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Let $f$ be a harmonic function. Prove that $\overline{f}$ is harmonic.

I need help to write a rigorous proof. Thank you

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    @yagmour: that is not the definition... It is exactly backwards: **if** $f_{xx}+f_{yy}=0$, **then** $f$ is (by definition) harmonic.2012-09-21

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We have $f_{xx}=\partial_{xx} u(x,y)+i\partial_{xx}v(x,y)$ and the same for the derivative in $y$, hence $\Delta f(x,y)=\Delta u(x,y)+i\Delta v(x,y)$. As $u$ and $v$ admit real values, so does $\Delta u$ and $\Delta v$, hence $\Delta u=0=\Delta v$.

As $\Delta\bar f(x,y)=\Delta u(x,y)-i\Delta v(x,y)$, we get that $\bar f$ is harmonic if (and only if) so is $f$.