# Weierstrass–Enneper parameterization

In mathematics, the Weierstrass–Enneper parameterization of minimal surfaces is a classical piece of differential geometry.

Alfred Enneper and Karl Weierstrass studied minimal surfaces as far back as 1863.

Let ${\displaystyle f}$ and ${\displaystyle g}$ be functions on either the entire complex plane or the unit disk, where ${\displaystyle g}$ is meromorphic and ${\displaystyle f}$ is analytic, such that wherever ${\displaystyle g}$ has a pole of order ${\displaystyle m}$, ${\displaystyle f}$ has a zero of order ${\displaystyle 2m}$ (or equivalently, such that the product ${\displaystyle fg^{2}}$ is holomorphic), and let ${\displaystyle c_{1},c_{2},c_{3}}$ be constants. Then the surface with coordinates ${\displaystyle (x_{1},x_{2},x_{3})}$ is minimal, where the ${\displaystyle x_{k}}$ are defined using the real part of a complex integral, as follows:

{\displaystyle {\begin{aligned}x_{k}(\zeta )&{}=\mathrm {Re} \left\{\int _{0}^{\zeta }\varphi _{k}(z)\,dz\right\}+c_{k},\qquad k=1,2,3\\\varphi _{1}&{}=f(1-g^{2})/2\\\varphi _{2}&{}=if(1+g^{2})/2\\\varphi _{3}&{}=fg\end{aligned}}}

The converse is also true: every nonplanar minimal surface defined over a simply connected domain can be given a parametrization of this type.[1]

For example, Enneper's surface has f(z) = 1, g(z) = zm.

## Parametric surface of complex variables

The Weierstrass-Enneper model defines a minimal surface ${\displaystyle X}$ (${\displaystyle \mathbb {R} ^{3}}$) on a complex plane (${\displaystyle \mathbb {C} }$). Let ${\displaystyle \omega =u+vi}$ (the complex plane as the ${\displaystyle uv}$ space), the Jacobian matrix of the surface can be written as a column of complex entries:

${\displaystyle \mathbf {J} ={\begin{bmatrix}\left(1-g^{2}(\omega )\right)f(\omega )\\i\left(1+g^{2}(\omega )\right)f(\omega )\\2g(\omega )f(\omega )\end{bmatrix}}}$
where ${\displaystyle f(\omega )}$ and ${\displaystyle g(\omega )}$ are holomorphic functions of ${\displaystyle \omega }$.

The Jacobian ${\displaystyle \mathbf {J} }$ represents the two orthogonal tangent vectors of the surface:[2]

${\displaystyle \mathbf {X_{u}} ={\begin{bmatrix}\operatorname {Re} \mathbf {J} _{1}\\\operatorname {Re} \mathbf {J} _{2}\\\operatorname {Re} \mathbf {J} _{3}\end{bmatrix}}\;\;\;\;\mathbf {X_{v}} ={\begin{bmatrix}-\operatorname {Im} \mathbf {J} _{1}\\-\operatorname {Im} \mathbf {J} _{2}\\-\operatorname {Im} \mathbf {J} _{3}\end{bmatrix}}}$

The surface normal is given by

${\displaystyle \mathbf {\hat {n}} ={\frac {\mathbf {X_{u}} \times \mathbf {X_{v}} }{|\mathbf {X_{u}} \times \mathbf {X_{v}} |}}={\frac {1}{|g|^{2}+1}}{\begin{bmatrix}2\operatorname {Re} g\\2\operatorname {Im} g\\|g|^{2}-1\end{bmatrix}}}$

The Jacobian ${\displaystyle \mathbf {J} }$ leads to a number of important properties: ${\displaystyle \mathbf {X_{u}} \cdot \mathbf {X_{v}} =0}$, ${\displaystyle \mathbf {X_{u}} ^{2}=\operatorname {Re} (\mathbf {J} ^{2})}$, ${\displaystyle \mathbf {X_{v}} ^{2}=\operatorname {Im} (\mathbf {J} ^{2})}$, ${\displaystyle \mathbf {X_{uu}} +\mathbf {X_{vv}} =0}$. The proofs can be found in Sharma's essay: The Weierstrass representation always gives a minimal surface.[3] The derivatives can be used to construct the first fundamental form matrix:

