Stratified Morse theory
Mark Goresky, Robert A. Macpherson · Proceedings of symposia in pure mathematics · 1983
Suppose that X is a topological space, f is a real valued function on X, and c is a real number. Then we will denote by X ≤c the subspace of points x in X such that f(x)≤c. The fundamental problem of Morse theory is to study the topological changes in the space X ≤c as the number c varies.