Complex Analysis: Theory and Applications · Problem 10.3.5

Applications of Harmonic Functions — Howell & Matthews

Mathematics — Complex Analysis · poisson-integral-formula

Problem

Show that the function ϕ(x,y)\phi(x,y) given by Poisson's integral formula is harmonic by applying Leibniz's rule, which permits you to write (∂2∂x2+∂2∂y2)ϕ(x,y)=1π∫−∞∞U(t)[(∂2∂x2+∂2∂y2)y(x−t)2+y2]dt.\left(\frac{\partial^2}{\partial x^2} + \frac{\partial^2}{\partial y^2}\right)\phi(x,y) = \frac{1}{\pi}\int_{-\infty}^{\infty} U(t)\left[\left(\frac{\partial^2}{\partial x^2} + \frac{\partial^2}{\partial y^2}\right)\frac{y}{(x-t)^2+y^2}\right]dt.

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