Ordinary reductions of abelian varieties

TL;DR

The paper reduces conjectures about primes of ordinary reduction for abelian varieties to a simpler Galois representation conjecture, proving this for certain compatible systems and establishing a key trace integrality property.

Contribution

It introduces a new slope estimate and reduces the problem of ordinary primes to a trace integrality conjecture, proving this for specific Galois systems.

Findings

Reduction of conjectures to Galois representation trace integrality

Proved trace integrality for certain abelian Galois systems

Established a new slope estimate for non Hodge-Witt abelian varieties

Abstract

I show that a conjecture of Joshi-Rajan on primes of Hodge-Witt reduction and in particular a conjecture of Jean-Pierre Serre on primes of good, ordinary reduction for an abelian variety over a number field follows from a certain conjecture on Galois rep- resentations which may perhaps be easier to prove (and I prove this conjecture for abelian compatible systems of a suitable type). This reduction (to a conjecture about certain sys- tems of Galois representations) is based on a new slope estimate for non Hodge-Witt abelian varieties. In particular for any abelian variety over a number field with at least one prime of good ordinary or split toric reduction, I show that the conjecture of Joshi-Rajan and the conjecture of Serre on ordinary reductions can be reduced to proving that a certain rational trace of Frobenius is in fact an integer. The assertion that this trace is an integer is proved for abelian systems of Galois representations (of suitable type).