?url_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.au=+Petrich+Mario&rft.aufirst=Mario&rft.aulast=Petrich&rft.issue=3-4&rft.atitle=Semigroups+of+periodic+endomorphisms+of+a+free+monoid&rft.title=Acta+scientiarum+mathematicarum&rft.volume=59&rft.pages=357-380&rft.issn=0001-6969&rft.date=1994&rft.genre=article