Brezis's refined topological-degree estimate for circle maps

Let ficeC0(S1;S1)f ice C^0({\mathbb S}^1;{\mathbb S}^1) be a continuous map, and let degf\deg f denote its topological degree. There should exist constants C>0C>0 and δ0>0\delta_0>0 such that, for every δ<δ0\delta<\delta_0,

degfCδf(s)f(t)>δdsdtst2.|\deg f| \leq C\delta \iint_{|f(s)-f(t)|>\delta} \frac{ds\,dt}{|s-t|^2}.

Brezis's conjecture. There exist constants C>0C>0 and δ0>0\delta_0>0 such that the displayed inequality holds for every continuous fC0(S1;S1)f\in C^0({\mathbb S}^1;{\mathbb S}^1) and every δ<δ0\delta<\delta_0. This question asks for a bound on the topological degree of a circle map in terms of a nonlocal integral; the paper proves an inequality inspired by this conjecture, while the conjectured estimate itself is presented as the motivating open problem.

Sources & referencesView supporting material

Primary source

Felipe Hernández, “Towards a refined estimate for topological degree in one dimension”, arXiv:2210.08138 (2022).

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