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Topological pair

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In mathematics, more specifically algebraic topology, a pair is shorthand for an inclusion of topological spaces . Sometimes is assumed to be a cofibration. A morphism from to is given by two maps and such that .

A pair of spaces is an ordered pair (X, A) where X is a topological space and A a subspace. The use of pairs of spaces is sometimes more convenient and technically superior to taking a quotient space of X by A. Pairs of spaces occur centrally in relative homology,[1] homology theory and cohomology theory, where chains in are made equivalent to 0, when considered as chains in .

Heuristically, one often thinks of a pair as being akin to the quotient space .

There is a functor from the category of topological spaces to the category of pairs of spaces, which sends a space to the pair .

A related concept is that of a triple (X, A, B), with BAX. Triples are used in homotopy theory. Often, for a pointed space with basepoint at x0, one writes the triple as (X, A, B, x0), where x0BAX.[1]

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