Complex Analysis: Theory and Applications · Problem 6.6.8

Taylor and Laurent Series — Howell & Matthews

Mathematics — Complex Analysis · harmonic-functions-mean-value

Problem

Let u(x,y)u(x,y) be harmonic for all (x,y)(x,y). Show that u(x0,y0)=12π∫02πu ⁣(x0+Rcos⁡θ,  y0+Rsin⁡θ)dθ,u(x_0,y_0) = \frac{1}{2\pi}\int_0^{2\pi} u\!\left(x_0 + R\cos\theta,\; y_0 + R\sin\theta\right)d\theta, where R>0R > 0. Hint: Let f(z)=u(x,y)+iv(x,y)f(z) = u(x,y) + iv(x,y), where vv is a harmonic conjugate of uu.

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