Warwick Algebraic Topology Seminar 23/24
A list of the seminar talks of the previous years can be found here.
Term 3
The talks will take place on Tuesday at 5pm (note the change of tim from the other terms), in B3.02.
Date  Speaker  Affiliation  Title  Notes 

Apr 23  Thomas Wasserman  University of Oxford  CohenMacaulay simplicial complexes and duality groups  
Apr 30  Stephen Theriault  University of Southampton  Homotopy theoretic properties of Poincaré Duality complexes  
May 7  Elizabeth Tatum  University of Bonn  
May 14  Christian Dahlhausen  Heidelberg University  
May 21  Cheuk Yu Mak  University of Southampton  
May 28  Rhiannon Savage  University of Oxford  
Jun 4  
Jun 11  Jack Davies  University of Bonn  
Jun 18  Ramon Flores  Universidad de Sevilla  
Jun 25 
Abstracts
Thomas Wasserman: CohenMacaulay simplicial complexes and duality groups
Duality groups are groups that admit a Poincarédualitylike relationship between their cohomology and their homology twisted by a module known as the dualising module. Many interesting groups are (virtually) duality groups, like free groups and their (outer) automorphisms, mapping class groups, and the general linear groups over the integers. Knowing duality is useful for homology computations, and the dualising module often has a nice interpretation. In this talk I will discuss work with Ric Wade where we explore the relationship between groups acting on simplicial complexes that are locally CohenMacaulay (meaning that their local homology is concentrated in a single degree and free) and groups having duality, and the implications this has for the dualising module.
Stephen Theriault: Homotopy theoretic properties of Poincaré Duality complexes
Let M be a simplyconnected closed Poincaré Duality complex of dimension n. The goal is to gain insight into the homotopy theory of M by determining properties of its based loop space. An approach will be outlined that has had some success for particular families of Poincaré Duality complexes.
We will go on to discuss a new development. Rationally, it is known that if X is the (n1)skeleton of M then the based loops on M retracts off the based loops on X, provided the rational cohomology ring of M is not generated by a single element. We will show that under certain conditions this is true integrally. Families for which the integral statement holds include momentangle manifolds and quasitoric manifolds.
Term 2
The talks will take place on Tuesday at 4pm. Some of the talks will be in person in B3.02, some will be given by a remote speaker on MS Teams and streamed in B3.02.
Date  Speaker  Affiliation  Title  Notes 

Jan 9  Doosung Park  University of Wuppertal  Syntomic cohomology and real topological cyclic homology  
Jan 16  Ian Leary  University of Southampton  The spectrum of the Chern subring for a fusion system  
Jan 23  Juan Omar Gomez Rodriguez  Bielefeld University  Decompositions of the stable module category for infinite groups  
Jan 30  Alexey Elagin  University of Edinburgh  Thick subcategories on smooth curves and orbifold curves  
Stephen Theriault  University of Southampton  Homotopy theoretic properties of Poincare Duality complexes  Talk cancelled  
Feb 13  Adam Thomas  University of Warwick 
Symmetric spaces for real compact Lie groups: an algebraist's viewpoint 

Feb 20  Magdalena Kedziorek  Radboud University Nijmegen  An algebraic model for rational SO(2)spectra  
Feb 27  Marco Linton  University of Oxford  The relation gap conjecture for groups with cyclic relation modules  
Mar 5  Isambard Goodbody  University of Glasgow  Reflexivity and Hochschild cohomology  
Mar 12  Matthew Pressland  University of Glasgow  Positroid varieties via triangulated categories 
Abstracts
Doosung Park: Syntomic cohomology and real topological cyclic homology
In this talk, I will show that real topological cyclic homology admits a complete exhaustive filtration whose graded pieces are equivariant suspensions of syntomic cohomology. Combined with the announced results of AntieauKrauseNikolaus and HarpazNikolausShah, this would lead to the computation of the equivariant slices of the real Ktheory of Z/p^n after a certain suspension. The key ingredients of the proof are a real refinement of the HochschildKostantRosenberg filtration and the computation of real topological Hochschild homology of perfectoid rings in my joint work with Hornbostel.
