Partition Algebras and the Invariant Theory of the Symmetric Group

Abstract

The symmetric group $\mathsf{S}_n$ and the partition algebra $\mathsf{P}_k(n)$ centralize one another in their actions on the $k$-fold tensor power $\mathsf{M}_n^{\otimes k}$ of the $n$-dimensional permutation module $\mathsf{M}_n$ of $\mathsf{S}_n$. The duality afforded by the commuting actions determines an algebra homomorphism $\Phi_{k,n}: \mathsf{P}_k(n) \to \mathsf{End}_{\mathsf{S}_n}(\mathsf{M}_n^{\otimes k})$ from the partition algebra to the centralizer algebra $\mathsf{End}_{\mathsf{S}_n}(\mathsf{M}_n^{\otimes k})$, which is a surjection for all $k, n \in \mathbb{Z}_{\ge 1}$, and an isomorphism when $n \ge 2k$. We present results that can be derived from the duality between $\mathsf{S}_n$ and $\mathsf{P}_k(n)$; for example, (i) expressions for the multiplicities of the irreducible $\mathsf{S}_n$-summands of $\mathsf{M}_n^{\otimes k}$, (ii) formulas for the dimensions of the irreducible modules for the centralizer algebra $\mathsf{End}_{\mathsf{S}_n}(\mathsf{M}_n^{\otimes k})$, (iii) a bijection between vacillating tableaux and set-partition tableaux, (iv) identities relating Stirling numbers of the second kind and the number of fixed points of permutations, and (v) character values for the partition algebra $\mathsf{P}_k(n)$. When $2k >n$, the map $\Phi_{k,n}$ has a nontrivial kernel which is generated as a two-sided ideal by a single idempotent. We describe the kernel and image of $\Phi_{k,n}$ in terms of the orbit basis of $\mathsf{P}_k(n)$ and explain how the surjection $\Phi_{k,n}$ can also be used to obtain the fundamental theorems of invariant theory for the symmetric group.