A reduction of Semigroup DLP to classic DLP

We present a polynomial-time reduction of the discrete logarithm problem (DLP) in any periodic (or torsion) semigroup (Semigroup DLP) to the classic DLP in a subgroup of the same semigroup. It follows that Semigroup DLP can be solved in polynomial time by quantum computers, and that Semigroup DLP has subexponential complexity whenever the classic DLP in the corresponding groups has subexponential complexity. We also consider several natural constructions of nonperiodic semigroups, and provide polynomial time solutions for the DLP in these semigroups.