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arXiv · 2507.20604

Maps preserving the sum-to-difference ratio in characteristic $p$

Sunil Chebolu·Apoorva Khare·Anindya Sen
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Abstract

Given a field $\mathbb{F}$, we introduce a novel group $SD(\mathbb{F})$ of its self-maps: the solutions $f \colon \mathbb{F} \twoheadrightarrow \mathbb{F}$ to the functional equation $f \left( (x+y)/(x-y) \right) = (f(x) + f(y))/(f(x) - f(y))$ for all $ x \neq y$ in $\mathbb{F}$. We compute this group for all fields algebraic over $\mathbb{F}_p$. In particular, this group distinguishes $\mathbb{F}_5$ among all finite fields $\mathbb{F}_q$, and in fact among all subfields of $\overline{\mathbb{F}_q}$.

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Sunil Chebolu, Apoorva Khare, Anindya Sen. 2025-07-28. Maps preserving the sum-to-difference ratio in characteristic $p$. https://arxiv.org/abs/2507.20604

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