• [1] S. Cohn-Vossen, Zwei Sätze über die Starrheit der Einflächen, Nachr. Ges. Wiss Göttingen Math.-Phys. Kl. (1927) pp. 125-134.
• [2] H. Hopf, Lectures on differential geometry in the large, University of Michigan.
• [3] E. Hopf, Elementare Bemerkungen über die Lösungen partieller Differential-gleichungen Zweiter Ordnung vom elliptischen Typus, Sitzgsber. Preuss. Akad. vol. 19 (1927) pp. 147-152.
• [4] S. Bochner, Vector fields and Ricci curvature, Bull. Amer. Math. Soc. 52 (1946), 776–797. MR 0018022 (8,230a), http://dx.doi.org/10.1090/S0002-9904-1946-08647-4
• [5] Shiing-shen Chern, An elementary proof of the existence of isothermal parameters on a surface, Proc. Amer. Math. Soc. 6 (1955), 771–782. MR 0074856 (17,657g), http://dx.doi.org/10.1090/S0002-9939-1955-0074856-1
• [6] A. D. Alexandrov, On a class of closed surfaces, Mat. Sb. vol. 4 (1938) pp. 69-77 (Russian, with German abstract).

Bibliography

[1]

S. Cohn-Vossen, Zwei Sätze über die Starrheit der Einflächen, Nachr. Ges. Wiss Göttingen Math.-Phys. Kl. (1927) pp. 125-134.

[2]

H. Hopf, Lectures on differential geometry in the large, University of Michigan.

[3]

E. Hopf, Elementare Bemerkungen über die Lösungen partieller Differential-gleichungen Zweiter Ordnung vom elliptischen Typus, Sitzgsber. Preuss. Akad. vol. 19 (1927) pp. 147-152.

[4]

S. Bochner, Vector fields and Ricci curvature, Bull. Amer. Math. Soc. vol. 52 (1946) pp. 776-797. MR 0018022 (8:230a)

[5]

S. S. Chern, An elementary proof of the existence of isothermal parameters on a surface, Proc. Amer. Math. Soc. vol. 6 (1955) pp. 771-782. MR 0074856 (17:657g)

[6]

A. D. Alexandrov, On a class of closed surfaces, Mat. Sb. vol. 4 (1938) pp. 69-77 (Russian, with German abstract).