# CM theory, maximal hyperelliptic curves, and Chebyshev polynomials

## TL;DR

This paper investigates hyperelliptic curves defined by Chebyshev polynomials over finite fields, identifying new maximal cases and conditions that prevent maximality, using advanced algebraic and computational techniques.

## Contribution

It introduces new maximality results and criteria for hyperelliptic curves associated with Chebyshev polynomials, expanding understanding of their properties over finite fields.

## Key findings

- New maximal cases for hyperelliptic curves over finite fields.
- Conditions ruling out maximality for certain (d,q) pairs.
- A novel method for proving maximality in specific characteristics.

## Abstract

This paper studies hyperelliptic curves $\cH_d$ corresponding to $y^2=\varphi_d(x)$ over finite fields, with $\varphi_d(x)$ a Chebyshev polynomial. Starting from the case where $d=\ell$ is an odd prime number, new cases $(d,q)$ are presented where $\cH_d$ is maximal over the finite field $\FF_{q^2}$ of cardinality $q^2$. In addition, new conditions ruling out the possibility that $\cH_d/\FF_{q^2}$ is maximal for given $(d,q)$, are presented. The arguments involve a mix of results on slopes of Frobenius, explicit descriptions of abelian subvarieties of the jacobian of $\cH_d$ with complex multiplication, and a technique from the theory of $2$-descent on jacobians of hyperelliptic curves. In particular, the method used here to prove maximality in characteristics $p\equiv 1\bmod 4$ for $d\equiv 1\bmod 4$ a prime number, deserves attention, as it differs from earlier maximality arguments for other curves. Using the new results as well as extensive calculations with Magma, we pose some questions. A positive answer would completely classify the pairs $(q,d)$ resulting in maximality.