Simply branched covers of curves and wild conductor exponents

Abstract

We use deformation theory to show that a cover of smooth, projective, geometrically connected curves $\pi\colon C\to D$ over a $p$-adic field can be $p$-adically perturbed to obtain a nearby simply branched cover $\pi'\colon C'\to D$ and that, if $D=\mathbb{P}^1$ and $\pi^*\mathcal{O}_{\mathbb{P}^1}(1)$ is very ample, then we may take $C'=C$. As an application, we give a formula for the wild conductor exponent of a curve at a prime $p>d$ in terms of the ramification data of any degree $d$ cover $C\to\mathbb{P}^1$.