H-space

In mathematics, an H-space is a homotopy-theoretic version of a generalization of the notion of topological group, in which the axioms on associativity and inverses are removed.

Definition

An H-space consists of a topological space X, together with an element e of X and a continuous map μ : X × X → X, such that μ(e, e) = e and the maps x ↦ μ(x, e) and x ↦ μ(e, x) are both homotopic to the identity map through maps sending e to e. This may be thought of as a pointed topological space together with a continuous multiplication for which the basepoint is an identity element up to basepoint-preserving homotopy.

One says that a topological space X is an H-space if there exists e and μ such that the triple (X, e, μ) is an H-space as in the above definition. Alternatively, an H-space may be defined without requiring homotopies to fix the basepoint e, or by requiring e to be an exact identity, without any consideration of homotopy. In the case of a CW complex, all three of these definitions are in fact equivalent.

Examples and properties

The standard definition of the fundamental group, together with the fact that it is a group, can be rephrased as saying that the loop space of a pointed topological space has the structure of an H-group, as equipped with the standard operations of concatenation and inversion. Furthermore, a continuous basepoint-preserving map of pointed topological spaces induces an H-homomorphism of the corresponding loop spaces; this reflects the group homomorphism on fundamental groups induced by a continuous map.

It is straightforward to verify that, given a pointed homotopy equivalence from an H-space to a pointed topological space, there is a natural H-space structure on the latter space. As such, the existence of an H-space structure on a given space is only dependent on its pointed homotopy type.