Integral cohomology of the Generalized Kummer fourfold

We describe the integral cohomology of the Generalized Kummer fourfold giving an explicit basis, using Hilbert scheme cohomology and tools developed by Hassett and Tschinkel. Then we apply our results to a IHS variety with singularities, obtained by a partial resolution of the Generalized Kummer quotiented by a symplectic involution. We calculate the Beauville--Bogomolov form of this new variety, presenting the first example of such a form that is odd.Comment: 40 pages, to appear in Algebraic Geometr

Integral cohomology of quotients via toric geometry

We describe the integral cohomology of a compact complex manifold $X$ quotiented by a cyclic group $G$ of prime order with only isolated fixed points. As a preliminary step, we investigate the integral cohomology of toric blow-ups of quotients of $\mathbb{C}^n$. We also provide necessary and sufficient conditions for having the spectral sequence of equivariant cohomology of $(X,G)$ degenerated at the second page. As an application, we compute the Beauville--Bogomolov form of Hilbert schemes of points on a K3 surface quotiented by automorphisms of orders 5 and 7.Comment: 32 page

Construction of G_2-instantons via twisted connected sums

We propose a method to construct G_2-instantons over a compact twisted connected sum G_2-manifold, applying a gluing result of S\'a Earp and Walpuski to instantons over a pair of 7-manifolds with a tubular end (see arXiv:1310.7933). In our example, the moduli spaces of the ingredient instantons are non-trivial, and their images in the moduli space over the asymptotic cross-section K3 surface intersect transversely. Such a pair of asymptotically stable holomorphic bundles is obtained using a twisted version of the Hartshorne-Serre construction, which can be adapted to produce other examples. Moreover, their deformation theory and asymptotic behaviour are explicitly understood, results which may be of independent interest.Comment: 22 pages. Final version to appear in Mathematical Research Letter