# JAWS Past Talks 2018/19

**2018 Term 1**

Week 1, 05/10/18

### Aurelio Carlucci (University of Oxford)

**Moduli spaces of stable pairs on the conifold and quiver representations**

A quiver is a collection of arrows, that is an oriented graph. They appear in different branches of mathematics and theoretical physics, as a means to encode, in an efficient way, combinatorial information arising from algebraic or categorical data. In certain cases, they allow to describe moduli spaces appearing in algebraic geometry.

This talk is going to be an introduction on the latter aspect, with a perspective on my current research. We will start by considering classical examples, like the Kronecker quiver or the trefle quiver, and the corresponding moduli spaces, hinting at the applications to enumerative geometry.

We will move then to the topic of representations of the path algebra associated to a quiver, and the notion of stability introduced by King; there is indeed a remarkable translation, in the quiver case, of the more general notion of stability for an abelian category.

In particular, Nagao and Nakajima proved that some classes of moduli spaces of sheaves on the resolved conifold, appearing in enumerative geometry, can be interpreted as moduli spaces of representations of a certain framed quiver, allusively called conifold quiver. We will unravel the details of this association, showing how simple computations can lead to the characterisation of stability conditions for representations of the conifold quiver.

This leads to an explicit coordinate description of the moduli spaces of stable pairs on the resolved conifold: we confirm the results for the known cases, and we complement the information already obtained by a more geometric approach in a new case.

Week 2, 8/10/2018

### Lorenzo De Biase (Cardiff University)

**Generalised braid categorification**

Ordinary braid group Brn is a well-known algebraic structure which encodes configurations of n non-touching strands (“braids”) up to continuous transformations (“isotopies”). A classical result of Khovanov and Thomas states that this group acts categorically on the space Fln of complete flags in Cn. Generalised braids are the braids whose strands are allowed to touch in a certain way. They have multiple endpoint configurations and can be non-invertible, thus forming a category rather than a group. In this talk I will present some progress that have been made towards extending the result of Khovanov and Thomas to the categorification of the generalised braid category.

Week 3, 15/10/2018

### Lucas das Dores (University of Liverpool)

**Symmetric powers of varieties and rational equivalence of 0-cycles**

For any quasi-projective variety X over a field k, its d-th symmetric power is the quotient of the its d-th Cartesian product by the action of the permutation group of d elements.

In particular, if the field is algebraically closed, the k-points of the d-th symmetric powers of the variety correspond to a zero-cycle of degree d on X. Moreover, rational equivalence of zero cycles is encoded by rational curves on symmetric powers of X.

In this talk, we review a classical result by Mumford stating that if X is a nonsingular projective surface with positive geometric genus, its group of 0-cycles of degree 0 modulo rational equivalence is not finite dimensional.

Week 4, 22/10/2018

### Peter Spacek (University of Kent)

**Mirror Symmetry for Homogeneous Spaces**

In this talk we will take a look at mirror symmetry at the level of the small quantum cohomology, specialized to the case of homogeneous spaces. We will introduce Gromov-Witten invariants and how they are involved in the small quantum cohomology using complex projective space as an illustration. We will stay with projective space, but now consider it as a homogeneous space, and work out a method to construct a mirror pair for it: that is, a variety whose coordinate ring modulo relations defined by a rational function models the small quantum cohomology of projective space. Finally, if time allows, we will try to give some insights how the construction works for more general homogeneous spaces.

Week 5, 29/10/2018

### Alessandro Bigazzi (University of Warwick)

**Degenerations of Conic Bundles in any Characteristic**

This talk aims to introduce informally how we can employ the recent degeneration method by Voisin to a wide class of rationality problems. After discussing the general method and its difficulties, the talk will focus on applications to conic bundles, with special regard to mixed characteristic degenerations. The special case of char = 2 will be highlighted as it was result of original research together with A. Auel, Ch. Boehning, H.Ch. Graf von Bothmer.

