Algebraic Topology
《Algebraic Topology》 · 第 427 页 · Section 4.B
For \(f:S^{2n-1}\to S^n\), if \(\alpha\in H^n(C_f)\) and \(\beta\in H^{2n}(C_f)\) are generators, the integer \(H(f)\) in \(\alpha^2=H(f)\beta\) is called the Hopf invariant of \(f\).