## Arkiv för Matematik

- Ark. Mat.
- Volume 32, Number 2 (1994), 475-492.

### Quadrature surfaces as free boundaries

#### Abstract

This paper deals with a free boundary porblem connected with the concept “quadrature surface”. Let Ω⊂**R**^{n} be a bounded domain with a *C*^{2} boundary and μ a measure compactly supported in Ω. Then we say ∂Ω is a quadrature surface with respect to μ if the following overdetermined Cauchy problem has a solution.
$\Delta u = - \mu in \Omega ,u = 0 and \frac{{\partial u}}{{\partial v}} = - 1 on \partial \Omega .$

Applying simple techniques, we derive basic inequalities and show uniform boundedness for the set of solutions. Distance estimates as well as uniqueness results are obtained in special cases, e.g. we show that if ∂Ω and ∂*D* are two quadrature surfaces for a fixed measure μ and Ω is convex, then *D*⊂Ω. The main observation, however, is that if ∂Ω is a quadrature surface for μ≥0 and *x*ε∂Ω, then the inward normal ray to ∂Ω at *x* intersects the convex hull of supp μ. We also study relations between quadrature surfaces and quadrature domains. *D* is said to be a quadrature domain with respect to a mesure μ if there is a solution to the following overdetermined Cauchy problem:
$\Delta u = 1 - \mu in D, andu = |\nabla u| = 0 on \partial D.$

Finally, we apply our results to a problem of electrochemical machining.

#### Note

The author is grateful to Professor H. S. Shapiro for valuable suggestions. He also thanks Professor B. Gustafsson for his constructive criticism, which led to improvement of some technical details.

#### Article information

**Source**

Ark. Mat., Volume 32, Number 2 (1994), 475-492.

**Dates**

Received: 18 November 1991

First available in Project Euclid: 31 January 2017

**Permanent link to this document**

https://projecteuclid.org/euclid.afm/1485898298

**Digital Object Identifier**

doi:10.1007/BF02559582

**Mathematical Reviews number (MathSciNet)**

MR1318543

**Zentralblatt MATH identifier**

0827.31004

**Rights**

1994 © Institut Mittag-Leffler

#### Citation

Shahgholian, Henrik. Quadrature surfaces as free boundaries. Ark. Mat. 32 (1994), no. 2, 475--492. doi:10.1007/BF02559582. https://projecteuclid.org/euclid.afm/1485898298