Laplace's equation in the exterior of a convex polygon. The equilateral triangle

Authors:
A. Charalambopoulos, G. Dassios and A. S. Fokas

Journal:
Quart. Appl. Math. **68** (2010), 645-660

MSC (2000):
Primary 35C15, 35J05, 35J25

DOI:
https://doi.org/10.1090/S0033-569X-2010-01168-X

Published electronically:
September 21, 2010

MathSciNet review:
2761508

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Abstract: A general method for studying boundary value problems for linear and for integrable nonlinear partial differential equations in two dimensions was introduced in Fokas, 1997. For linear equations in a convex polygon (Fokas and Kapaev (2000) and (2003), and Fokas (2001)), this method: (a) expresses the solution in the form of an integral (generalized inverse Fourier transform) in the complex -plane involving a certain function (generalized direct Fourier transform) that is defined as an integral along the boundary of the polygon, and (b) characterizes a generalized Dirichlet-to-Neumann map by analyzing the so-called *global relation*. For simple polygons and simple boundary conditions, this characterization is *explicit*. Here, we extend the above method to the case of elliptic partial differential equations in the exterior of a convex polygon and we illustrate the main ideas by studying the Laplace equation in the exterior of an equilateral triangle.

Regarding (a), we show that whereas is identical with that of the interior problem, the contour of integration in the complex -plane appearing in the formula for depends on . Regarding (b), we show that the global relation is now replaced by a *set* of appropriate relations which, in addition to the boundary values, also involve certain additional unknown functions. In spite of this significant complication we show that, for certain simple boundary conditions, the exterior problem for the Laplace equation can be mapped to the solution of a Dirichlet problem formulated in the interior of a convex polygon formed by three sides.

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Additional Information

**A. Charalambopoulos**

Affiliation:
Department of Material Science and Engineering, University of Ioannina, Greece

**G. Dassios**

Affiliation:
Department of Applied Mathematics and Theoretical Physics, University of Cambridge, United Kingdom

**A. S. Fokas**

Affiliation:
Department of Applied Mathematics and Theoretical Physics, University of Cambridge, United Kingdom

DOI:
https://doi.org/10.1090/S0033-569X-2010-01168-X

Keywords:
Laplace equation,
equilateral triangle,
exterior domain

Received by editor(s):
January 29, 2009

Received by editor(s) in revised form:
February 18, 2009

Published electronically:
September 21, 2010

Additional Notes:
The second author is on leave from the University of Patras and ICE-HT/FORTH Greece. His current address is the Division of Applied Mathematics, Department of Chemical Engineering, University of Patras, GR 265 04 Patras, Greece

The present work was performed under the Marie Curie Chair of Excellence Project BRAIN, granted to the second and the third author by the European Commission under code number EXC 023928

Article copyright:
© Copyright 2010
Brown University