My book says that for any holomorphic function $f(z)=u(x,y)+iv(x,y)$, $u$ and $v$ satisfy Laplace's equation.
$f$ is holomorphic $\implies$ * $u_x=v_y$ and $u_y=-v_x$, so $u_{xx}=v_{yx}=v_{xy}=(-u_y)_y=-u_{yy}$. My question is in regards to symmetry of the mixed partials $v_{yx}=v_{xy}$. From what I understand, if $f$ is holomorphic, all we know (from the Cauchy-Riemann eqs) is that the first partials for $u$ and $v$ satisfy *, but nothing about the existence or continuity of the mixed partials. Is this just an extra implicit assumption that $u$ and $v$ satisfy Clairaut's theorem, or does this actually follow from $f$ being holomorphic?
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Summary of comments by Daniel Fischer: