Mojmír Kretínský Vojtech Rehák Jan Strejcek Refining the Undecidability Border of Weak Bisimilarity. 17-36 2006 149 Electr. Notes Theor. Comput. Sci. 1 http://dx.doi.org/10.1016/j.entcs.2005.11.014 db/journals/entcs/entcs149.html#KretinskyRS06