Semisimple Algebras and PI-Invariants of Finite Dimensional Algebras

Abstract

Let $\Gamma$ be a $T$-ideal of identities of an affine PI-algebra over an algebraically closed field $F$ of characteristic zero. Consider the family $\mathcal{M}_{\Gamma}$ of finite dimensional algebras $\Sigma$ with $Id(\Sigma) = \Gamma$. By Kemer's theory it is known that such $\Sigma$ exists. We show there exists a semisimple algebra $U$ which satisfies the following conditions. $(1)$ There exists an algebra $A \in \mathcal{M}_{\Gamma}$ with Wedderburn-Malcev decomposition $A \cong U \oplus J_{A}$, where $J_{A}$ is the Jacobson's radical of $A$ $(2)$ If $B \in \mathcal{M}_{\Gamma}$ and $B \cong B_{ss} \oplus J_{B}$ is its Wedderburn-Malcev decomposition then $U$ is a direct summand of $B_{ss}$. We refer to $U$ as the unique minimal semisimple algebra corresponding to $\Gamma$. We fully extend this result to the non-affine $G$-graded setting where $G$ is a finite group. In particular we show that if $A$ and $B$ are finite dimensional $G_{2}:= \mathbb{Z}_{2} \times G$-graded simple algebras then they are $G_{2}$-graded isomorphic if and only if $E(A)$ and $E(B)$ are $G$-graded PI-equivalent, where $E$ is the unital infinite dimensional Grassmann algebra and $E(A)$ is the Grassmann envelope of $A$.