The congruence of tangents to a system of curves on a surface
Compositio Mathematica, Tome 13 (1956-1958), pp. 247-256. 
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Kamalamma, K. N. The congruence of tangents to a system of curves on a surface. Compositio Mathematica, Tome 13 (1956-1958), pp. 247-256. http://www.numdam.org/item/CM_1956-1958__13__247_0/

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