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What are some examples of functions that are differentiable (everywhere) in $\mathbb{R^2}$, but that are not differentiable in the complex plane? We got an example for homework, $f(z)=2xy$, and I was wondering if there were any others.

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    Related: [How is $\mathbb{C}$ different than $\mathbb{R}^2$?](http://math.stackexchange.com/q/5108)2012-02-10

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As some commentors have pointed out, there are many, many other such functions. All that's required is that your function fail to satisfy the Cauchy-Riemann equations -- that is, if $f(z) = u(z) + iv(z)$, the function $f$ will fail to be complex differentiable just in case one of the following fails:

$u_x(z) = v_y(z)$

$u_y(z) = -v_x(z)$.

Here are a few classic examples that are easily seen to be real-differentiable:

$f(z) = \overline z$

$f(z) = |z|$

EDIT: although the latter fails to be real-differentiable on the axes (my mistake).

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    $|z|$ is real differentiable at most points on the axes. The only point where it is not differentiable is the origin.2012-02-12