no. 1
Intermediate flexibility of surfaces
Compositio Mathematica, Tome 25 (1972) no. 1, pp. 71-78. 
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     author = {Helfenstein, H. G. and Katz, E.},
     title = {Intermediate flexibility of surfaces},
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     number = {1},
     year = {1972},
     mrnumber = {320962},
     zbl = {0237.53047},
     language = {en},
     url = {http://archive.numdam.org/item/CM_1972__25_1_71_0/}
}
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Helfenstein, H. G.; Katz, E. Intermediate flexibility of surfaces. Compositio Mathematica, Tome 25 (1972) no. 1, pp. 71-78. http://archive.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.