Tensor products of Lie nilpotent associative algebras and applications to codimension sequences

Abstract

Let $G$ and $H$ be Lie nilpotent associative algebras over a field $K$ such that in addition $H$ satisfies the identity $[x_1, x_2] \cdots [x_{2k-1}, x_{2k}]=0$ for some $k \geq 2$. In this paper, extending results of Deryabina and Krasilnikov, we show that the tensor product $G \otimes H$ is again a Lie nilpotent associative algebra. Moreover, we give a lower and an upper bound on the minimal value of $q$ for which $[x_1, \dots, x_{q+1}] = 0$ is an identity for $G\otimes H$. In the case when $H$ satisfies the identities $[x_1, x_2, x_3] = 0$ and $[x_1, x_2][x_3, x_4] = 0$ and $\operatorname{char}K \neq 3$, we determine a better upper bound for $q$, which in many cases is equal to the minimal index of Lie nilpotency for $G\otimes H$. As a corollary, we reprove a result of Drensky saying that any product of Grassmann algebras of the form $E\otimes E_{i_1}\otimes \cdots \otimes E_{i_s}$ or $E_{j_1} \otimes E_{j_2} \otimes \cdots \otimes E_{j_t}$, where $E$ denotes the Grassmann algebra over a countable dimensional vector space and $E_r$ denotes the Grassmann algebra over an $r$-dimensional vector space, satisfies an identity of the form $[x_1, \dots, x_{q+1}] = 0$. We also provide several particular cases in which the minimal value of $q$ can be explicitly computed. As an application, we consider a field of characteristic zero, the variety $\mathfrak{N}_p$ of Lie nilpotent associative algebras of index at most $p$ and the corresponding relatively free algebras of finite rank, $F_n(\mathfrak{N}_p)$. We exhibit many explicit irreducible $S_n$-modules in the $S_n$-module decomposition of the space of proper multilinear polynomials of degree $n$ in $F_n(\mathfrak{N}_p)$ for any $p$. This gives a lower bound for the dimensions of the spaces of multilinear and proper multilinear polynomials of degree $n$ in $F_n(\mathfrak{N}_p)$.