Ian Leary: The spectrum of the Chern subring for a fusion system
(Joint work with Jason Semeraro) Around 1970 Quillen gave a complete description of the prime ideal spectrum for the modp cohomology of a finite group $G$ in terms of its elementary abelian subgroups. Around 1995 David Green and I gave an analogous result for the subring of cohomology generated by Chern classes of representations of $G$.
A (saturated) fusion system is a sort of `finite group at a single prime $p$'. An analogue of Quillen's theorem has been proved for fusion systems. Jason Semeraro and I give an analogues of my result with David Green and of some related results.
I intend that the talk will be accessible to someone who has seen neither Quillen's theorem nor fusion systems previously.
Juan Omar Gomez Rodriguez: Decompositions of the stable module category for infinite groups
In this talk, we will present a homotopytheoretic interpretation of the stable module category for infinite groups over a commutative noetherian ring. For groups acting on trees with finite isotropy, we use this approach to exhibit a decomposition of the stable category in terms of their finite subgroups. As an application, we give a complete description of the Picard group of the stable category for discrete ptoral groups over a field.
Alexey Elagin: Thick subcategories on smooth curves and orbifold curves
I will talk about the classification problem for thick triangulated subcategories in triangulated categories, such as derived categories of coherent sheaves on algebraic varieties, or derived categories of modules over algebras. In some cases such classification is possible, but in general thick subcategories are abundant and hardly controllable. Nevertheless, one can classify thick subcategories in the derived category of a smooth projective curve _up to an equivalence_: every thick subcategory has a simple explicit description in terms of some quiver. Moreover, one can describe all quivers that arise in this way. If time permits, I will explain how these results generalise to weighted projective curves.
Stephen Theriault: Homotopy theoretic properties of Poincare Duality complexes
Let M be a simplyconnected closed Poincare Duality complex of dimension n. The goal is to gain insight into the homotopy theory of M by determining properties of its based loop space. An approach will be outlined that has had some success for particular families of Poincare Duality complexes.
We will go on to discuss a new development. Rationally, it is known that if X is the (n1)skeleton of M then the based loops on M retracts off the based loops on X, provided the rational cohomology ring of M is not generated by a single element. We will show that under certain conditions this is true integrally. Families for which the integral statement holds include momentangle manifolds and quasitoric manifolds.
Adam Thomas: Symmetric spaces for real compact Lie groups: an algebraist's viewpoint
I have been fortunate enough to be introduced to some topics in algebraic topology by working with John Jones and Dmitriy Rumynin since arriving at Warwick. This talk will be my, heavily algebraic, take on our joint work. My punchline will be a uniform approach to calculating the rational cohomology ring of Rosenfeld Projective Planes. In fact, the approach is applicable to all homogenous spaces of Lie groups G/H where H has the same rank as G. I hope to make it accessible (and enjoyable) for algebraists, topologists and everyone in between!
Magdalena Kedziorek: An algebraic model for rational SO(2)spectra
The category of Gspectra, for any compact Lie group G is very interesting, but at the same time very complicated. A big part of the interesting information comes from the internally encoded group action while one of the main complications comes from working over the integers. The first step in our understanding is to simplify this category by working over rationals. This removes the complexity coming from ordinary stable homotopy theory while leaving much of the information about group G still in the picture.
In this talk, I will discuss what we know about algebraic models for rational SO(2)spectra, concentrating on our understanding of the most naive and the most genuine levels of commutativity in this setting. This talk is based on joint work with Dave Barnes and John Greenlees and ongoing work with John Greenlees.
Marco Linton: The relation gap conjecture for groups with cyclic relation modules
If F is a free group and F/N is a presentation of the group G, there is a natural way to turn the abelianisation of N into a left ZGmodule, known as the relation module of the presentation. A presentation F/N is said to have relation gap if its relation module has strictly fewer ZGmodule generators than N has normal generators. Infinite relation gaps were found by BestvinaBrady, but the existence of finite relation gaps remains an open problem, closely connected to questions on the homotopy types of finite 2complexes such as Wall's D(2) problem. I will first motivate the problem and survey what is known and what is not known. Then I will present a solution to the relation gap and relation lifting problems for certain groups with cyclic relation module.
