On some Diophantine systems involving symmetric polynomials

Abstract

Let ${\sigma }_i(x_1,\dots ,x_n)={\sum }_{1\le k_1<k_2<\dots <k_i\le n}{x}_{k_1}\dots x_{k_i}$ be the $i$-th elementary symmetric polynomial. In this note we generalize and extend the results obtained in a recent work of Zhang and Cai \cite{ZC,ZC2}. More precisely, we prove that for each $n\ge 4$ and rational numbers $a,b$ with $ab\ne 0$, the system of diophantine equations \begin{equation*} σ_{1}(x_{1},\ldots, x_{n})=a, \quad σ_{n}(x_{1},\ldots, x_{n})=b, \end{equation*} has infinitely many solutions depending on $n-3$ free parameters. A similar result is proved for the system \begin{equation*} σ_{i}(x_{1},\ldots, x_{n})=a, \quad σ_{n}(x_{1},\ldots, x_{n})=b, \end{equation*} with $n\ge 4$ and $2\le i<n$. Here, $a,b$ are rational numbers with $b\ne 0$. We also give some results concerning the general system of the form \begin{equation*} σ_{i}(x_{1},\ldots, x_{n})=a, \quad σ_{j}(x_{1},\ldots, x_{n})=b, \end{equation*} with suitably chosen rational values of $a,b$ and $i<j<n$. Finally, we present some remarks on the systems involving three different symmetric polynomials.

to appear in Math. Comp