Multivariate $P$- and $Q$-polynomial structures of generalized Johnson association schemes

Abstract

Let $\mathfrak X$ be a commutative association scheme of class $d$ on $X$. We construct the generalized Johnson association scheme $\mathfrak J_{n,h}(\mathfrak X)$ on partial maps from $[n]$ to $X$ with domains of size $h$. For $1\le h\le n-1$, we prove that $\mathfrak J_{n,h}(\mathfrak X)$ has $(d+1)$-variate $P$- and $Q$-polynomial structures. In particular, its multivariate $Q$-polynomial property answers affirmatively a problem posed for generalized Johnson association schemes arising from finite Gelfand pairs. We also determine the primitive idempotents and the two eigenmatrices, expressing the first eigenmatrix as a product of an Eberlein polynomial and a multivariate Krawtchouk polynomial, and the second eigenmatrix as a product of a Hahn polynomial and a multivariate Krawtchouk polynomial. For the multivariate factors, we use Mizukawa--Tanaka $(d+1,2d+2)$-hypergeometric functions. In an appendix, we identify the normalized columns of the second eigenmatrix with the known spherical functions in the finite Gelfand pair case. We also discuss the orthogonality relations and the imprimitivity arising from partial maps with the same domain, and explicitly describe the multivariate $P$- and $Q$-polynomial structures inherited by the blocks and the quotient.