Undecidability of a simple fragment of a positive theory with a single constant for a free semigroup of rank two (Q1582961)

This paper is a note about the algorithmic nature of some simple fragments of positive theories with few constants for noncyclic semigroups. The authors prove 3 theorems and 4 corollaries on the undecidability of the positive theory with the constants \(a_1, a_2,\dots,a_{n-1}\) for the free semigroup of rank \(n\).