattices

Abstract: We will discuss the notion of a length function on a group, focusing on lattices in Lie groups, such as SL_n(Z), and discuss how techniques from dynamics can help to understand some important questions about these groups.



Mike Willis

Date:  February 9

Title: Spectral Jones-Wenzl projectors and differences between Khovanov homology and stable homotopy

Abstract: Categorified Jones-Wenzl projectors P_n are widely studied, with endomorphisms related to the Khovanov homology of torus links. We will discuss the lifts of such projectors to the category of spectra, focusing on certain properties of P_n that fail to lift. These are some of the first structural differences found between Khovanov homology and Khovanov stable homotopy, and include the surprising negative answer to a question of Lawson-Lipshitz-Sarkar asking whether topological Hochschild homology for tangle spectra can be used to define a spectral invariant for links in S1xS2. This work is joint with Matt Stoffregen.



Alex Zupan

Date:  February 16

TitleThe square knot bounds infinitely many non-isotopic ribbon disks

Abstract: A knot K in S^3 is (smoothly) slice if K is the boundary of a properly embedded disk D in B^4, and K is ribbon if this disk can be realized without any local maxima with respect to the radial Morse function on B^4.  In dimension three, a knot K with nice topology – that is, a fibered knot – bounds a unique fiber surface up to isotopy.  Thus, it is natural to wonder whether this sort of simplicity could extend to the set of ribbon disks for K, arguably the simplest class of surfaces bounded by a knot in B^4.  Surprisingly, we demonstrate that the square knot, one of the two non-trivial ribbon knots with the lowest crossing number, bounds infinitely many distinct ribbon disks up to isotopy.  This is joint work with Jeffrey Meier.



Sally Collins

Date:  February 23

TitleHomology cobordism & smooth knot concordance

Abstract: The 0-surgeries of two knots K1 and K2 are homology cobordant rel meridians if there exists an integer homology cobordism X between them such that the two positive knot meridians are in the same homology class of X. It is a natural question to ask: if two knots have 0-surgeries related in this sense, must they be smoothly concordant? We give a pair of rationally slice knots as counterexample, and along the way expand upon an involutive knot Floer homology technique for obstructing torsion in the smooth concordance group first introduced by Hom, Kang, Park, and Stoffregen. No previous knowledge of Heegaard Floer theory will be assumed.

Eugen Rogozinnikov

Date:  March 1

Title:Parametrizing spaces of positive representations

Abstract: Higher Techmüller theory deals with spaces of representations of the fundamental group of a surface into a reductive Lie group $G$, modulo the conjugation, especially with the connected components (called higher Teichmüller spaces) that consist entirely of injective representations with discrete image.

In the last two decades in works of Fock, Goncharov, Burger, Iozzi, Guichard, Wienhard, and others researchers, it was discovered that the most interesting higher Teichmüller spaces are emerging from the groups $G$ having a positive structure, i.e. certain submonoid $G_+$ with no invertible non-unit elements. Some of these submonoids have been known since 1930’s as totally positive matrices and then generalized by Lustzig for split real Lie groups. However it left out a large class of non-split reductive Lie groups such as $SO(p,q)$. O. Guichard and A. Wienhard filled this gap in 2018 by introducing the Theta-positivity, which also includes submonoids $SO(p,q)_+$ sitting in a unipotent group of $SO(p,q)$ and $Sp(2n,R)_+$ which is the set of upper uni-triangular block 2x2-matrices with a symmetric positive definite matrix in the upper right corner.
In my talk, I introduce the Theta-positivity for Lie groups and explain how the spaces of positive representations of the fundamental group of a punctured surface into a Lie group with a positive structure can be parametrized, and how we can describe the topology of these spaces using this parametrization. This is a joint work with O. Guichard and A. Wienhard.


Francesco Costantino

Date: March 7

Title:Stated skein modules of 3-manifolds and TQFTs

Abstract:  After reviewing the definition of stated skein modules for surfaces and 3 manifolds, I will detail how this recent notion allows to relate topological constructions (related to cut and paste techniques) to algebraic ones (braided tensor products of algebra objects in braided categories for instance). I will explain how the stated skein algebra of some special surfaces provides a topological description for some notable algebras (e.g. the quantised functions ring $O_q(\mathfrak{sl_2})$ or its ``transmutation’’ BSL_2(q)). Then I will describe how stated skein moduli of 3-manifolds fit into a TQFT framework albeit a non completely standard one. If time permits I will also discuss some unexpected non injectivity results in dimension 3. (Joint work with Thang Le)




Other relevant information.

Previous semesters:

Fall 2022Spring 2022Fall 2021, Spring 2021, Fall 2020Spring 2020Fall 2019Spring 2019, Fall 2018, Spring 2018, Fall 2017, Spring 2017, Fall 2016, Spring 2016, Fall 2015, Spring 2015, Fall 2014, Spring 2014, Fall 2013, Spring 2013, Fall 2012, Spring 2012, Fall 2011, 2010/11, Spring 2010, Fall 2009, Spring 2009, Fall 2008, Spring 2008, Fall 2007, Spring 2007, Fall 2006.

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