Regularity of $f\,dt$ at the place of $K_\alpha$ over $t=\infty$, in terms of the chart at infinity

Let $m\ge1$ be an integer and $\alpha\in\mathbb C$ with $\alpha\ne\overline\alpha$, and put $h_\alpha(t)=-t(t^m+1)(\alpha t^m+\overline\alpha)\in\mathbb C[t]$. Let $K_\alpha=\mathbb C(t)[W]/(W^2-h_\alpha)$ be the function field of the double cover $w^2=h_\alpha(t)$, where $t$ and $w$ denote the classes of $t$ and $W$; in the setting of the note this is the function field of $V_\alpha$, read in the chart of equation (7) with $w=t(t^m+1)Y$. Write $\Omega_{K_\alpha/\mathbb C}$ for the $K_\alpha$-vector space of Kähler differentials of $K_\alpha$ over $\mathbb C$ and $dx$ for the differential of $x\in K_\alpha$.

For the place over $t=\infty$, put $h_{\overline\alpha}(s)=-s(s^m+1)(\overline\alpha s^m+\alpha)$ and let $K_{\overline\alpha}=\mathbb C(s)[W]/(W^2-h_{\overline\alpha})$ be the function field of the conjugate cover ${w'}^2=h_{\overline\alpha}(s)$, where $s$ and $w'$ denote the classes of $s$ and $W$. Since $h_{\overline\alpha}(0)=0$, the point $(s,w')=(0,0)$ lies on this cover; let $\mathcal O'_{(0,0)}\subset K_{\overline\alpha}$ be the localization of $\mathbb C[s][W]/(W^2-h_{\overline\alpha})$ at the maximal ideal of this point (the kernel of $s\mapsto0$, $w'\mapsto0$), a discrete valuation ring, whose uniformizers are the generators of its maximal ideal. Let $\varphi\colon K_\alpha\to K_{\overline\alpha}$ be the isomorphism of $\mathbb C$-algebras with $\varphi(t)=1/s$ and $\varphi(w)=w'\,s^{-(m+1)}$, the chart $s=1/t$, $w'=w\,s^{m+1}$ at infinity. The place of $K_\alpha$ over $t=\infty$ has valuation ring $\mathcal O_\infty=\varphi^{-1}(\mathcal O'_{(0,0)})$.

Differentials of $K_{\overline\alpha}$ are written in the same way, in $\Omega_{K_{\overline\alpha}/\mathbb C}$. A differential $\omega\in\Omega_{K_\alpha/\mathbb C}$ is regular at infinity if its transport $\varphi_{\ast}\omega\in\Omega_{K_{\overline\alpha}/\mathbb C}$, determined by $\varphi_{\ast}(g\,dx)=\varphi(g)\,d\varphi(x)$, is regular at $(0,0)$: for every uniformizer $u'$ of $\mathcal O'_{(0,0)}$ and every $g'\in K_{\overline\alpha}$ with $\varphi_{\ast}\omega=g'\,du'$ one has $g'\in\mathcal O'_{(0,0)}$.

Let $h_{\overline\alpha}'$ be the derivative of $h_{\overline\alpha}$. For every $f\in K_\alpha$,

The element on the right is the coefficient of the transported differential $\varphi_{\ast}(f\,dt)$ against the uniformizer $w'$ of $\mathcal O'_{(0,0)}$. This is the regularity criterion at the place over $t=\infty$, which is then simplified to the condition $\varphi(f)/{w'}^3\in\mathcal O'_{(0,0)}$; in this way it enters the formal proof of the genus clause of Lemma 4 of the note (the normalization of $V_\alpha$ has genus $m$) and, through the case $m=2$, of Remark 5.

Formalization Note: the hypotheses $m\ge1$ and $\alpha\ne\overline\alpha$ are Fact instances; the condition $|\alpha|=1$ of the note is not assumed. In the formal statement the coefficient appears as $\varphi(f)\cdot\bigl(s^2h_{\overline\alpha}'(s)\bigr)^{-1}\cdot(-2w')$.