arXiv subjects

Gilles Christol

Publications and source records attributed to Gilles Christol.

3 recordsLinked to original sources

arXiv2018

Fast Coefficient Computation for Algebraic Power Series in Positive Characteristic

We revisit Christol's theorem on algebraic power series in positive characteristic and propose yet another proof for it. This new proof combines several ingredients and advantages of existing proofs, which make it very well-suited for algorithmic purposes. We apply the construction used in the new proof to the design of a new efficient algorithm for computing the $N$th coefficient of a given algebraic power series over a perfect field of characteristic~$p$. It has several nice features: it is more general, more natural and more efficient than previous algorithms. Not only the arithmetic complexity of the new algorithm is linear in $\log N$ and quasi-linear in~$p$, but its dependency with respect to the degree of the input is much smaller than in the previously best algorithm. {Moreover, when the ground field is finite, the new approach yields an even faster algorithm, whose bit complexity is linear in $\log N$ and quasi-linear in~$\sqrt{p}$}.

math.NT↗

arXiv2016

Fast Computation of the Nth Term of an Algebraic Series over a Finite Prime Field

We address the question of computing one selected term of an algebraic power series. In characteristic zero, the best algorithm currently known for computing the $N$th coefficient of an algebraic series uses differential equations and has arithmetic complexity quasi-linear in $\sqrt{N}$. We show that over a prime field of positive characteristic $p$, the complexity can be lowered to $O(\log N)$. The mathematical basis for this dramatic improvement is a classical theorem stating that a formal power series with coefficients in a finite field is algebraic if and only if the sequence of its coefficients can be generated by an automaton. We revisit and enhance two constructive proofs of this result for finite prime fields. The first proof uses Mahler equations, whose sizes appear to be prohibitively large. The second proof relies on diagonals of rational functions; we turn it into an efficient algorithm, of complexity linear in $\log N$ and quasi-linear in $p$.

cs.SC↗

arXiv2000

Sur le théorème de l'indice des équations différentielles p-adiques. III

This paper works out the structure of singular points of p-adic differential equations (i.e. differential modules over the ring of functions analytic in some annulus with external radius 1). Surprisingly results look like in the formal case (differential modules over a one variable power series field) but proofs are much more involved. However, unlike in the Turritin theorem, even after ramification, in the p-adic theory there are irreducible objects of rank >1. The first part is devoted to the definition of p-adic slopes and to a decomposition along p-adic slopes theorem. The case of slope 0 (p-adic analogue of the regular singular case) was already studied in the paper with the same title but number II [Ann. of Math. (2) 146 (1997), 345-410]. The second part states several index existence theorems and index formulas. As a consequence, vertices of the Newton polygon built from p-adic slopes are proved to have integral components (analogue of the Hasse-Arf theorem). After the work of the second author, existence of index implies finitness of p-adic (Monsky-Washnitzer) cohomology for affine varieties over finite fields. The end of the paper outlines the construction of a p-adic-coefficient category over curves (over a finite field) with all needed finitness properties. In the paper with the same title but number IV [Invent. Math. 143 (2001), 629-672], further insights are given.

math.NT↗