Let $u$ be an Harmonic function in $B(0,a)$ in $R^3$
we define $I(x)=x\dfrac{a^2}{|x|^2} $
Let $w(x) = u(I(x))$.
Is there a way to prove that $w$ is harmonic without making too much computation?
If not I will make them my self. Thanks for your help!