1. S. I. Adyan, On the embeddability of semigroups in groups, DAN SSSR(2) 133 (1960), 255–257.
2. , S. I. AdyanDefining relations and algorithmic problems for groups and semigroups, Trudy Mat.Inst. AN SSSR 85 (1966).
3. J. Alonso, Inégalités isopérimetriques et quasi-isométries, C. R. Acad. Sci. Paris Série 1, 331 (1991), 761–764.
4. W. A. Bogley and S. J. Pride, Aspherical relative presentations, Proc. Edin. Math. Soc. 35 (1992), 1–39.
5. , W. A. Bogley and S. J. PrideCalculating generators of Π2, Two-Dimensional Homotopy and Combinatorial Group Theory (C. Hog-Angeloni, W. Metzler and A. Sieradski, eds.), London Mathematical Society Lecture Notes Series, no. 197, Cambridge University Press, 1993, pp. 157–188.