On the second fundamental theorem of invariant theory for the orthosymplectic supergroup

Abstract

The first fundamental theorem of invariant theory for the orthosymplectic supergroup ${\rm OSp}(V)$ (where $V$ has superdimension $(m|2n)$) in the endomorphism algebra setting states that there is a surjective algebra homomorphism $F_r^r: B_r(m-2n)\rightarrow {\rm{End}}_{{\rm OSp}(V)}(V^{\otimes r})$ from the Brauer algebra of degree $r$ to the endomorphism algebra of $V^{\otimes r}$ over ${\rm OSp}(V)$. The second fundamental theorem in this setting seeks to describe ${\rm Ker} F_r^r$ as a $2$-sided ideal of $B_r(m-2n)$. We show that ${\rm Ker} F_r^r\neq 0$ if and only if $r\geq r_c:=(m+1)(n+1)$, and present a basis and a dimension formulae for ${\rm Ker} F_r^r$. As a 2-sided ideal, ${\rm Ker} F_r^r$ for any $r\ge r_c$ is generated by ${\rm Ker} F_{r_c}^{r_c}$, for which a set of generators is explicitly constructed in terms of Brauer diagrams. As applications of these results, we obtain the necessary and sufficient conditions for the endomorphism algebra $\rm{End}_{{\mathfrak {osp}}(V)}(V^{\otimes r})$ over the orthosymplectic Lie superalgebra ${\mathfrak {osp}}(V)$ to be isomorphic to $B_r(m-2n)$, and give new proofs for the main theorems in recent papers of G. Lehrer and R. Zhang on the second fundamental theorem of invariant theory for the orthogonal and symplectic groups.