Poincaré-Hopf theorem

Let $M$ be a smooth compact manifold with boundary $W=\partial M$, and let $X$ be a vector field on $M$ with isolated zeros such that $X$ points outwards at all points in the boundary $W$.

Then the sum of the indices of the zeros of $V$ (see Singular point, index of a) is equal to the Euler characteristic of $M$.

This is the generalization proved by H. Hopf, in 1926, of the two-dimensional version owed to H. Poincaré (1881, 1885) (see Poincaré theorem).

References

[a1]	J.W. Milnor, "Topology from the differentiable viewpoint" , Univ. Virginia Press (1965) pp. 35
[a2]	N.G. Lloyd, "Degree theory" , Cambridge Univ. Press (1978) pp. 33