Beyond the Erdős discrepancy problem in function fields

Klurman, Oleksiy; Mangerel, Alexander P.; Teräväinen, Joni

Abstract

We characterize the limiting behavior of partial sums of multiplicative functions f:Fq[t]→S1. In contrast to the number field setting, the characterization depends crucially on whether the notion of discrepancy is defined using long intervals, short intervals, or lexicographic intervals. Concerning the notion of short interval discrepancy, we show that a completely multiplicative f:Fq[t]→{-1, +1} with q odd has bounded short interval sums if and only if f coincides with a “modified" Dirichlet character to a prime power modulus. This confirms the function field version of a conjecture over Z that such modified characters are extremal with respect to partial sums. Regarding the lexicographic discrepancy, we prove that the discrepancy of a completely multiplicative sequence is always infinite if we define it using a natural lexicographic ordering of Fq[t]. This answers a question of Liu and Wooley. Concerning the long sum discrepancy, it was observed by the Polymath 5 collaboration that the Erdős discrepancy problem admits infinitely many completely multiplicative counterexamples on Fq[t]. Nevertheless, we are able to classify the counterexamples if we restrict to the class of modified Dirichlet characters. In this setting, we determine the precise growth rate of the discrepancy, which is still unknown for the analogous problem over the integers.

Citation

Klurman, O., Mangerel, A. P., & Teräväinen, J. (2024). Beyond the Erdős discrepancy problem in function fields. Mathematische Annalen, 389(3), 2959-3008. https://doi.org/10.1007/s00208-023-02700-z

• Journal Article Type: Article
• Acceptance Date: Aug 9, 2023
• Online Publication Date: Sep 12, 2023
• Publication Date: Jul 1, 2024
• Deposit Date: Oct 25, 2023
• Publicly Available Date: Jan 9, 2024
• Journal: Mathematische Annalen
• Print ISSN: 0025-5831
• Electronic ISSN: 1432-1807
• Publisher: Springer
• Peer Reviewed: Peer Reviewed
• Volume: 389
• Issue: 3
• Pages: 2959-3008
• DOI: https://doi.org/10.1007/s00208-023-02700-z
• Public URL: https://durham-repository.worktribe.com/output/1817328