Curves of constant diameter and inscribed polygons

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