Sharp estimates of the curvature of some free boundaries in two dimensions

• We prove that the Cauchy transform of a positive measure on the interval )-1,1) subset of R in the complex plane maps the exterior of the unit disc onto a domain Omega subset of C which can be written as a union of discs centered on the real axis. This is applied to the obstacle problem, partial balayage, quadrature domains and Hele-Shaw flow moving boundary problems, and we obtain sharp estimates of the curvature of free boundaries appearing in such problems.

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quadrature domains

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