arXiv subjects
Publications and source records attributed to Xavier Vidaux.
By fully describing the lattice of subfields of some towers of number fields built by iterating square roots, we obtain infinitely many fields, each of them either contradicts Julia Robinson's problem (obtaining a JR-number $4$ which is not a minimum) or gives a Julia Robinson number strictly between four and infinity. This improves a previous result by M. Castillo and the same authors.
The analogue of Hilbert's tenth problem over $\mathbb{Q}$ asks for an algorithm to decide the existence of rational points in algebraic varieties over this field. This remains as one of the main open problems in the area of undecidability in number theory. Besides the existence of rational points, there is also considerable interest in the problem of effectivity: one asks whether the sought rational points satisfy determined height bounds, often expressed in terms of the height of the coefficients of the equations defining the algebraic varieties under consideration. We show that, in fact, Hilbert's tenth problem over $\mathbb{Q}$ with (finitely many) height comparison conditions is undecidable.
We investigate the problem of deciding whether a system of linear equations, together with divisibility conditions on the variables, has a solution over holomorphy subrings of global fields. We obtain decidability results when we allow poles at a cofinite set of places, and undecidability results when at a finite set of places.
Given a prime $p\ge5$ and an integer $s\ge1$, we show that there exists an integer $M$ such that for any quadratic polynomial $f$ with coefficients in the ring of integers modulo $p^s$, such that $f$ is not a square, if a sequence $(f(1),\dots,f(N))$ is a sequence of squares, then $N$ is at most $M$. We obtain this result by reducing to the case where $f$ has an invertible dominant coefficient.
We compare three different characterizations, due respectively to R. Dedekind, K. Uchida, and H. Lüneburg, of when $\mathbb Z[θ]$ is the ring of integers of $\mathbb Q(θ)$, and apply our results to some concrete $2$-towers of number fields.
We prove an analogue of Hilbert's Tenth Problem for complex meromorphic functions. More precisely, we prove that the set of integers is positive existentially definable in fields of complex meromorphic functions in several variables over the language of rings, together with constant symbols for two of the independent variables and the set of constants, a unary relation symbol for non-zero functions, and a unary relation symbol for evaluation at a fixed point (a place). We obtain a similar result for analytic functions, where the place appears in the language as a binary predicate. In both cases, we only require the functions to be meromorphic (or analytic) on a set containing $\mathbb C$ in one of the variables (it can be germs in all the other variables).
For rings $\mathcal{O}_K$ of totally real algebraic integers, J. Robinson defined a set which is always $\{+\infty\}$ or of the form $[λ,+\infty)$ or $(λ,+\infty)$ for some real number $λ\ge4$. All known examples give either $\{+\infty\}$ or $[4,+\infty)$. In this paper, we construct infinitely many fields such that the set is an interval, but not equal to $[4,+\infty)$.
We present a concept of uniform encodability of theories and develop tools related to this concept. As an application we obtain general undecidability results which are uniform for large families of structures. In the way, we define uniformly in the characteristic the equivalence relation "$x\sim y$ if and only if $x$ is an iterate of $y$ throuh the Frobenius map, or vice versa" in large classes of function fields and in polynomial rings.
We give a new characterization of generalized Büchi sequences (sequences whose sequence of squares has constant second difference $(a)$, for some fixed integer $a$) of length 3 over the integers and a strategy for attacking Büchi's n Squares Problem. Known characterizations of integer Büchi sequences of length 3 are actually characterizations over the rationals, plus some divisibility criterions that keep integer sequences.
Büchi's problem asks whether there exists a positive integer $M$ such that any sequence $(x_n)$ of at least $M$ integers, whose second difference of squares is the constant sequence $(2)$, satisifies $x_n^2=(x+n)^2$ for some $x\in\Z$. A positive answer to Büchi's problem would imply that there is no algorithm to decide whether or not an arbitrary system of quadratic diagonal forms over $\Z$ can represent an arbitrary given vector of integers. We give explicitly an infinite family of polynomial parametrizations of non-trivial length $4$ Büchi sequences of integers. In turn, these parametrizations give an explicit infinite family of curves (which we suspect to be hyperelliptic) with the following property: any integral point on one of these curves would give a length $5$ non-trivial Büchi sequence of integers (it is not known whether any such sequence exists).
Büchi's $n$ Squares Problem asks for an integer $M$ such that any sequence $(x_0,...,x_{M-1})$, whose second difference of squares is the constant sequence $(2)$ (i.e. $x^2_n-2x^2_{n-1}+x_{n-2}^2=2$ for all $n$), satisfies $x_n^2=(x+n)^2$ for some integer $x$. Hensley's problem for $r$-th powers (where $r$ is an integer $\geq2$) is a generalization of Büchi's problem asking for an integer $M$ such that, given integers $ν$ and $a$, the quantity $(ν+n)^r-a$ cannot be an $r$-th power for $M$ or more values of the integer $n$, unless $a=0$. The analogues of these problems for rings of functions consider only sequences with at least one non-constant term. Let $K$ be a function field of a curve of genus $g$. We prove that Hensley's problem for $r$-th powers has a positive answer for any $r$ if $K$ has characteristic zero, improving results by Pasten and Vojta. In positive characteristic $p$ we obtain a weaker result, but which is enough to prove that Büchi's problem has a positive answer if $p\geq 312g+169$ (improving results by Pheidas and the second author).