Past exam of the mathematics course of the University of Cambridge / 2006 / iii / Paper 16 / 6 / b

Give the smooth curve its complex orientation. The cellular homology of complex projective space gives $H_2({\mathbb{CP}}^2;\mathbb{Z})=\mathbb{Z}$, generated by the class $[L]$ of a projective line. Thus $[C]=D[L]$ for a unique integer $D$, its homological degree of a smooth complex plane curve. Two distinct projective lines meet transversely in one point with positive sign, so $[L]\cdot [L]=1$. Therefore the degree $D$ is the intersection number $[C]\cdot [L]$ with any transverse projective line.

For the displayed curve, take $d\ge 1$ and put $F=x^d+y^d-z^d$. If $d\ge 2$, its three partial derivatives cannot vanish simultaneously at a projective point; if $d=1$, the equation defines a line and is also smooth. Intersect with $L=\{y=0\}$. Every intersection has $z\ne 0$, so in the chart $z=1$ the points areThere are exactly $d$ distinct points. At each, $\partial F/\partial x=d{\zeta }^{d-1}\ne 0$. In affine coordinates the curve is a graph $x=x(y)$, whereas the line is $y=0$; their tangent lines are transverse. For $d\ge 2$ the curve tangent at the point is the $y$-direction, while for $d=1$ it is the direction $dx+dy=0$, still transverse to $dy=0$.

Each intersection contributes $+1$, not merely an unsigned count: both tangent spaces are complex lines, and the real determinant of the complex-linear isomorphism from their direct sum to the ambient tangent space is the squared modulus of its complex determinant, hence positive. Therefore $[C]\cdot [L]=d$, andThis computes the degree by explicit transverse intersections, without using Bézout's theorem to assume the answer.