Some Combinatorial Properties of Free Semigroups
Ki Hang Kim, Mohan S. Putcha, F.W. Roush · Journal of the London Mathematical Society · 1977
Many problems of a combinatorial nature can be formulated about free semigroups. We study two in particular: 1. How many words of length n are not powers of words of shorter length? 2. Let P be a finite set of words. How many words of length n do not contain any word in P as a segment?