Maximal curves over finite fields: examples

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A (smooth, complete, geometrically irreducible) curve LaTeX: C defined over a finite field LaTeX:  {\mathbb F}_q is called maximal, if the cardinality of the set of LaTeX: {\mathbb F}_q-rational points on LaTeX: C equals LaTeX: q+1+gm. Here LaTeX:  g denotes the genus of the curve, and LaTeX:  m is the largest integer not exceeding LaTeX:  \sqrt{4q} .

We consider the following question: given a curve over LaTeX:  {\mathbb Z}[\frac{1}{N}] , for which prime powers LaTeX:  q coprime with LaTeX:  N , is LaTeX:  C\otimes {\mathbb F}_q maximal over LaTeX: {\mathbb F}_q}?

Although in general this is a difficult problem (already for genus 1!), it turns out that in certain examples one can give a complete answer. In the talk this is done for a particular hyperelliptic curve of genus 3.

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