Partition algebras $\mathsf{P}_k(n)$ with $2k>n$ and the fundamental theorems of invariant theory for the symmetric group $\mathsf{S}_n$

Abstract

Assume $\mathsf{M}_n$ is the $n$-dimensional permutation module for the symmetric group $\mathsf{S}_n$, and let $\mathsf{M}_n^{\otimes k}$ be its $k$-fold tensor power. The partition algebra $\mathsf{P}_k(n)$ maps surjectively onto the centralizer algebra $\mathsf{End}_{\mathsf{S}_n}(\mathsf{M}_n^{\otimes k})$ for all $k, n \in \mathbb{Z}_{\ge 1}$ and isomorphically when $n \ge 2k$. We describe the image of the surjection $\Phi_{k,n}:\mathsf{P}_k(n) \to \mathsf{End}_{\mathsf{S}_n}(\mathsf{M}_n^{\otimes k})$ explicitly in terms of the orbit basis of $\mathsf{P}_k(n)$ and show that when $2k > n$ the kernel of $\Phi_{k,n}$ is generated by a single essential idempotent $\mathsf{e}_{k,n}$, which is an orbit basis element. We obtain a presentation for $\mathsf{End}_{\mathsf{S}_n}(\mathsf{M}_n^{\otimes k})$ by imposing one additional relation, $\mathsf{e}_{k,n} = 0$, to the standard presentation of the partition algebra $\mathsf{P}_k(n)$ when $2k > n$. As a consequence, we obtain the fundamental theorems of invariant theory for the symmetric group $\mathsf{S}_n$. We show under the natural embedding of the partition algebra $\mathsf{P}_n(n)$ into $\mathsf{P}_k(n)$ for $k \ge n$ that the essential idempotent $\mathsf{e}_{n,n}$ generates the kernel of $\Phi_{k,n}$. Therefore, the relation $\mathsf{e}_{n,n} = 0$ can replace $\mathsf{e}_{k,n} = 0$ when $k \ge n$.