1. 1. H. Kneser, Geschlossene Flachen in dreidimensionalen Munigfaltigkeiten, Jbhr. Deutsch. Math. Verein. vol. 38 (1929) pp. 248-260.
2. 2. A. G. Kurosch, Gruppentheorie, Berlin (1953; translated from Russian).
3. Affine structures in 3-manifolds. V. The triangulation theorem and Hauptvermutung;Moise, Edwin E.;Ann. of Math. (2),1952
4. On the number of generators of a free product;Neumann, B. H.;J. London Math. Soc.,1943
5. On Dehn’s lemma and the asphericity of knots;Papakyriakopoulos, C. D.;Ann. of Math. (2),1957