An algebraic characterization of multiplicative semigroups of real valued functions. T. Head — 1977 Semigroup forum
Quotient monoids and greatest commutative monoid images of several types. T. Head — 1979 Semigroup forum
Retracts of free monoids are nowhere dense with respect to finite group topologies and p-adic topologies. T. Head; W. Forys — 1991 Semigroup forum