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Curves of constant diameter and inscribed polygons

Mathieu Baillif
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Abstract

A simple closed curve in the Euclidean plane is said to have property C_n(R) if at each point we can inscribe a unique regular $n$-gon with edges length $R$. C_2(R) is equivalent to having constant diameter. We show that smooth curves satisfying C_n(R) other than the circle do exist for all n, and that the circle is the only $C^2$ regular curve satisfying C_2(R) and C_4(R') where $R'=R/\sqrt{2}$. In an addendum, we show that the last assertion holds for any R and R'. The proofs use only elementary differential calculus and geometry.

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Mathieu Baillif. 2012-02-10. Curves of constant diameter and inscribed polygons. https://arxiv.org/abs/math/0504300

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