Highly Versal Torsors

Let $G$ be a linear algebraic group over an infinite field $k$. Loosely speaking, a $G$-torsor over $k$-variety is said to be versal if it specializes to every $G$-torsor over any $k$-field. The existence of versal torsors is well-known. We show that there exist $G$-torsors that admit even stronger versality properties. For example, for every $d\in\mathbb{N}$, there exists a $G$-torsor over a smooth quasi-projective $k$-scheme that specializes to every torsor over a quasi-projective $k$-scheme after removing some codimension-$d$ closed subset from the latter. Moreover, such specializations are abundant in a well-defined sense. Similar results hold if we replace $k$ with an arbitrary base-scheme. In the course of the proof we show that every globally generated rank-$n$ vector bundle over a $d$-dimensional $k$-scheme of finite type can be generated by $n+d$ global sections. When $G$ can be embedded in a group scheme of unipotent upper-triangular matrices, we further show that there exist $G$-torsors specializing to every $G$-torsor over any affine $k$-scheme. We show that the converse holds when $\operatorname{char} k=0$. We apply our highly versal torsors to show that, for fixed $m,n\in\mathbb{N}$ and a fixed semilocal $\mathbb{Z}[\frac{1}{n},e^{2\pi i/n}]$-ring $R$ with infinite residue fields, the symbol length of Azumaya algebras over $R$ having degree $m$ and period $n$ is uniformly bounded. Under mild assumptions on $R$, e.g., if $R$ is local, the bound depends only on $m$ and $n$, and not on $R$.Comment: 41 pages. Comments are welcom

Rationally Isomorphic Hermitian Forms and Torsors of Some Non-Reductive Groups

Let $R$ be a semilocal Dedekind domain. Under certain assumptions, we show that two (not necessarily unimodular) hermitian forms over an $R$-algebra with involution, which are rationally ismorphic and have isomorphic semisimple coradicals, are in fact isomorphic. The same result is also obtained for quadratic forms equipped with an action of a finite group. The results have cohomological restatements that resemble the Grothendieck--Serre conjecture, except the group schemes involved are not reductive. We show that these group schemes are closely related to group schemes arising in Bruhat--Tits theory.Comment: 27 pages. Changes from previous version: Section 5 was split into two sections, several proofs have been simplified, other mild modification