# Cofibration

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A triple , where are topological spaces and is an imbedding with the following property, known as the homotopy extension property with respect to polyhedra: For any polyhedron , any mapping and any homotopy with there exists a homotopy such that where If this property holds with respect to any topological space, then the cofibration is known as a Borsuk pair (in fact, the term "cofibration" is sometimes also used in the sense of "Borsuk pair" ). The space is called the cofibre of . The mapping cylinder construction converts any continuous mapping into a cofibration and makes it possible to construct a sequence of topological spaces in which ( is the suspension of ) is the cofibre of the mapping — being converted into a cofibration, is the cofibre of the mapping , etc. If is a cofibration of pointed spaces, then for any pointed polyhedron the induced sequence is an exact sequence of pointed sets; all terms of this sequence, from the fourth onward, are groups, and from the seventh onward — Abelian groups.

How to Cite This Entry:
Cofibration. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Cofibration&oldid=13585
This article was adapted from an original article by A.F. Kharshiladze (originator), which appeared in Encyclopedia of Mathematics - ISBN 1402006098. See original article