Isambard Goodbody: Reflexivity and Hochschild cohomology
Smooth and proper DGcategories are noncommutative models for smooth and proper schemes. They also include finite dimensional algebras of finite global dimension. Kuznetsov and Shinder defined reflexive DGcategories as a (vast) generalisation; they include all projective schemes and all finite dimensional algebras. Smooth and proper DGcategories can also be characterised as the dualizable objects in the monoidal category of DGcategories localised at Morita equivalences. I’ll explain how by using a monoidal characterisation of reflexive DGcategories one can prove that the Hochschild cohomology of a reflexive DGcategory is isomorphic to that of its derived category of cohomologically finite modules.
Matthew Pressland: Positroid varieties via triangulated categories
The totally nonnegative Grassmannian is an important object in several stories, including Lusztig's total positivity, and the calculation of scattering amplitudes via the amplituhedron. It has a cell decomposition, described by Postnikov, in which each cell is obtained by intersecting the totally nonnegative Grassmannian with a particular subvariety of the full Grassmannian: a socalled open positroid variety.
A useful tool in studying totally positive spaces is Fomin–Zelevinsky's theory of cluster algebras. A recent result of Galashin and Lam is that the coordinate ring of (the cone on) an open positroid variety has the structure of such an algebra, in two different but closely related ways, confirming a longstanding expectation. Muller and Speyer conjectured a precise relationship (quasicoincidence) between these two cluster structures, which makes them equivalent from the point of view of total positivity. In this talk, I will explain how to prove their conjecture. Perhaps surprisingly, the proof is highly dependent on additive categorification, or in other words, on homological algebra in exact and triangulated categories.
Term 1
The talks will take place on Tuesday at 4pm. Some of the talks will be in person in B3.03, some will be given by a remote speaker on MS Teams and streamed in B3.03.
Date  Speaker  Affiliation  Title  Notes 

Oct 3  University of Southampton 
postponed, train strikes 

Oct 10  Özgür Bayındır  City University of London  Algebraic Ktheory of the twoperiodic first Morava Ktheory  
Oct 17  Daniel Kasprowski  University of Southampton 
Stable equivalence relations of 4manifolds 

Oct 24  Itamar Mor  Queen Mary University of London  Profinite Galois descent in K(n)local homotopy theory  
Oct 31  Bastiaan Cnossen  University of Regensburg  Genuine sheaves on differentiable stacks  
Nov 7  Scott Balchin  Queen's University Belfast  A jaunt through the tensortriangular geometry of rational G spectra for G profinite or compact Lie  
Nov 14  Kaif Hilman  Max Planck Institute Bonn  Equivariant hermitian Ktheory and a periodicity result for C_2Ltheory  
Nov 21  Emel Yavuz  Queen's University Belfast  C_2Equivariant Orthogonal Calculus  
Nov 28  Eric Finster  University of Birmingham  A Topos Theoretic View of Goodwillie Calculus  
Dec 5  Irakli Patchkoria  University of Aberdeen  Posets of finite abelian subgroups and Morava Ktheory  
Dec 12  Anna Marie Bohmann  Vanderbilt University  Scissors congruence and the Ktheory of covers 
Abstracts
Özgür Bayındır: Algebraic Ktheory of the twoperiodic first Morava Ktheory
Using a root adjunction formalism developed in an earlier work and logarithmic THH, we obtain a simplified computation of the algebraic Ktheory of the complex Ktheory spectrum. Furthermore, our computational methods also provide the algebraic Ktheory of the twoperiodic Morava Ktheory spectrum of height 1.
Daniel Kasprowski: Stable equivalence relations of 4manifolds
Kreck’s modified surgery gives an approach to classify 2nmanifolds up to stable diffeomorphism, i.e., up to a connected sum with copies of S^n x S^n. In dimension 4, we use a combination of modified and classical surgery to compare the stable diffeomorphism classification with other stable equivalence relations. Most importantly, we consider homotopy equivalence up to connected sum with copies of S^n x S^n. This is joint work with John Nicholson and Simona Veselá.
Itamar Mor: Profinite Galois descent in K(n)local homotopy theory
Using condensed mathematics, I give a construction of the K(n)local E_nAdams spectral sequence as a HFPSS for the continuous action of the Morava stabiliser group. A modified version gives a spectral sequence computing the Picard and Brauer groups of K(n)local spectra.
