Intermediate flexibility of surfaces
Compositio Mathematica, Tome 25 (1972) no. 1, pp. 71-78. 
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     title = {Intermediate flexibility of surfaces},
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Helfenstein, H. G.; Katz, E. Intermediate flexibility of surfaces. Compositio Mathematica, Tome 25 (1972) no. 1, pp. 71-78. http://www.numdam.org/item/CM_1972__25_1_71_0/

N.W. Efimov [1] Flächenverbiegung im Grossen, Berlin 1957.

T. Rado [2] The mathematical theory of rigid surfaces, Lecture notes, University of North Carolina, 1954.