1. The homotopy groups of a triad;Blakers;(I) Ann. Math.,1951
2. Structures simpliciales;Cartier;Sem. Bourbaki,1959
3. On the homotopy groups of unions of spaces;Hilton;Comment. Math. Helv.,1955
4. On the connection between the fundamental groups of some connected spaces;van Kampen;Amer. J. Math.,1933
5. A combinatorial definition of homotopy groups;Kan;Ann. Math.,1958