Counting Lie ideals of niltriangular matrices

Abstract

We give an explicit finite sum for the number of ideals of the Lie algebra of strictly lower triangular $n\times n$ matrices over $\mathbb F_q$, valid for every prime power $q$. The sum runs over integer compositions, with weights expressed using ordinary and Gaussian binomial coefficients. A contraction bijection transforms Gagnon's configuration sum into a weighted enumeration of nonnesting partitions, with antichains of intervals chosen independently in each block. We derive a Stieltjes continued fraction for the block weights. Lagrange inversion and the coefficient formula for Stieltjes--Rogers polynomials then yield the explicit sum.