“…Cayley graphs of semigroups are closely related to automata theory, as explained in the monograph [37] and papers [35,39,45]. This is a very large topic, and so without trying to be complete we indicate only a few pertinent papers [20,34,38,58,59,61,62,63,64,65,66,67,68,71,72,80,81] just to illustrate.…”