An analytic function $f(z)$ of complex variable $z=x+iy$ may be written as $f(z)=u(x,y)+iv(x,y)$. Then $u(x,y)$ and $v(x,y)$ must satisfy
- $\dfrac{\partial u}{ \partial x} = \dfrac{\partial v}{ \partial y} \text{ and } \dfrac{\partial u}{ \partial y} = \dfrac{\partial v}{ \partial x} \\$
- $\dfrac{\partial u}{ \partial x} = \dfrac{\partial v}{ \partial y} \text{ and } \dfrac{\partial u}{ \partial y} = – \dfrac{\partial v}{ \partial x} \\$
- $\dfrac{\partial u}{ \partial x} = – \dfrac{\partial v}{ \partial y} \text{ and } \dfrac{\partial u}{ \partial y} = \dfrac{\partial v}{ \partial x} \\$
- $\dfrac{\partial u}{ \partial x} = – \dfrac{\partial v}{ \partial y} \text{ and } \dfrac{\partial u}{ \partial y} = – \dfrac{\partial v}{ \partial x} $