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Homotopy theory begins with the homotopy groups πn (X) , which are the natural higher-dimensional analogs of the fundamental group. These higher homotopy groups have certain formal similarities with homology groups. For example, πn (X) turns out to be always abelian for n ≥ 2 , and there are relative homotopy groups fitting into a long exact sequence just like the long exact sequence of homology groups. |