6 papers
Optimality of the Prym-Tyurin construction for
Philip Engel, Olivier de Gaay Fortman, Stefan Schreieder
We prove that on a very general principally polarized abelian 6-fold, the smallest multiple of the minimal curve class which can be represented by an algebraic cycle is 6.
Combinatorics and Hodge theory of degenerations of abelian varieties: A survey of the Mumford construction
Philip Engel, Olivier de Gaay Fortman, Stefan Schreieder
We survey the Mumford construction of degenerating abelian varieties, with a focus on the analytic version of the construction, and its relation to toric geometry. Moreover, we stu…
Descent for algebraic stacks
Olivier de Gaay Fortman
We prove that algebraic stacks satisfy 2-descent for fppf coverings. We generalize Galois descent for schemes to stacks, by considering the case where the fppf covering is a finite…
On the topology of real algebraic stacks
Emiliano Ambrosi, Olivier de Gaay Fortman
Motivated by questions arising in the theory of moduli spaces in real algebraic geometry, we develop a range of methods to study the topology of the real locus of a Deligne-Mumford…
Non-arithmetic hyperbolic orbifolds attached to unitary Shimura varieties
Olivier de Gaay Fortman
We develop a new method of constructing non-arithmetic lattices in the projective orthogonal group for every integer larger than one. The technique is to consi…
Real moduli spaces and density of non-simple real abelian varieties
Olivier de Gaay Fortman
For fixed and a family of polarized abelian varieties of dimension over , we give a criterion for the density in the parameter space of those abelian varietie…