# When is a function satisfying the Cauchy-Riemann equations holomorphic?

It is, of course, one of the first results in basic complex analysis that a holomorphic function satisfies the Cauchy-Riemann equations when considered as a differentiable two-variable real function. I have always seen the converse as: if $f$ is continuously differentiable as a function from $U \subset \mathbb{R}^2$ to $\mathbb{R}^2$ and satisfies the Cauchy-Riemann equations, then it is holomorphic (see e.g. Stein and Shakarchi, or Wikipedia). Why is the $C^1$ condition necessary? I don't see where this comes in to the proof below.

Assume that $u(x,y)$ and $v(x,y)$ are continuously differentiable and satisfy the Cauchy-Riemann equations. Let $h=h_1 + h_2i$. Then
\begin{equation*} u(x+h_1, y+h_2) - u(x,y) = \frac{\partial u}{\partial x} h_1 + \frac{\partial u}{\partial y}h_2 + o(|h|) \end{equation*} and \begin{equation*} v(x+h_1, y+h_2) - v(x,y) = \frac{\partial v}{\partial x} h_1 + \frac{\partial v}{\partial y} h_2 + o(|h|). \end{equation*} Multiplying the second equation by $i$ and adding the two together gives \begin{align*} (u+iv)(z+h)-(u+iv)(z) &= \frac{\partial u}{\partial x} h_1 + i \frac{\partial v}{\partial x} h_1 + \frac{\partial u}{\partial y} h_2 + i \frac{\partial v}{\partial y} h_2 + o(|h|)\\\ &= \left( \frac{\partial u}{\partial x} + i \frac{\partial v}{\partial x} \right) (h_1+i h_2) + o(|h|). \end{align*} Now dividing by $h$ gives us the desired result.

Does there exist a differentiable but not $C^1$ function $f: U \rightarrow \mathbb{R}^2$ which satisfies the Cauchy-Riemann equations and does NOT correspond to a complex-differentiable function?

• The $C^1$ condition in this proof is used as follows: $C^1 \implies$ (Frechet) differentiable $\implies$ the "directional derivative" local linear approximation dot product formula $[\frac{\partial u}{\partial x}, \frac{\partial u}{\partial x}]\cdot [h_1,h_2]+o(|h|)$ (see e.g. Theorems 2.17 and 2.19 in Folland's Advanced Calculus)
– D.R.
Feb 2, 2023 at 7:25

See When is a Function that Satisfies the Cauchy-Riemann Equations Analytic? J. D. Gray and S. A. Morris The American Mathematical Monthly Vol. 85, No. 4 (Apr., 1978), pp. 246-256.

• So that is why the question's title sounded vaguely familiar... :D Dec 30, 2010 at 2:20
• I wonder if there have been any advancements in the past 44 years since this paper (and 11 years since this answer). Mar 23, 2022 at 9:38

There's also the Looman–Menchoff theorem.

• Where can I find proof of this theorem? Dec 7, 2020 at 11:12
• TeX only works inside $-signs here. In order to get italic text and bold text text you can enclose it in single and double asterisks: *italic text* and **bold text**. – t.b. Jun 20, 2011 at 16:51 • Just for completeness:$L^p_{\text{loc}}$suffices for any$p>0\$ as proven in link.springer.com/article/10.1007/BF02807221 Sep 27, 2016 at 10:37