Let $\displaystyle u: \Omega \longrightarrow \mathbb{R}$ be a harmonic function and $\displaystyle f:\Omega_1 \longrightarrow \Omega$ a diffeomorphism. Is $\displaystyle uf$ (composition) harmonic in $\displaystyle \Omega_1$? I know I could compute the laplacian, but is there any other method to do it?.