Modules of higher order invariants

Abstract:

Let $k$ be an algebraically closed field of characteristic $p\geq 0$. Let $A$ be a commutative $k$-algebra with multiplicative identity and let $M$ be an $A$-module. Let $G$ be a linear algebraic group acting rationally on both $A$ and $M$. In this paper we study $A^{G}$-modules of $n$th order invariants, $I_{n}(M,G)$. The $I_{n}(M,G)$ are defined inductively by $I_{0}(M,G)=\{0\}$ and $I_{n}(M,G)$ = $\{m\in M:g\cdot m-m\in I_{n-1}(M,G)$ for all $g\in G\}$. We show that some fundamental problems concerning these modules can be reduced to the case $I_{n}(k[G],G)$ where $G$ acts on itself by right translation. We study the questions as to when $I_{n}(M,G)$ is a finitely generated $A^{G}$-module and how the $I_{n}(M,G)$ are related to equivariant mappings. For the classical case of $\mathbb {G}_{a}$ acting on binary forms, we describe the $I_{n}(M,G)$ and determine when they are Cohen-Macaulay.