Unlike the wreath product, the block product is not associative at the level of varieties. All decomposition theorems involving block products, such as the bilateral version of Krohn–Rhodes' theorem, have always assumed a right-to-left bracketing of the operands. We consider here the left-to-right bracketing, which is generally weaker. More precisely, we are interested in characterizing for any pseudovarieties of monoids U, V the smallest pseudovariety W that contains U and such that W □ V = W. This allows us to obtain new decomposition results for a number of important varieties such as DA, DO and DA * G. We apply these results to characterize the regular languages definable with generalized first-order sentences using only two variables and to shed new light on recent results on regular languages recognized by bounded-depth circuits with a linear number of wires and regular languages with small communication complexity.
On the Non-deterministic Communication Complexity of Regular Languages