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arXiv · 2607.24263

Signs of Square-Free Fourier Coefficients of Half-Integral weight cusp forms and the Congruent Number Problem

Wei Tao·Guo Xuejun
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Abstract

We study the sign distribution of square-free Fourier coefficients of half-integral weight cusp forms. We prove that, for half-integral weight cusp forms \(f\) satisfying the eigenform conditions and having a nonzero cuspidal Shimura lift, the numbers of positive and negative square-free Fourier coefficients up to \(X\) are both \(\gg_{f,\varepsilon}X^{4/7-\varepsilon}\). As an application to the congruent-number problem, we consider the weight \(3/2\) cusp form whose Shimura lift is the weight \(2\) newform attached to $ E:y^2=x^3-x$. We prove that each each sign occurs among its square-free Fourier coefficients at \(\gg_{\varepsilon}X^{4/7-\varepsilon}\) odd square-free integers \(t\leq X\). In particular, the square-free Fourier coefficients change sign infinitely often.

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Wei Tao, Guo Xuejun. 2026-07-27. Signs of Square-Free Fourier Coefficients of Half-Integral weight cusp forms and the Congruent Number Problem. https://arxiv.org/abs/2607.24263

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