Let ${\displaystyle K}$  be an abstract simplicial complex.

Suppose that ${\displaystyle \tau ,\sigma }$  are two simplices of ${\displaystyle K}$  such that the following two conditions are satisfied:

1. ${\displaystyle \tau \subseteq \sigma ,}$  in particular ${\displaystyle \dim \tau <\dim \sigma ;}$ 
2. ${\displaystyle \sigma }$  is a maximal face of ${\displaystyle K}$  and no other maximal face of ${\displaystyle K}$  contains ${\displaystyle \tau ,}$ 

then ${\displaystyle \tau }$  is called a free face.

A simplicial collapse of ${\displaystyle K}$  is the removal of all simplices ${\displaystyle \gamma }$  such that ${\displaystyle \tau \subseteq \gamma \subseteq \sigma ,}$  where ${\displaystyle \tau }$  is a free face. If additionally we have ${\displaystyle \dim \tau =\dim \sigma -1,}$  then this is called an elementary collapse.

A simplicial complex that has a sequence of collapses leading to a point is called collapsible. Every collapsible complex is contractible, but the converse is not true.

This definition can be extended to CW-complexes and is the basis for the concept of simple-homotopy equivalence.^[3]

• Complexes that do not have a free face cannot be collapsible. Two such interesting examples are R. H. Bing's house with two rooms and Christopher Zeeman's dunce hat; they are contractible (homotopy equivalent to a point), but not collapsible.
• Any n-dimensional PL manifold that is collapsible is in fact piecewise-linearly isomorphic to an n-ball.^[1]

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