[B2] T.F. Banchoff, Triple points and singularities of projections of smoothly immersed surfaces, Proc. Amer. Math. Soc. 46 (1974) 402-406.The author proves that the number of triple points in an immersion of a smooth surface is congruent modulo 2 to the Euler characteristic of the surface, and that the homology class of the double set of an immersion is a topological invariant. He gives a geometric interpretation of Whitney duality.
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