Free involutive Hom-semigroups and Hom-associative algebras
  In this paper we construct free Hom-semigroups when its unary operation is multiplicative and is an involution. Our method of construction is by bracketed words. As a consequence, we obtain free Hom-associative algebras generated by a set under the same conditions for the unary operation. 
Li Guo and Shanghua Zheng
math.RA math.GR