Homotopy group

Homotopy groups are tools used in algebraic topology to classify topological spaces. The different ways to map an n-sphere continuously into a given topological space are divided into equivalence classes, called homotopy classes. The set of homotopy classes of maps of the n-sphere into a space may be endowed with a group structure by a means analogous to the concatenation operation used to construct the fundamental group; this group is usually denoted <math>\pi_n</math>. However, as long as <math>n \geq 2</math>, the homotopy groups <math>\pi_n(X)</math> are abelian groups.

Homotopy groups are notoriously difficult to compute, in contrast with homology and cohomology groups, where are generally computable: even the higher homotopy groups of spheres are not fully understood. For example, the group <math>\pi_3(S^2)</math> is isomorphic to the group of integers, generated by the Hopf fibration. Spectral sequences are an important tool in the computation of higher homotopy groups.