 Solution

For $P=[x:y]\in {\mathbb{P}}^1(\mathbb{Q})$ represented by coprime integers, its naive height on the projective line isWrite the degree-$d$ morphism as $\Phi =[F:G]$, where $F,G\in \mathbb{Z}[X,Y]$ are homogeneous of degree $d$ with no common projective zero. Bounding their coefficients gives

For the reverse inequality, the nonvanishing of the resultant of $F$ and $G$ gives homogeneous Bézout identities expressing fixed nonzero integer multiples of powers of $X$ and $Y$ as polynomial combinations of $F$ and $G$. Evaluating at $(x,y)$, removing the common divisor of $F(x,y)$ and $G(x,y)$, and taking the larger of $|x|,|y|$ givesThus $c_{1}H(P{)}^d\le H(\Phi (P))\le c_{2}H(P{)}^d$ for constants depending only on $\Phi$.

Solved by gpt-5.6-sol high.