8 citations · 35 across the 20 of their papers we have counts for
26 papers · 1 filter
Generalizations of quasielliptic curves
Cesar Hilario, Stefan Schröer
We generalize the notion of quasielliptic curves, which have infinitesimal symmetries and exist only in characteristic two and three, to a remarkable hierarchy of regular curves ha…
Sign involutions on para-abelian varieties
Jakob Bergqvist, Thuong Dang, Stefan Schröer
We study the so-called sign involutions on twisted forms of abelian varieties, and show that such a sign involution exists if and only if the class in the Weil--Châtelet group is a…
The inverse Galois problem for connected algebraic groups
Michel Brion, Stefan Schröer
We show that each connected group scheme of finite type over an arbitrary ground field is isomorphic to the component of the identity inside the automorphism group scheme of some p…
Albanese maps for open algebraic spaces
Stefan Schröer
We show that for each algebraic space that is separated and of finite type over a field, and whose affinization is connected and reduced, there is a universal morphism to a para-ab…
Loops on schemes and the algebraic fundamental group
Kay Rülling, Stefan Schröer
In this note we give a re-interpretation of the algebraic fundamental group for proper schemes that is rather close to the original definition of the fundamental group for topologi…
The resolution property holds away from codimension three
Siddharth Mathur, Stefan Schröer
The purpose of this paper is to verify a conjecture of Gross under mild hypothesis: all reduced, separated, and excellent schemes have the resolution property away from a closed su…
The structure of Frobenius kernels for automorphism group schemes
Stefan Schröer, Nikolaos Tziolas
We establish structure results for Frobenius kernels of automorphism group schemes for surfaces of general type in positive characteristics. It turns out that there are surprisingl…
Discriminant groups of wild cyclic quotient singularities
Dino Lorenzini, Stefan Schröer
Let p be prime. We describe explicitly the resolution of singularities of several families of wild Z/pZ-quotient singularities in dimension two, including families that generalize…
Para-abelian varieties and Albanese maps
Bruno Laurent, Stefan Schröer
We construct for every proper algebraic space over a ground field an Albanese map to a para-abelian variety, which is unique up to unique isomorphism. This holds in the absence of…
Moret-Bailly families and non-liftable schemes
Damian Roessler, Stefan Schröer
Generalizing the Moret-Bailly pencil of supersingular abelian surfaces to higher dimensions, we construct for each field of characteristic p>0 a smooth projective variety with triv…
Rigidified torsor cocycles, hypercoverings and bundle gerbes
Stefan Schröer
We give a geometric interpretation of sheaf cohomology for higher degrees n in terms of torsors on the member of degree d=n-1 in hypercoverings of type r=n-2, endowed with an addit…
There is no Enriques surface over the integers
Stefan Schröer
We show that there is no family of Enriques surfaces over the ring of integers. This extends non-existence results of Minkowski for families of finite étale schemes, of Tate and Og…
The Deligne-Illusie Theorem and exceptional Enriques surfaces
Stefan Schröer
Building on the results of Deligne and Illusie on liftings to truncated Witt vectors, we give a criterion for non-liftability that involves only the dimension of certain cohomology…
The maximal unipotent finite quotient, unusual torsion in Fano threefolds, and exceptional Enriques surfaces
Andrea Fanelli, Stefan Schröer
We introduce and study the maximal unipotent finite quotient for algebraic group schemes in positive characteristics. Applied to Picard schemes, this quotient encodes unusual torsi…
Moderately ramified actions in positive characteristic
Dino Lorenzini, Stefan Schröer
In characteristic two and dimension two, wild Z/2Z-actions on k[[u,v]] ramified precisely at the origin were classified by Artin, who showed in particular that they induce hypersur…
Del Pezzo surfaces and Mori fiber spaces in positive characteristic
Andrea Fanelli, Stefan Schröer
We settle a question that originates from results and remarks by Kollár on extremal ray in the minimal model program: In positive characteristics, there are no Mori fibrations on t…
Total separable closure and contractions
Stefan Schröer
We show that on integral normal separated schemes whose function field is separably closed, for each pair of points the intersection of the resulting local schemes is local. This e…
On equivariant formal deformation theory
Stefan Schröer, Yukihide Takayama
Using the set-up of deformation categories of Talpo and Vistoli, we re-interpret and generalize, in the context of cartesian morphisms in abstract categories, some results of Rim c…
Enriques surfaces with normal K3-like coverings
Stefan Schröer
We analyze the structure of simply-connected Enriques surface in characteristic two whose K3-like covering is normal, building on the work of Ekedahl, Hyland and Shepherd-Barron. W…
The p-radical closure of local noetherian rings
Stefan Schröer
Given a local noetherian ring whose formal completion is integral, we introduce and study the -radical closure . Roughly speaking, this is the largest purely i…
Geometry on totally separably closed schemes
Stefan Schröer
We prove, for quasicompact separated schemes over ground fields, that Cech cohomology coincides with sheaf cohomology with respect to the Nisnevich topology. This is a partial gene…
Points in the fppf topology
Stefan Schröer
Using methods from commutative algebra and topos-theory, we construct topos-theoretical points for the fppf topology of a scheme. These points are indexed by both a geometric point…
Wild quotient surface singularities whose dual graphs are not star-shaped
Hiroyuki Ito, Stefan Schroeer
We obtain some results that answer certain questions of Lorenzini on wild quotient singularities in dimension two. Using Kato's theory of log structures and log regularity, we prov…
Wildly Ramified Actions and Surfaces of General Type Arising from Artin-Schreier Curves
Hiroyuki Ito, Stefan Schroeer
We analyse the diagonal quotient for products of certain Artin--Schreier curves. The smooth models are almost always surfaces of general type, with Chern slopes tending asymptotica…