Hori-mological projective duality

Kuznetsov has conjectured that Pfaffian varieties should admit non-commutative crepant resolutions which satisfy his Homological Projective Duality. We prove half the cases of this conjecture, by interpreting and proving a duality of non-abelian gauged linear sigma models proposed by Hori.Comment: 55 pages. V2: slightly rewritten to take advantage of the `non-commutative Bertini theorem' recently proved by the authors and Van den Bergh. V3: lots of changes in exposition following referees' comments. Section 5 has been mostly cut because it was boring. To appear in Duke Math. J. V3: added funder acknowledgemen

The homological projective dual of Sym^2 P(V)

We study the derived category of a complete intersection X of bilinear divisors in the orbifold Sym^2 P(V). Our results are in the spirit of Kuznetsov's theory of homological projective duality, and we describe a homological projective duality relation between Sym^2 P(V) and a category of modules over a sheaf of Clifford algebras on P(Sym^2 V^vee). The proof follows a recently developed strategy combining variation of GIT stability and categories of global matrix factorisations. We begin by translating D^b(X) into a derived category of factorisations on an LG model, and then apply VGIT to obtain a birational LG model. Finally, we interpret the derived factorisation category of the new LG model as a Clifford module category. In some cases we can compute this Clifford module category as the derived category of a variety. As a corollary we get a new proof of a result of Hosono and Takagi, which says that a certain pair of nonbirational Calabi-Yau 3-folds have equivalent derived categories.Comment: 54 pages, Ph.D. thesi

A non-commutative Bertini theorem

We prove a version of the classical 'generic smoothness' theorem with smooth varieties replaced by non-commutative resolutions of singular varieties. This in particular implies a non-commutative version of the Bertini theorem.Comment: 6 pages. v2: added funder acknowledgement. Published in J. Noncommutative Geometr

Fano varieties with torsion in the third cohomology group

We construct first examples of Fano varieties with torsion in their third cohomology group. The examples are constructed as double covers of linear sections of rank loci of symmetric matrices, and can be seen as higher-dimensional analogues of the Artin--Mumford threefold. As an application, we answer a question of Voisin on the coniveau and strong coniveau filtrations of rationally connected varieties.Comment: 18 page

Donaldson-Thomas theory

Donaldson-Thomas invariants are integers assigned to a smooth, projective threefold X. These integers are defined by a virtual count of points on a Hilbert scheme of curves on X, and are invariant under deformations of X. We give an introduction to this theory, and explain its relation to the similar curve-counting theory of Gromov-Witten invariants. We present different techniques for calculating Donaldson-Thomas invariants and for generalizing the invariants to nonprojective X. In the last chapter we calculate the Donaldson-Thomas invariants of a threefold X with a map to a surface S having fibres isomorphic to a fixed elliptic curve E

Automorphisms of Hilbert schemes of points on surfaces

We show that every automorphism of the Hilbert scheme of $n$ points on a weak Fano or general type surface is natural, i.e. induced by an automorphism of the surface, unless the surface is a product of curves and $n=2$. In the exceptional case there exists a unique non-natural automorphism. More generally, we prove that any isomorphism between Hilbert schemes of points on smooth projective surfaces, where one of the surfaces is weak Fano or of general type and not equal to the product of curves, is natural. We also show that every automorphism of the Hilbert scheme of $2$ points on $\mathbb{P}^n$ is natural.Comment: 20 pages; added reference to related work of Hayash

Universal Polynomials for Tautological Integrals on Hilbert Schemes

We show that tautological integrals on Hilbert schemes of points can be written in terms of universal polynomials in Chern numbers. The results hold in all dimensions, though they strengthen known results even for surfaces by allowing integrals over arbitrary "geometric" subsets (and their Chern-Schwartz-MacPherson classes). We apply this to enumerative questions, proving a generalised G\"ottsche Conjecture for all singularity types and in all dimensions. So if L is a sufficiently ample line bundle on a smooth variety X, in a general subsystem P^d of |L| of appropriate dimension the number of hypersurfaces with given singularity types is a polynomial in the Chern numbers of (X,L). When X is a surface, we get similar results for the locus of curves with fixed "BPS spectrum" in the sense of stable pairs theory.Comment: 44 pages, minor changes and correction

A proof of the Donaldson-Thomas crepant resolution conjecture

We prove the crepant resolution conjecture for Donaldson-Thomas invariants of hard Lefschetz CY3 orbifolds, formulated by Bryan-Cadman-Young, interpreting the statement as an equality of rational functions. In order to do so, we show that the generating series of stable pair invariants on any CY3 orbifold is the expansion of a rational function. As a corollary, we deduce a symmetry of this function induced by the derived dualising functor. Our methods also yield a proof of the orbifold DT/PT correspondence for multi-regular curve classes on hard Lefschetz CY3 orbifolds.Comment: 66 pages, 1 figure. Comments welcome. v2: references adde