Intermediate flexibility of surfaces
Compositio Mathematica, Volume 25 (1972) no. 1, p. 71-78
@article{CM_1972__25_1_71_0,
     author = {Helfenstein, H. G. and Katz, E.},
     title = {Intermediate flexibility of surfaces},
     journal = {Compositio Mathematica},
     publisher = {Wolters-Noordhoff Publishing},
     volume = {25},
     number = {1},
     year = {1972},
     pages = {71-78},
     zbl = {0237.53047},
     mrnumber = {320962},
     language = {en},
     url = {http://www.numdam.org/item/CM_1972__25_1_71_0}
}
Helfenstein, H. G.; Katz, E. Intermediate flexibility of surfaces. Compositio Mathematica, Volume 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.