A topological proof of Grushko's theorem on free products

Grushko, I.: On the bases of a free product of groups. Mat. Sbornik8, 169?182 (1940).

Higgins, P.J.: Presentations of groupoids, with applications to groups. Proc. Cambridge Philos. Soc.60, 7?20 (1964).

Kneser, H.: Geschlossene Flächen in dreidimensionalen Mannigfaltigkeiten. Jahresb. Deutsch. Math.-Ver.38, 248?260 (1929).

Kurosh, A.G.: The Theory of Groups, vol. II. New York: Chelsea 1956.

Papakyriakopoulos, C.D.: Topology of 3-dimensional manifolds (to appear).

Stallings, J.R.: Some topological proofs and extensions of Gru?ko's theorem. Thesis Princeton University 1959.

Stallings, J.R.: Manifolds whose fundamental groups are free products (to appear).

Kampen, E.R. van: On the connection between the fundamental groups of some related spaces. Amer. J. Math.55, 261?267 (1933).

Wagner, D.H.: On free products of groups. Trans. Amer. Math. Soc.84, 352?378 (1957).