The University of Arizona
Please note that this event has ended!

Newton Polygons of Abelian $L$-Functions on Curves

Algebra and Number Theory Seminar

Newton Polygons of Abelian $L$-Functions on Curves
Series: Algebra and Number Theory Seminar
Location: ENR2-S395
Presenter: James Upton, UC San Diego

Let $U$ be a smooth, affine curve over a finite field of characteristic $p > 2$. Let $\rho:\pi_1(U) \to \mathbb{C}^\times$ be a character of order $p^n$. If $\rho\neq 1$, then the Artin $L$-function $L(\rho,s)$ is a polynomial, and a theorem of Kramer-Miller states that its $p$-adic Newton polygon $\mathrm{NP}(\rho)$ is bounded below by a certain Hodge polygon $\mathrm{HP}(\rho)$ which is defined in terms of local monodromy invariants. In this talk we discuss the interaction between the polygons $\mathrm{NP}(\rho)$ and $\mathrm{HP}(\rho)$. Our main result states that if $U$ is ordinary, then $\mathrm{NP}(\rho)$ and $\mathrm{HP}(\rho)$ share a vertex if and only if there is a corresponding vertex shared by certain "local" Newton and Hodge polygons associated to each ramified point of $\rho$. As an application, we give a local criterion that is necessary and sufficient for $\mathrm{NP}(\rho)$ and $\mathrm{HP}(\rho)$ to coincide. This is joint work with Joe Kramer-Miller.