Experiments on the Brauer map in High Codimension

Abstract

Using twisted and formal-local methods, we prove that every separated algebraic space which is the (open/flat) pushout of affine schemes has enough Azumaya algebras. As a corollary we show that, under mild hypothesis, every cohomological Brauer class is representable by an Azumaya algebra away from a closed subset of codimension $\ge 3$, generalizing an early result of Grothendieck. Next, we show that $\mathrm{Br}(X)={\mathrm{Br}}^{\prime }(X)$ when $X$ is an algebraic space obtained from a quasi-projective scheme by contracting a curve. This is the first method of descending an Azumaya algebra along a Chow cover which works in all dimensions. As a corollary, we prove that cohomological Brauer classes are geometric on arbitrary separated surfaces. In higher dimensions, we obtain the first examples of non-quasi-projective algebraic spaces with $\mathrm{Br}(X)={\mathrm{Br}}^{\prime }(X)$.

v3: Minor changes and corrections. To appear in Algebra & Number Theory

Cited by in corpus (2)

• Locally free twisted sheaves of infinite rank
• Pinching Azumaya algebras