1. G.Baumslag, J. L.Dyer and C. F.Miller, Homology of finitely presented groups. Topology (to appear).
2. R. C.Lyndon and P. E.Schupp, Combinatorial Group Theory. Berlin-Heidelberg-New York 1977.
3. H.Neumann, Varieties of Groups. New York 1967.
4. M. O. Rabin, Computable algebra, general theory and theory of computable fields. Trans. Amer. Math. Soc.95, 341?360 (1960).