Symmetrization of a Cauchy-like kernel on curves

Abstract

Given a curve $\Gamma\subset \mathbb C$ with specified regularity, we investigate boundedness and positivity for a certain three-point symmetrization of a Cauchy-like kernel $K_{\Gamma}$ whose definition is dictated by the geometry and complex function theory of the domains bounded by $\Gamma$. Our results show that $\mathtt S[\text{Re} K_{\Gamma}]$ and $\mathtt S[\text{Im} K_{\Gamma}]$ (namely, the symmetrizations of the real and imaginary parts of $K_{\Gamma}$) behave very differently from their counterparts for the Cauchy kernel previously studied in the literature. For instance, the quantities $\mathtt S[\text{Re} K_{\Gamma}](\mathbf z)$ and $\mathtt S[\text{Im} K_{\Gamma}](\mathbf z)$ can behave like $\frac32c^2(\mathbf z)$ and $-\frac12c^2(\mathbf z)$, where $\mathbf z$ is any three-tuple of points in $\Gamma$ and $c(\mathbf z)$ is the Menger curvature of $\mathbf z$. For the original Cauchy kernel, an iconic result of M. Melnikov gives that the symmetrized forms of the real and imaginary parts are each equal to $\frac12c^2(\mathbf z)$ for all three-tuples in $\mathbb C$.