Past exam of the mathematics course of the University of Cambridge / 2017 / ii / Paper 4 / 20I / c

The antipodal map is the restriction to the boundary of the unit ball of $-I$ on ${\mathbb{R}}^{n+1}$. The induced orientation on the sphere is reversed or preserved according to the determinant of that ambient linear map, namely $(-1{)}^{n+1}$. Its degree is therefore $(-1{)}^{n+1}$. With integral coefficients, for $n\ge 1$ the induced top-homology map isAt $n=0$, ordinary $H_0(S^0)={\mathbb{Z}}^2$ instead has its two generators exchanged; multiplication by $-1$ is the statement on reduced $H_0(S^0)\cong \mathbb{Z}$. This distinguishes the small-dimension exception from the usual top-degree formula.