1. Test sets and checking words for homomorphic equivalence;Culik;J. Comput. System Sci.,1980
2. Primitive words and languages associated to PCP;Lipponen;EATCS Bull.,1994
3. M. Lipponen, Post Correspondence Problem: words possible as primitive solutions, to appear.
4. M. Lipponen, On F-prime solutions of the Post Correspondence Problem, in preparation.
5. PCP-prime words and primality types;Mateescu;RAIRO Inform. Théor.,1993