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Séminaire de Théorie Algorithmique des Nombres

Towards computing the canonical lift of an ordinary elliptic curve in medium characteristic

Damien Robert

( IMB )

-

le 05 avril 2022 à 10:00

Satoh's algorithm for counting the number of points of an elliptic curve E/FqE/\mathbb F_q with q=pnq=p^n is the fastest known algorithm when pp is fixed: it computes the invertible eigenvalue »» of the Frobenius to pp-adic precision mm in time O~(p2nm)\tilde{O}(p^2 n m). Since by Hasse's bound, recovering χπ\chi_{\pi} requires working at precision m=O(n)m=O(n), the point counting complexity is of O~(p2n2)\tilde{O}(p^2 n^2), quasi-quadratic in the degree nn. Unfortunately, the term p2p^2 in the complexity makes Satoh's algorithm suitable only for smaller pp. For medium sized pp, one can use Kedlaya's algorithm which cost O~(pn2m)\tilde{O}(p n^2 m) or a variant by Harvey's which cost O~(p1/2n5/2m+n4m)\tilde{O}(p^{1/2} n^{5/2} m + n^4 m), which have a better complexity on pp but a worse one on nn. For large pp, the SEA algorithm costs O~(log4q)\tilde{O}(log^4 q). In this talk, we improve the dependency on pp of Satoh's algorithm while retaining the dependency on nn to bridge the gap towards medium characteristic. We develop a new algorithm with a complexity of O~(pnm)\tilde{O}(p n m). In the particular case where we are furthermore provided with a rational point of pp-torsion, we even improve this complexity to O~(p1/2nm)\tilde{O}(p^{1/2} n m). This is a joint work with Abdoulaye Maiga.