6. Corollary 2.1.1

If we represent $f (x + i y) = u (x, y) + i v (x, y)$ ,
:?:
then $u, v$ are harmonic. i.e.

\frac{\delta {#2u} }{\delta^2x}+\frac{\delta^2u}{\delta^2y}=0

This is a corollary of 3. Theorem 2.1.1 - Cauchy-Riemann (And the theorem of Goursat).