The homotopy groups of the simplicial mapping space between algebras

Abstract

Let $\ell$ be a commutative ring with unit. To every pair of $\ell$-algebras $A$ and $B$ one can associate a simplicial set $\mathrm{hom}(A,B^{\Delta })$ so that ${\pi }_0\mathrm{hom}(A,B^{\Delta })$ equals the set of polynomial homotopy classes of morphisms from $A$ to $B$. We prove that ${\pi }_n\mathrm{hom}(A,B^{\Delta })$ is the set of homotopy classes of morphisms from $A$ to $B^{S_n}$, where $B^{S_n}$ is the ind-algebra of polynomials on the $n$-dimensional cube with coefficients in $B$ vanishing at the boundary of the cube. This is a generalization to arbitrary dimensions of a theorem of Cortiñas-Thom, which addresses the cases $n\le 1$. As an application we give a simplified proof of a theorem of Garkusha that computes the homotopy groups of his matrix-unstable algebraic KK-theory space in terms of polynomial homotopy classes of morphisms.

16 pages