Let K be a simplicial complex, and suppose that s is a simplex in K. We say that s has a free face t if t is a face of s and t has no other cofaces. We call (s, t) a free pair. If we remove s and t from K, we obtain another simplicial complex, which we call an elementary collapse of K. A sequence of elementary collapses is called a collapse. A simplicial complex that has a collapse to a point, implying all other points were in free pairs, is called collapsible.
This definition can be extended to CW-complexes and is the basis for the concept of simple-homotopy equivalence.
Read more about this topic: Collapse (topology)
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