The Canonical Quadratic Pair on Clifford Algebras over Schemes

Abstract

Working over an arbitrary base scheme $S$, we define the canonical quadratic pair on the Clifford algebra associated to an Azumaya algebra with quadratic pair. Given an Azumaya algebra $\mathcal{A}$ with quadratic pair, i.e., with an orthogonal involution and a semi-trace, its associated Clifford algebra's canonical involution is only orthogonal in certain cases, namely when $\mathrm{deg}(\mathcal{A})$ is divisible by $8$ or when both $2=0$ over $S$ and $\mathrm{deg}(\mathcal{A})$ is divisible by $4$. When $\mathrm{deg}(\mathcal{A}) \geq 8$, our definition of the canonical quadratic pair on the Clifford algebra is extended from previous work of Dolphin and Qu\'eguiner-Mathieu, who worked over fields of characteristic $2$. When $\mathrm{deg}(\mathcal{A})=4$, we show that no canonical quadratic pair exists.