Let M be a compact manifold and f a smooth function with isolated critical points. If Q is a quadratic form of signature (p,q), I define sig(Q)=p−q. I define χ(f) as:
χ(f)=∑x,df(x)=0sig(Hess(f)(x))
where Hess(f)(x) is the Hessian of f at the point x.
Then we have, ∀f:
χ(f)=Cst=χ(M)
Have we defined a topological invariant of the manifold?