Answering an open problem in papers by Marcus and Pǎun (1994), we give here two examples of infinite (nonalmost periodic) words ω such that the set of subwords appearing infinitely many times in ω is context-free nonregular (in fact, one language is linear nonregular and one is context-free nonlinear).
Authors
Ilie L
Journal
Discrete Applied Mathematics, Vol. 63, No. 3, pp. 277–279