Minimal Presentations and Efficiency of Semigroups

S

is said to be efficient if it can be defined by a presentation (A | R) with |R| -|A|=rank(H ₂(S)). In this paper we demonstrate certain infinite classes of both efficient and inefficient semigroups. Thus, finite abelian groups, dihedral groups D _{2 n} with n even, and finite rectangular bands are efficient semigroups. By way of contrast we show that finite zero semigroups and free semilattices are never efficient. These results are compared with some well-known results on the efficiency of groups.