Two-element factors and finite bases for additively idempotent semirings

Abstract

For every additively idempotent semiring $S$ in the constant-free signature, $S$ is finitely based if and only if $S\times E$ is finitely based when $E=M_2$ or $T_2$. For each $E\in\{L_2,R_2,D_2,N_2\}$, both implications fail in general: we give a finitely based semiring whose product with $E$ is nonfinitely based and a nonfinitely based semiring whose product with $E$ is finitely based.