Constructions of representations of o(2n + 1, C) that imply Molev and Reiner-Stanton lattices are strongly Sperner

• Authors: Robert G. Donnelly;Scott J. Lewis;Robert Pervine
• Venue: Discrete Mathematics
• Year: 2003

Two infinite families of distributive lattices parameterized by positive integers n and k are considered. The first family of lattices, here denoted LRSB(k,2n), was introduced by Reiner and Stanton (J. Algebraic Combin. 7 (1998) 91) as the distributive lattices Good(k,2n) of certain partitions. There, Reiner and Stanton showed that these lattices are rank symmetric and rank unimodal and conjectured that they are strongly Sperner. The second family of lattices introduced here is denoted LMolB(k, 2n) because of its connection to certain representation constructions of the odd orthogonal Lie algebras obtained by Molev (J. Phys. A 33 (2000) 4143). For fixed n and k, the two lattices have the same rank generating function, but the lattices are isomorphic as posets if and only if k = 1. In this paper, the lattices LRSB(k,2n) and LMolB(k,2n) are used to produce two different constructions of the irreducible representation of the odd orthogonal Lie algebra o(2n + 1, C) isomorphic to the largest irreducible component in the kth symmetric power of the defining representation of o(2n + 1, C). Constructions of the analogous infinite family of irreducible representations of G2 are obtained as a special case. These constructions use the elements of the lattices to index bases for the representing spaces, and explicit formulas for the matrix entries of the representing matrices for certain Lie algebra generators are given. These constructions together with a result of Proctor imply that both lattices are rank symmetric, rank unimodal, and strongly Sperner.