Aldo de Luca Amy Glen Luca Q. Zamboni Rich, Sturmian, and trapezoidal words. 569-573 2008 407 Theor. Comput. Sci. 1-3 http://dx.doi.org/10.1016/j.tcs.2008.06.009 db/journals/tcs/tcs407.html#LucaGZ08