Dimitry S. Ananichev Alessandra Cherubini Mikhail V. Volkov Image reducing words and subgroups of free groups. 77-92 2003 307 Theor. Comput. Sci. 1 http://dx.doi.org/10.1016/S0304-3975(03)00093-8 db/journals/tcs/tcs307.html#AnanichevCV03