Week 7, 12/11/2018

### Matt Booth (University of Edinburgh)

**Deformation Theory**

Deformation theory can be thought of as the study of infinitesimal thickenings of geometric or algebraic objects. Equivalently, it is the study of infinitesimal neighbourhoods in moduli spaces. In this talk, I'll provide an introduction to some deformation-theoretic concepts. I'll explain Deligne's philosophy that in characteristic zero, deformation problems are 'controlled' by differential graded Lie algebras (dglas). I'll talk about derived deformation theory, and how it clarifies some concepts within the classical theory, in particular the proofs of Lurie and Pridham that dglas are equivalent to derived moduli problems via Koszul duality. Time permitting, I'll talk about an application of noncommutative deformation theory to birational geometry in the form of the Donovan-Wemyss contraction algebra, and I will mention how derived noncommutative deformations also fit into the picture.

Week 8, 19/11/2018

### Gaia Comaschi (Universite Lille-I)

**Pfaffian representations of cubic threefolds and instanton bundles**

Given a hypersurface X ⊂ P^n, we can determine whether its equation might be expressed as the determinant of a matrix of linear forms, by showing the existence of certain ACM sheaves on X. One of the most effcient way to produce these sheaves is to use Serre's correspondence, starting from AG subschemes of X. In this talk I will treat the case where X is a cubic threefold. I will illustrate how we can construct explicitly AG curves corresponding to Pfaffian representations of X and how Serre's correspondence yields a component of the moduli space of instanton

bundles on X.

Week 9, 26/11/2018

### Aeran Fleming (University of Liverpool)

**Multipoint Seshadri constants and explicit Kähler packings of projective complex manifolds.**

In this talk I will start by introducing the concept of multipoint Seshadri constants and discuss their relationship withNagata's conjecture. In the remaining time I will introduce the notions of Symplectic and Kähler packings and end by showing that there is a direct connection between the multipoint Seshardi constant and the existence of Kähler packings.

Week 10, 3/10/2018

### James Plowman (University of Warwick)

**The Chow-Witt group of a scheme with dualising complex**

Chow Witt groups were originally introduced by Barge & Morel as a cohomological object which they conjectured would serve as a good target for a theory of Euler classes - these classes have since been constructed in many cases of interest. In this talk we aim to define the Chow-Witt group for possibly singular schemes similarly to the description of the ordinary Chow group as cycles modulo rational equivalence.

**2018 Term 2**

Week 2, 14/01/19

### Angelica Simonetti (UMass Amherst)

**On cusps singularities and their smoothability**

Given an isolated surface singularity it is natural to study its deformation theory and look for conditions for it to be smoothable: it turns out that for normal surface cusp singularities there's a nice result initially conjectured by Looijenga relating the existence of smoothings of such singularities to the combinatorics of certain special rational surfaces. After a general introduction on cusp singularities I will discuss Looijenga's theorem, giving a sketch of its proof.

Week 3, 21/01/19

### Dominic Foord (University of Liverpool)

**Birational Rigidity of higher dimensional Fano spaces**

In this talk we will review some recent results in the area of Birational Rigidity. We will begin by recalling the main definitions and theorems behind the theory, before studying the case of a cyclic cover with a singular point, showing the Birational Superrigidity of a general such variety. We will also review some more recent results using the techniques discussed.

Week 5, 04/02/19

Tim Grange (Loughborough University)

**Blow-ups of products of projective spaces**

The classification of Mori Dream Spaces is an open problem in algebraic geometry. Castravet and Tevelev gave a criterion to determine whether the blow-up of products of projective spaces of the form (ℙ^a)^b at points in general position are Mori Dream Spaces. The case of arbitrary products of projective spaces remains unknown. In this talk, I will discuss progress in this direction and give some positivity results.

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Week 6, 11/02/2019

### Matthias Nickel (Goethe University Frankfurt)

**Newton-Okounkov bodies of exceptional curve valuations**

Newton-Okounkov bodies are convex bodies associated to line bundles on projective varieties that capture positivity properties of the line bundle in question. Let $p$ be a closed point in ${\mathbb{P}}_{\mathbb{C}}^^2$ and consider a surface obtained by a sequence of finitely many blowups of points where we start with $p$ and always blow up a point in the exceptional divisor created last.

Our result is an explicit description of the Newton-Okounkov body of the pullback of $\mathcal{O}_{\mathbb{P}^^2}(1)$ with respect to the flag given by the last exceptional divisor and a point on it.