The weak commutativity construction for Lie algebras

Abstract: We study the analogue of Sidki's weak commutativity construction, defined originally for groups, in the category of Lie algebras. This is the quotient $\chi(\mathfrak{g})$ of the Lie algebra freely generated by two isomorphic copies $\mathfrak{g}$ and $\mathfrak{g}^{\psi}$ of a fixed Lie algebra by the ideal generated by the brackets $[x,x^{\psi}]$, for all $x$. We exhibit an abelian ideal of $\chi(\mathfrak{g})$ whose associated quotient is a subdirect sum in $\mathfrak{g} \oplus \mathfrak{g} \oplus \mathfrak{g}$ and we give conditions for this ideal to be finite dimensional. We show that $\chi(\mathfrak{g})$ has a subquotient that is isomorphic to the Schur multiplier of $\mathfrak{g}$. We prove that $\chi(\mathfrak{g})$ is finitely presentable or of homological type $FP_2$ if and only if $\mathfrak{g}$ has the same property, but $\chi(\mathfrak{f})$ is not of type $FP_3$ if $\mathfrak{f}$ is a non-abelian free Lie algebra.