Polynomial parametrizations of length $4$ Büchi sequences

Abstract

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 $x\in \mathbb{Z}$. 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 $\mathbb{Z}$ 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).

arXiv comment17 pages, 1 figure