Bivariate Affine $q$-Krawtchouk Polynomials and Association Schemes over Galois Rings

We introduce regularized bivariate affine $q$-Krawtchouk polynomials and show that they give the first eigenmatrix of a translation association scheme on ${\mathrm{Mat}}_{d\times n}(\mathrm{GR}(p^2,r))$. The relations are defined by Smith type, and the corresponding eigenvalues are written as character sums over Smith type classes. The proof is based on recurrence relations obtained from the transition numbers describing how Smith types change when one row and one column are added to a matrix. These recurrences identify the character sums with the regularized bivariate affine $q$-Krawtchouk polynomials.