# Cauchy-Riemann equations

The following system of partial differential equations

 $\frac{\partial u}{\partial x}=\frac{\partial v}{\partial y},\quad\frac{% \partial u}{\partial y}=-\frac{\partial v}{\partial x},$

where $u(x,y),v(x,y)$ are real-valued functions defined on some open subset of $\mathbb{R}^{2}$, was introduced by Riemann[1] as a definition of a holomorphic function. Indeed, if $f(z)$ satisfies the standard definition of a holomorphic function, i.e. if the complex derivative

 $f^{\prime}(z)=\lim_{\zeta\rightarrow 0}\frac{f(z+\zeta)-f(z)}{\zeta}$

exists in the domain of definition, then the real and imaginary parts of $f(z)$ satisfy the Cauchy-Riemann equations. Conversely, if $u$ and $v$ satisfy the Cauchy-Riemann equations, and if their partial derivatives are continuous, then the complex valued function

 $f(z)=u(x,y)+iv(x,y),\quad z=x+iy,$

possesses a continuous complex derivative.

## References

1. 1.

D. Laugwitz, Bernhard Riemann, 1826-1866: Turning points in the Conception of Mathematics, translated by Abe Shenitzer. Birkhauser, 1999.

Title Cauchy-Riemann equations CauchyRiemannEquations 2013-03-22 12:55:36 2013-03-22 12:55:36 rmilson (146) rmilson (146) 5 rmilson (146) Definition msc 30E99 Holomorphic