References
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- H. Hopf, Uber die Curvatura integra geschlossener Hyperflachen, Mathematische Annalen 95 (1925-1926), 340-365. https://doi.org/10.1007/BF01206615
- J. Milnor, Morse Theory, Princeton University Press, 1963.
- J. Milnor, Lectures on the h-cobordism Theorem, Princeton Press, 1965.
- T. Sakkalis, On a theorem of H. Hopf, Internat. J. Math. Math. Sci. 13 (1990), no. 4, 813-816. https://doi.org/10.1155/S0161171290001132