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arXiv · 2502.02147

On Siegel's problem and Dwork's conjecture for $G$-functions

Javier Fresán·Yeuk Hay Joshua Lam·Yichen Qin
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Abstract

We answer in the negative Siegel's problem for $G$-functions, as formulated by Fischler and Rivoal. Roughly, we prove that there are $G$-functions that cannot be written as polynomial expressions in algebraic pullbacks of hypergeometric functions; our examples satisfy differential equations of order two, which is the smallest possible. In fact, we construct infinitely many non-equivalent rank-two local systems of geometric origin which are not algebraic pullbacks of hypergeometric local systems, thereby providing further counterexamples to Dwork's conjecture and answering a question by Krammer. The main ingredients of the proof are a Lie algebra version of Goursat's lemma, the monodromy computations of hypergeometric local systems due to Beukers and Heckman, as well as results on invariant trace fields of Fuchsian groups.

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Javier Fresán, Yeuk Hay Joshua Lam, Yichen Qin. 2025-02-04. On Siegel's problem and Dwork's conjecture for $G$-functions. https://arxiv.org/abs/2502.02147

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