relative homotopy group

《Algebraic Topology》 · 第 122 页 · Section 6.1

For a pointed pair \((X,A,*)\), the relative homotopy set \(\pi_n(X,A,*)\) is represented by maps \((I^n,J^{n-1},0)\to(X,A,*)\), and it is a group for \(n\ge2\).