The Constant Width Measure Set, the Spherical Measure Set and   isoperimetric equalities for planar ovals
  In this paper we introduce and study sets: the Constant Width Measure Set and the Spherical Measure Set, which measure the constant width property and the spherical property of planar regular simple curves, respectively.   Using these sets and the Wigner caustic, we find the exact relations between the length and the area bounded by a closed regular simple convex planar curve $M$. Namely, the following equalities are fulfilled: \begin{align} \label{AbstractEq1} L_{M}^2 &=4\pi A_M+8\pi\left|\widetilde{A}_{E_{\frac{1}{2}}(M)}\right|+\pi\left|\widetilde{A}_{CWMS(M)}\right|,\\ \label{AbstractEq2} L_{M}^2 &=4\pi A_M+4\pi\left|\widetilde{A}_{SMS(M)}\right|, \end{align} where $L_{M}, A_{M}, \widetilde{A}_{E_{\frac{1}{2}}(M)}, \widetilde{A}_{CWMS(M)}, \widetilde{A}_{SMS(M)}$ are the length of $M$, the area bounded by $M$, the oriented area of the Wigner caustic of $M$, the oriented area of the Constant Width Measure Set of $M$ and the oriented area of the Spherical Measure Set of $M$, respectively. The second isoperimetric equality holds also if $M$ is a regular closed simple curve.   We also present a new proof of Four-Vertex Theorem for generic ovals based on the number of singular points of the $SMS$. 
M. Zwierzy\'nski
math.DG