Effective Generalized Fermat equation of signature $(2p,2q,r)$ with odd narrow class number

Abstract

Fix a rational prime $r\ge 5$. In this article, we study the integer solutions of the generalized Fermat equation of signature $(2p,2q,r)$, namely $x^{2p}+y^{2q}=z^r$, where the primes $p,q\ge 5$ are varying. For each rational prime $r\ge 5$, we first establish a condition on the solutions of the $S$-unit equation over $\mathbb{Q}({\zeta }_r+{\zeta }_r^{-1})$ such that there exists a constant $V_r>0$ (depending on $r$) for which the equation $x^{2p}+y^{2q}=z^r$ with $p,q\ge V_r$ has no non-trivial primitive integer solutions. Then for each rational prime $r\ge 2$, we prove that every elliptic curve over $\mathbb{Q}({\zeta }_r+{\zeta }_r^{-1})$ is modular. As an application of this, we prove that the above constant $V_r$ is effectively computable. Finally, we provide a criterion for $r$ such that the equation $x^{2p}+y^{2q}=z^r$ with $p,q\ge V_r$ has no non-trivial primitive integer solutions when the narrow class number of $\mathbb{Q}({\zeta }_r+{\zeta }_r^{-1})$ is odd.

15 pages