Specht module

In mathematics, a Specht module is one of the representations of symmetric groups studied by Wilhelm Specht (1935). They are indexed by partitions, and in characteristic 0 the Specht modules of partitions of n form a complete set of irreducible representations of the symmetric group on n points.

Definition

Fix a partition λ of n. A tabloid is an equivalence class of labellings of the Young diagram of shape λ, where two labellings are equivalent if one is obtained from the other by permuting the entries of each row. Denote by $\{T\}$ the equivalence class of a tableau $T$. The symmetric group on n points acts on the set of tableaux of shape λ (i.e., on the set of labellings of the Young diagram). Consequently, it acts on tabloids, and on the module V with the tabloids as basis. For each Young tableau T of shape λ, form the element

$E_T=\sum_{\sigma\in Q_T}\epsilon(\sigma)\{\sigma(T)\} \in V$

where Q_T is the subgroup of permutations, preserving (as sets) all columns of T and $\epsilon(\sigma)$ is the sign of the permutation σ . The Specht module of the partition λ is the module generated by the elements E_T as T runs through all tableaux of shape λ.

The Specht module has a basis of elements E_T for T a standard Young tableau.

Structure

Over fields of characteristic 0 the Specht modules are irreducible, and form a complete set of irreducible representations of the symmetric group.

A partition is called p-regular if it does not have p parts of the same (positive) size. Over fields of characteristic p>0 the Specht modules can be reducible. For p-regular partitions they have a unique irreducible quotient, and these irreducible quotients form a complete set of irreducible representations.