Bastiaan Cnossen: Genuine sheaves on differentiable stacks
Cohomology theories for equivariant spaces typically only depend on the associated quotient stacks X//G. It would thus be desirable to have a flexible framework for cohomology theories for stacks. In this talk, I will present such a framework, following ideas from motivic homotopy theory. The main result is a version of relative Poincaré duality for differentiable stacks, which generalizes Poincaré duality for smooth manifolds, Atyah duality for equivariant manifolds, and the Wirthmüller isomorphism in equivariant stable homotopy theory.
Scott Balchin: A jaunt through the tensortriangular geometry of rational G spectra for G profinite or compact Lie
In this talk, I will report on joint work with BarnesBarthel and BarthelGreenlees which analyses the category of rational G equivariant spectra for G a profinite group or compact Lie group respectively. In particular, I will focus on a series of results regarding the Balmer spectra of these categories, and how the topology of these topological spaces informs structural results regarding the category.
Kaif Hilman: Equivariant hermitian Ktheory and a periodicity result for C_2Ltheory
Building upon the recent foundational advances in hermitian Ktheory by a set of nine authors and the formalism of parametrised higher categories by a set of five authors, I will introduce in this talk a Gequivariant enhancement of the hermitian theory via the notion of a GPoincare category for finite groups G. As an application, I will indicate how one can use said enhancement to deduce a new periodicity result for Ltheory when G is the cyclic group of order 2.
Emel Yavuz: C_2Equivariant Orthogonal Calculus
Orthogonal homotopy calculus is the branch of functor calculus involving the study of functors from the category of finite dimensional real vector spaces to the category of pointed topological spaces. Using it, one can construct a Taylor tower of approximations to such functors, consisting of polynomial functors, and the layers of the tower are characterised by orthogonal spectra, making them much easier to compute.
A natural question is; what happens when the functors come with a group action? Such functors are of great interest, as they arise naturally within algebraic topology, for example the functor V \mapsto BO(V) where V is a Grepresentation. After an introduction to orthogonal calculus, I will discuss the main constructions and theorems of a C_2equivariant orthogonal calculus, that works for functors from finite dimensional C_2inner product spaces to C_2spaces.
Eric Finster: A Topos Theoretic View of Goodwillie Calculus
I will describe a framework for understanding the unstable version of Goodwillie’s calculus of functors from a topostheoretic perspective which builds on an analogy between higher topos theory and commutative algebra. In particular, I will describe how both Goodwilile’s original ”homotopy calculus” as well as the ”orthogonal calculus” of Michael Weiss can be understood in this framework. Along the way, we will see emerge a picture of the topos of nexcisive functors as classifying ”nnilpotent” objects. This is joint with with M. Anel, G. Biedermann and A. Joyal.
Irakli Patchkoria: Posets of finite abelian subgroups and Morava Ktheory
A result of K. Brown says that for a nice enough discrete group G, the orbifold Euler characteristic of G and the equivariant Euler characteristic of the poset of its nontrivial finite subgroups have the same fractional parts. We will present an analogous result for the poset of nontrivial finite abelian subgroups for which we will use Morava Ktheory. After presenting some computations, we will discuss potential applications in number theory analogous to Brown’s results on denominators of special values of zeta functions.
Anna Marie Bohmann: Scissors congruence and the Ktheory of covers
Scissors congruence, the subject of Hilbert's Third Problem, asks for invariants of polytopes under cutting and pasting operations. One such invariant is obvious: two polytopes that are scissors congruent must have the same volume, but Dehn showed in 1901 that volume is not a complete invariant. Trying to understand these invariants leads to the notion of the scissors congruence group of polytopes, first defined the 1970s. Elegant recent work of Zakharevich allows us to view this as the zeroth level of a series of higher scissors congruence groups.
In this talk, I'll discuss some of the classical story of scissors congruence and then describe a way to build the higher scissors congruence groups via Ktheory of covers, a new framework for such constructions. We'll also see how to relate coinvariants and Ktheory to produce concrete nontrivial elements in the higher scissors congruence groups. This work is joint with Gerhardt, Malkiewich, Merling and Zakharevich.