Vesa Halava Jarkko Kari Yuri Matiyasevich On post correspondence problem for letter monotonic languages. 2957-2960 2009 410 Theor. Comput. Sci. 30-32 http://dx.doi.org/10.1016/j.tcs.2009.01.040 db/journals/tcs/tcs410.html#HalavaKM09