Spiral shifting operators from the enumeration of finite-index submodules of $\mathbb{F}_q[[T]]^d$

Abstract: Fix an integer $d\geq 1$. Solomon counts index-$n$ submodules of the free module $R^d$ for any Dedekind domain $R$ that admits a Dedekind zeta function. The proof involves the Hermite normal form for matrices. We propose another normal form in the case where $R=\mathbb{F}_q[[T]]$, which unlike the Hermite normal form, recovers the Smith normal form. We use our normal form to give a uniform approach to Solomon's result and a refinement of it due to Petrogradsky, which counts finite-index submodules $M$ of $R^d$ such that $R^d/M$ has a given module structure. Our normal form and Hermite's can be interpreted as reduced Gr\"obner bases with respect to two different monomial orders. Counting with our normal form gives rise to a total distance'' statistics $W$ and certainspiral shifting'' operators $g_1,\dots,g_d$ on the set $X=\mathbb{N}^d$ of $d$-tuples of nonnegative integers. These combinatorial structures are of independent interest. We show that these operators commute, and collectively act on $X$ freely and transitively, which in particular gives a nontrivial self-bijection on $\mathbb{N}^d$. We give a formula on how $g_j$ interacts with the $W$-statistics on $X$, leading to a rational formula for the generating function for $W$-statistics over any forward orbit in $X$ under these operators.