(Krieger 2006) p. 287 Krieger, Dalia (2006). "On critical exponents in fixed points of non-erasing morphisms". In Ibarra, Oscar H.; Dang, Zhe (eds.). Developments in language theory : 10th international conference, DLT 2006, Santa Barbara, CA, USA, June 26-29, 2006 : proceedings. Berlin: Springer. pp. 280–291. ISBN978-3-540-35430-7. OCLC262693179.
Dăscălescu, Sorin; Năstăsescu, Constantin; Raianu, Șerban (2001). Hopf Algebra: An Introduction. Pure and Applied Mathematics. Vol. 235. New York, NY: Marcel Dekker. p. 363. ISBN0-8247-0481-9. Zbl0962.16026.