Module structure of the $K$-theory of polynomial-like rings

Abstract

Suppose $Γ$ is a submonoid of a lattice, not containing a line. In this note, we use the natural $Γ$-grading on the monoid algebra $R[Γ]$ to prove structural results about the relative $K$-theory $K(R[Γ], R)$. When $R$ contains a field, we prove a decomposition indexed by the rays in $Γ$, and a compatible action by the Witt vectors of $R$ for each $\mathbf N$-grading of $Γ$. In characteristic zero, there is additionally an action by Witt vectors for the truncation set $Γ$. Finally, we apply this to get a ray-like description of $K_*(R[x_1,...,x_n])$ proposed by J.\,Davis.