1. Blondel V.D. and Canterini V., Undecidable problems for probabilistic automata of fixed dimension.Theor. Comput. Syst.36(2003) 231–245.
2. Ehrenfeucht A., Karhumäki J. and Rozenberg G., The (generalized) Post Correspondence Problem with lists consisting of two words is decidable.Theoret. Comput. Sci.21(1982) 119–144.
3. Halava V. and Harju T., Undecibability of infinite Post Correspondence Problem for instances of size 9.RAIRO–Theor. Inf. Appl.40(2006) 551–557.
4. Halava V., Harju T. and Hirvensalo M., Binary (generalized) Post Correspondence Problem.Theoret. Comput. Sci.276(2002) 183–204.
5. Matiyasevich Y. and Sénizergues G., Decision problems for semi-Thue systems with a few rules.Theoret. Comput. Sci.330(2005) 145–169.