2022

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Time and place: , NHA B1120

Consider the singularity C^4/(Z/2), where Z/2 acts as the matrix diag(-1,-1,-1,-1). This singularity is special, in that it does not admit a crepant resolution. However, it does admit a so-called noncommutative crepant resolution, given by a Calabi-Yau 4 quiver. The moduli space of representations of this quiver turns out to share a lot of similarities with moduli spaces of sheaves over Calabi-Yau fourfolds, and it turns out that we can reuse techniques from studying moduli of sheaves to define and compute invariants of this moduli space of representations. In this talk, I will explain how these invariants can be defined, and give conjectures about the forms of these invariants. This talk is based on joint work with Raf Bocklandt.

Time and place: , NHA B1120
The variety of sums of powers, VSP(F, r) of a homogeneous form F of rank r is the closure in the Hilbert scheme of apolar schemes of length r. A bad limit is a scheme in the closure that is not apolar to F. I will discuss examples of bad limits, including examples for quadrics found by Joachim Jelisiejew that contradicts earlier results on polar simplicies. This is report on work in progress with Jelisiejew and Schreyer and with Grzegorz and Michal Kapustka.
Time and place: , NHA B1120
Counterexamples to the integral Hodge conjecture can arise either from torsion cohomology classes (as in Atiyah's and Hirzebruch's original counterexample from 1961) or from non-torsion classes (as first seen in Kollár's counterexample from 1991). After Voisin proved the IHC for uniruled threefolds, Schreieder found a unirational fourfold where the IHC fails. His construction of a non-algebraic Hodge class relies on abstract arguments with unramified cohomology. It was an open question whether this class is of torsion type. In this talk, I want to explain a new method that gives an explicit geometric description of the unramified cohomology class appearing in his argument. In particular, this approach allows to prove that Schreieder's unirational counterexample is of torsion type.
Time and place: , NHA B1120

As a consequence of the S-duality conjecture, Vafa and Witten conjectured certain symmetries concerning invariants derived from spaces of vector bundles on a closed Riemannian four-manifold. For a smooth complex projective surface X, a satisfying mathematical definition of Vafa-Witten invariants has been given by Tanaka and Thomas. Their invariants are a sum of two parts, one of which can be defined in terms of moduli spaces of stable vector bundles on X. Focusing on this instanton part of the VW invariants one can ask how it changes under blowing up the surface X. I will discuss joint work with Oliver Leigh and Yuuji Tanaka that answers this question.

Time and place: , NHA B1120

I will explain how a recent “universal wall-crossing” framework of Joyce works in equivariant K-theory, which I view as a multiplicative refinement of equivariant cohomology. Enumerative invariants, possibly of strictly semistable objects living on the walls, are controlled by a certain (multiplicative version of) vertex algebra structure on the K-homology groups of the ambient stack. In very special settings like refined Vafa-Witten theory, one can obtain some explicit formulas. For moduli stacks of quiver representations, this geometric vertex algebra should be dual in some sense to the quantum loop algebras that act on the K-theory of stable loci.

Time and place: , NHA B1120
Time and place: , NHA B1120

Abstract (PDF)

Time and place: , NHA B1120

Abstract (PDF)

Time and place: , NHA B1120

In 80s Weibel observed that K-theory is homotopy invariant on Fp-schemes up to p-torsion. His main tool was the action of the ring Witt vectors on nil-K-groups: NKi(R) = Ker(Ki(R[t]) → Ki(R)). We will revisit the proof and check that the same result holds for all finitary localizing invariants.

Time and place: , NHA B1120

I will explain how motivic homotopy theory can be used to attack problems regarding finite projective modules over smooth affine k-algebras. I will recall in particular the foundational theorem of Morel and Asok-Hoyois-Wendt, and the construction of the Barge-Morel Euler class. Time permitting, I will explain recent progress on Murthy's splitting conjecture.

Time and place: , NHA B1119

Abstract (PDF)

Time and place: , NHA B1119

I will discuss the question in the title. This is joint work with Alex Degtyarev and Ilia Itenberg. This will be a talk involving very classical topics in algebraic geometry. I will try to make the talk accessible to students at master- and PhD level.

Time and place: , NHA B1120
Hilbert schemes of points for a surface are a well studied subject with many famous results like Göttsche’s formula for its Betti numbers. A natural generalization comes from studying Grothendieck’s Quot-schemes and the associated enumerative invariants. Unlike the former, punctual Quot-schemes are smooth only virtually admitting perfect obstruction theories and virtual fundamental classes. This has recently been used to study invariants counting zero-dimensional quotients of trivial vector bundles by multiple authors who used virtual localization and therefore could not treat the case of a general vector bundle. We rely on other techniques which use a general wall-crossing framework of D. Joyce to study these. Our methods rely on existence of a Lie algebra coming from vertex algebras constructed out of topological data. I will explain how these arise naturally in the Quot-scheme setting and how one can obtain explicit invariants and study their symmetries.