In mathematics, particularly algebraic topology, a pair of spaces is an ordered pair (X, A) where X is a topological space and A a subspace (with the subspace topology).
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.
A related concept is that of a triple (X, A, B), with B ⊂ A ⊂ X. 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 x0 ∈ B ⊂ A ⊂ X.
Famous quotes containing the words pair of, pair and/or spaces:
“To get a man soundly saved it is not enough to put on him a pair of new breeches, to give him regular work, or even to give him a University education. These things are all outside a man, and if the inside remains unchanged you have wasted your labour. You must in some way or other graft upon the man’s nature a new nature, which has in it the element of the Divine.”
—William Booth (1829–1912)
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—Alfred Gingold, U.S. humorist. Items From Our Catalogue, “Tofu Innersoles,” Avon Books (1982)
“We should read history as little critically as we consider the landscape, and be more interested by the atmospheric tints and various lights and shades which the intervening spaces create than by its groundwork and composition.”
—Henry David Thoreau (1817–1862)