Towards computing the canonical lift of an ordinary elliptic curve in medium characteristic
Satoh's algorithm for counting the number of points of an elliptic curve
with
is the fastest known algorithm when
is fixed: it computes the invertible eigenvalue
of the Frobenius to
-adic precision
in time
. Since by Hasse's bound, recovering
requires working at precision
, the point counting complexity is of
, quasi-quadratic in the degree
.
Unfortunately, the term
in the complexity makes Satoh's algorithm suitable only for smaller
. For medium sized
, one can use Kedlaya's algorithm which cost
or a variant by Harvey's which cost
, which have a better complexity on
but a worse one on
. For large
, the SEA algorithm costs
.
In this talk, we improve the dependency on
of Satoh's algorithm while retaining the dependency on
to bridge the gap towards medium characteristic. We develop a new algorithm with a complexity of
. In the particular case where we are furthermore provided with a rational point of
-torsion, we even improve this complexity to
.
This is a joint work with Abdoulaye Maiga.