Hurewicz 1935 πn(X) abelian for n ≥ 2. Hurewicz theorem π_n(X) → H_n(X). Hard to compute (πn(S^k) for n > k mostly unknown). Stable-homotopy stabilisation. Bridges to spectra.
Hurewicz 1935 πn(X) abelian for n ≥ 2. Hurewicz theorem π_n(X) → H_n(X). Hard to compute (πn(S^k) for n > k mostly unknown). Stable-homotopy stabilisation. Bridges to spectra.