Collins & Zieschang 1993, p. 149. Collins, D. J.; Zieschang, H. (1993), "Combinatorial group theory and fundamental groups", Algebra VIII: Combinatorial Group Theory, Applications to Geometry, Encyclopaedia of Mathematical Sciences, vol. 58, Berlin: Springer, pp. 1–166, doi:10.1007/978-3-642-58013-0, ISBN3-540-54700-2, MR1265270
Collins & Zieschang 1993, Cor. 7.2.6. Collins, D. J.; Zieschang, H. (1993), "Combinatorial group theory and fundamental groups", Algebra VIII: Combinatorial Group Theory, Applications to Geometry, Encyclopaedia of Mathematical Sciences, vol. 58, Berlin: Springer, pp. 1–166, doi:10.1007/978-3-642-58013-0, ISBN3-540-54700-2, MR1265270
Collins 1969. Collins, Donald J. (1969), "Word and conjugacy problems in groups with only a few defining relations", Zeitschrift für Mathematische Logik und Grundlagen der Mathematik, 15 (20–22): 305–324, doi:10.1002/malq.19690152001, MR0263903
Borisov 1969. Borisov, V. V. (1969), "Simple examples of groups with unsolvable word problem", Akademiya Nauk SSSR. Matematicheskie Zametki, 6: 521–532, ISSN0025-567X, MR0260851
Knuth, D.; Bendix, P. (2014) [1970], "Simple word problems in universal algebras", in Leech, J. (ed.), Computational Problems in Abstract Algebra: Proceedings of a Conference Held at Oxford Under the Auspices of the Science Research Council Atlas Computer Laboratory, 29th August to 2nd September 1967, Springer, pp. 263–297, ISBN9781483159423
Collins & Zieschang 1993, p. 149. Collins, D. J.; Zieschang, H. (1993), "Combinatorial group theory and fundamental groups", Algebra VIII: Combinatorial Group Theory, Applications to Geometry, Encyclopaedia of Mathematical Sciences, vol. 58, Berlin: Springer, pp. 1–166, doi:10.1007/978-3-642-58013-0, ISBN3-540-54700-2, MR1265270
Collins & Zieschang 1993, Cor. 7.2.6. Collins, D. J.; Zieschang, H. (1993), "Combinatorial group theory and fundamental groups", Algebra VIII: Combinatorial Group Theory, Applications to Geometry, Encyclopaedia of Mathematical Sciences, vol. 58, Berlin: Springer, pp. 1–166, doi:10.1007/978-3-642-58013-0, ISBN3-540-54700-2, MR1265270
Collins 1969. Collins, Donald J. (1969), "Word and conjugacy problems in groups with only a few defining relations", Zeitschrift für Mathematische Logik und Grundlagen der Mathematik, 15 (20–22): 305–324, doi:10.1002/malq.19690152001, MR0263903
Borisov 1969. Borisov, V. V. (1969), "Simple examples of groups with unsolvable word problem", Akademiya Nauk SSSR. Matematicheskie Zametki, 6: 521–532, ISSN0025-567X, MR0260851