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Given a $C^2$ convex function $f$ and $u$ a harmonic function in an open subset of $\mathbb{R^2}$, how can I show that $f(u)$ is sub-harmonic?

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    use Jensen's inequality to show averages over balls smaller than value at center, (or bigger, whichever is right).2012-05-25

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You can also simply compute derivatives since everything is differentiable enough: $ (f(u))_x = f'\cdot u_x\Rightarrow (f(u))_{xx} = f'' u_x^2+f'u_{xx} $ and clearly, $(f(u))_{yy} = f'' u_y^2+f' u_{yy}$. As a result, $ \Delta f(u) = f'' (u_x^2+u_y^2)\geq 0 $ since $f$ is a convex $C^2$ function.

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    just that simple! Thanks very much.2012-05-25