On periodic points of free inverse monoid endomorphisms
  It is proved that the periodic point submonoid of a free inverse monoid endomorphism is always finitely generated. Using Chomsky's hierarchy of languages, we prove that the fixed point submonoid of an endomorphism of a free inverse monoid can be represented by a context-sensitive language but, in general, it cannot be represented by a context-free language. 
Emanuele Rodaro, Pedro V. Silva
math.GR cs.FL