arXiv subjects
Publications and source records attributed to Annamaria Iezzi.
We study endomorphism rings of principally polarized abelian surfaces over finite fields from a computational viewpoint with a focus on exhaustiveness. In particular, we address the cases of non-ordinary and non-simple varieties. For each possible surface type, we survey known results and, whenever possible, provide improvements and missing results.
We give an algorithm for computing an inseparable endomorphism of a supersingular elliptic curve $E$ defined over $\mathbb F_{p^2}$, which, conditional on GRH, runs in expected $O(p^{1/2}(\log p)^2(\log\log p)^3)$ bit operations and requires $O((\log p)^2)$ storage. This matches the time and storage complexity of the best conditional algorithms for computing a nontrivial supersingular endomorphism, such as those of Eisenträger-Hallgren-Leonardi-Morrison-Park and Delfs-Galbraith. Unlike these prior algorithms, which require two paths from $E$ to a curve defined over $\mathbb F_p$, the algorithm we introduce only requires one; thus when combined with the algorithm of Corte-Real Santos-Costello-Shi, our algorithm will be faster in practice. Moreover, our algorithm produces endomorphisms with predictable discriminants, enabling us to prove properties about the orders they generate. With two calls to our algorithm, we can provably compute a Bass suborder of $\operatorname{End}(E)$. This result is then used in an algorithm for computing a basis for $\operatorname{End}(E)$ with the same time complexity, assuming GRH. We also argue that $\operatorname{End}(E)$ can be computed using $O(1)$ calls to our algorithm along with polynomial overhead, conditional on a heuristic assumption about the distribution of the discriminants of these endomorphisms. Conditional on GRH and this additional heuristic, this yields a $O(p^{1/2}(\log p)^2(\log\log p)^3)$ algorithm for computing $\operatorname{End}(E)$ requiring $O((\log p)^2)$ storage.
We provide new examples of curves of genus 6 or 10 attaining the Serre bound. They all belong to the family of sextics introduced in [19] as a a generalization of the Wiman sextics [36] and Edge sextics [9]. Our approach is based on a theorem by Kani and Rosen which allows, under certain assumptions, to fully decompose the Jacobian of the curve. With our investigation we are able to update several entries in \url{http://www.manypoints.org} ([35]).
We use the Aubry-Perret bound for singular curves, a generalization of the Hasse-Weil bound, to prove the following curious result about rational functions over finite fields: Let $f(X),g(X)\in\Bbb F_q(X)\setminus\{0\}$ be such that $q$ is sufficiently large relative to $\text{deg}\, f$ and $\text{deg}\, g$, $f(\Bbb F_q)\subset g(\Bbb F_q\cup\{\infty\})$, and for ``most'' $a\in\Bbb F_q\cup\{\infty\}$, $|\{x\in \Bbb F_q:g(x)=g(a)\}|>(\text{deg}\, g)/2$. Then there exists $h(X)\in\Bbb F_q(X)$ such that $f(X)=g(h(X))$. A generalization to multivariate rational functions is also included.
We propose an algorithm for computing an isogeny between two elliptic curves $E_1,E_2$ defined over a finite field such that there is an imaginary quadratic order $\mathcal{O}$ satisfying $\mathcal{O}\simeq \operatorname{End}(E_i)$ for $i = 1,2$. This concerns ordinary curves and supersingular curves defined over $\mathbb{F}_p$ (the latter used in the recent CSIDH proposal). Our algorithm has heuristic asymptotic run time $e^{O\left(\sqrt{\log(|Δ|)}\right)}$ and requires polynomial quantum memory and $e^{O\left(\sqrt{\log(|Δ|)}\right)}$ classical memory, where $Δ$ is the discriminant of $\mathcal{O}$. This asymptotic complexity outperforms all other available method for computing isogenies. We also show that a variant of our method has asymptotic run time $e^{\tilde{O}\left(\sqrt{\log(|Δ|)}\right)}$ while requesting only polynomial memory (both quantum and classical).
Using an Euclidean approach, we prove a new upper bound for the number of closed points of degree 2 on a smooth absolutely irreducible projective algebraic curve defined over the finite field $\mathbb F\_q$.This bound enables us to provide explicit conditions on $q, g$ and $π$ for the non-existence of absolutely irreducible projective algebraic curves defined over $\mathbb F\_q$ of geometric genus $g$, arithmetic genus $π$ and with $N\_q(g)+π-g$ rational points.Moreover, for $q$ a square, we study the set of pairs $(g,π)$ for which there exists a maximal absolutely irreducible projective algebraic curve defined over $\mathbb F\_q$ of geometric genus $g$ and arithmetic genus $π$, i.e. with $q+1+2g\sqrt{q}+π-g$ rational points.
We give a construction of singular curves with many rational points over finite fields. This construction enables us to prove some results on the maximum number of rational points on an absolutely irreducible projective algebraic curve defined over Fq of geometric genus g and arithmetic genus $π$.