Whitehead theorem

In homotopy theory (a branch of mathematics), the Whitehead theorem states that if a continuous mapping f between CW complexes X and Y induces isomorphisms on all homotopy groups, then f is a homotopy equivalence. This result was proved by J. H. C. Whitehead in two landmark papers from 1949, and provides a justification for working with the concept of a CW complex that he introduced there. It is a model result of algebraic topology, in which the behavior of certain algebraic invariants (in this case, homotopy groups) determines a topological property of a mapping. In more detail, let X and Y be topological spaces. Given a continuous mapping and a point x in X, consider for any n ≥ 1 the induced homomorphism where πn(X,x) denotes the n-th homotopy group of X with base point x. (For n = 0, π0(X) just means the set of path components of X.) A map f is a weak homotopy equivalence if the function is bijective, and the homomorphisms f* are bijective for all x in X and all n ≥ 1. (For X and Y path-connected, the first condition is automatic, and it suffices to state the second condition for a single point x in X.) The Whitehead theorem states that a weak homotopy equivalence from one CW complex to another is a homotopy equivalence. (That is, the map f: X → Y has a homotopy inverse g: Y → X, which is not at all clear from the assumptions.) This implies the same conclusion for spaces X and Y that are homotopy equivalent to CW complexes. Combining this with the Hurewicz theorem yields a useful corollary: a continuous map between simply connected CW complexes that induces an isomorphism on all integral homology groups is a homotopy equivalence. A word of caution: it is not enough to assume πn(X) is isomorphic to πn(Y) for each n in order to conclude that X and Y are homotopy equivalent. One really needs a map f : X → Y inducing an isomorphism on homotopy groups. For instance, take X= S2 × RP3 and Y= RP2 × S3. Then X and Y have the same fundamental group, namely the cyclic group Z/2, and the same universal cover, namely S2 × S3; thus, they have isomorphic homotopy groups.