1. 1. S. Lefschetz, Algebraic topology, Princeton, 1942.
2. Annals of Mathematics Studies, No. 10;Lefschetz, Solomon,1942
3. 3. K. Borsuk, Fund. Math. vol. 24 (1935) pp. 249-258.
4. 4. Sze-Tsen Hu, C. R. Acad. Sci. URSS. vol. 57 (1947), pp. 231-234.
5. Princeton Mathematical Series, vol. 4;Hurewicz, Witold,1941