# Simultaneous representation of integers by a pair of ternary quadratic forms - With an application to index form equations in quartic number fields

István Gaál, Attila Petho, Michael Pohst

Research output: Contribution to journalArticle

32 Citations (Scopus)

### Abstract

Let Q1, Q2 ∈ ℤ[X, Y, Z] be two ternary quadratic forms and u1, u2 ∈ ℤ. In this paper we consider the problem of solving the system of equations (formula presented) According to Mordell  the coprime solutions of (formula presented) can be presented by finitely many expressions of the form x = fx(p, q), y = fy(p, q), z = fz(p, q), where fx,fy,fz ∈ ℤ[P, Q] are binary quadratic forms and p, q are coprime integers. Substituting these expressions into one of the equations of (1), we obtain a quartic homogeneous equation in two variables. If it is irreducible it is a quartic Thue equation, otherwise it can be solved even easier. The finitely many solutions p, q of that equation then yield all solutions x, y, z of (1). We also discuss two applications. In [ 8 ] we showed that the problem of solving index form equations in quartic numbers fields K can be reduced to the resolution of a cubic equation F(u, v) = i and a corresponding system of quadratic equations Q1(x, y, z) = u, Q2(x, y, z) = u, where F is a binary cubic form and Q1, Q2 are ternary quadratic forms. In this case the application of the new ideas for the resolution of (1) leads to quartic Thue equations which split over the same quartic field K. The second application is to the calculation of all integral points of an elliptic curve.

Original language English 90-104 15 Journal of Number Theory 57 1 https://doi.org/10.1006/jnth.1996.0035 Published - Mar 1996

### ASJC Scopus subject areas

• Algebra and Number Theory