This document contains the notes of a lecture I gave at the "Journées Nationales du Calcul Formel 1 " (JNCF) on January 2017. The aim of the lecture was to discuss low-level algorithmics for p-adic numbers. It is divided into two main parts: first, we present various implementations of p-adic numbers and compare them and second, we introduce a general framework for studying precision issues and apply it in several concrete situations.
(French) National Computer Algebra DaysTheory and Arithmetic Geometry. After Diophantine equations, other typical examples come from the study of number fields: we hope deriving interesting information about a number field K by studying carefully all its p-adic incarnations K ⊗ Q Q p . The ramification of K, its Galois properties, etc. can be -and are very often -studied in this manner [69,65]. The class field theory, which provides a precise description of all Abelian extensions 2 of a given number field, is also formulated in this language [66]. The importance of p-adic numbers is so prominent today that there is still nowadays very active research on theories which are dedicated to purely p-adic objects: one can mention for instance the study of p-adic geometry and p-adic cohomologies [6,58], the theory of p-adic differential equations [50], Coleman's theory of padic integration [24], the p-adic Hodge theory [14], the p-adic Langlands correspondence [5], the study of p-adic modular forms [34], p-adic ζ-functions [52] and L-functions [22], etc. The proof of Fermat's last Theorem by Wiles and Taylor [81, 78] is stamped with many of these ideas and developments. Over the last decades, p-adic methods have taken some importance in Symbolic Computation as well. For a long time, p-adic methods have been used for factoring polynomials over Q [56]. More recently, there has been a wide diversification of the use of p-adic numbers for effective computations: Bostan et al. [13] used Newton sums for polynomials over Z p to compute composed products for polynomials over F p ; Gaudry et al. [32] used p-adic lifting methods to generate genus 2 CM hyperelliptic curves; Kedlaya [49], Lauder [54] and many followers used p-adic cohomology to count points on hyperelliptic curves over finite fields; Lercier and Sir vent [57] computed isogenies between elliptic curves over finite fields using p-adic differential equations.The need to build solid foundations to the algorithmics of p-adic numbers has then emerged. This is however not straightforward because a single p-adic number encompasses an infinite amount of information (the infinite sequence of its digits) and then necessarily needs to be truncated in order to fit in the memory of a computer. From this point of view, p-adic numbers behave very similarly to real numbers and the questions that emerge when we are trying to implement p-adic numbers are often the same as the questions arising when dealing with rounding errors in the real setting [62,26,63]. The algorithmic study of p-adic numbers is then located at the frontier between Symbolic Computa...