Singular homology

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In algebraic topology, singular homology refers to the study of a certain set of algebraic invariants of a topological space ${\displaystyle X}$, the so-called homology groups ${\displaystyle H_{n}(X).}$ Intuitively, singular homology counts, for each dimension ${\displaystyle n}$, the ${\displaystyle n}$-dimensional holes of a space. Singular homology is a particular example of a homology theory, which has now grown to be a rather broad collection of theories. Of the various theories, it is perhaps one of the simpler ones to understand, being built on fairly concrete constructions (see also the related theory simplicial homology).

In brief, singular homology is constructed by taking maps of the standard n-simplex to a topological space, and composing them into formal sums, called singular chains. The boundary operation – mapping each ${\displaystyle n}$-dimensional simplex to its ${\displaystyle (n-1)}$-dimensional boundary – induces the singular chain complex. The singular homology is then the homology of the chain complex. The resulting homology groups are the same for all homotopy equivalent spaces, which is the reason for their study. These constructions can be applied to all topological spaces, and so singular homology is expressible as a functor from the category of topological spaces to the category of graded abelian groups.