Maurice Margenstern Kenichi Morita NP problems are tractable in the space of cellular automata in the hyperbolic plane. 99-128 2001 259 Theor. Comput. Sci. 1-2 db/journals/tcs/tcs259.html#MargensternM01 http://dx.doi.org/10.1016/S0304-3975(99)00328-X