${\displaystyle {\begin{bmatrix}\mathbf {X_{u}} \cdot \mathbf {X_{u}} &\;\;\mathbf {X_{u}} \cdot \mathbf {X_{v}} \\\mathbf {X_{v}} \cdot \mathbf {X_{u}} &\;\;\mathbf {X_{v}} \cdot \mathbf {X_{v}} \end{bmatrix}}={\begin{bmatrix}1&0\\0&1\end{bmatrix}}}$

and the second fundamental form matrix

${\displaystyle {\begin{bmatrix}\mathbf {X_{uu}} \cdot \mathbf {\hat {n}} &\;\;\mathbf {X_{uv}} \cdot \mathbf {\hat {n}} \\\mathbf {X_{vu}} \cdot \mathbf {\hat {n}} &\;\;\mathbf {X_{vv}} \cdot \mathbf {\hat {n}} \end{bmatrix}}}$

Finally, a point ${\displaystyle \omega _{t}}$ on the complex plane maps to a point ${\displaystyle \mathbf {X} }$ on the minimal surface in ${\displaystyle \mathbb {R} ^{3}}$ by

${\displaystyle \mathbf {X} ={\begin{bmatrix}\operatorname {Re} \int _{\omega _{0}}^{\omega _{t}}\mathbf {J} _{1}d\omega \\\operatorname {Re} \int _{\omega _{0}}^{\omega _{t}}\mathbf {J} _{2}d\omega \\\operatorname {Re} \int _{\omega _{0}}^{\omega _{t}}\mathbf {J} _{3}d\omega \end{bmatrix}}}$
where ${\displaystyle \omega _{0}=0}$ for all minimal surfaces throughout this paper except for Costa's minimal surface where ${\displaystyle \omega _{0}=(1+i)/2}$.

## Embedded minimal surfaces and examples

The classical examples of embedded complete minimal surfaces in ${\displaystyle \mathbb {R} ^{3}}$ with finite topology include the plane, the catenoid, the helicoid, and the Costa's minimal surface. Costa's surface involves Weierstrass's elliptic function ${\displaystyle \wp }$:[4]

${\displaystyle g(\omega )={\frac {A}{\wp '(\omega )}}}$
${\displaystyle f(\omega )=\wp (\omega )}$
where ${\displaystyle A}$ is a constant.[5]

### Helicatenoid

Choosing the functions ${\displaystyle f(\omega )=e^{-i\alpha }e^{\omega /A}}$ and ${\displaystyle g(\omega )=e^{-\omega /A}}$, a one parameter family of minimal surfaces is obtained.

${\displaystyle \varphi _{1}=e^{-i\alpha }\sinh \left({\frac {\omega }{A}}\right)}$
${\displaystyle \varphi _{2}=ie^{-i\alpha }\cosh \left({\frac {\omega }{A}}\right)}$
${\displaystyle \varphi _{3}=e^{-i\alpha }}$
${\displaystyle \mathbf {X} (\omega )=\operatorname {Re} {\begin{bmatrix}e^{-i\alpha }A\cosh \left({\frac {\omega }{A}}\right)\\ie^{-i\alpha }A\sinh \left({\frac {\omega }{A}}\right)\\e^{-i\alpha }\omega \\\end{bmatrix}}=\cos(\alpha ){\begin{bmatrix}A\cosh \left({\frac {\operatorname {Re} (\omega )}{A}}\right)\cos \left({\frac {\operatorname {Im} (\omega )}{A}}\right)\\-A\cosh \left({\frac {\operatorname {Re} (\omega )}{A}}\right)\sin \left({\frac {\operatorname {Im} (\omega )}{A}}\right)\\\operatorname {Re} (\omega )\\\end{bmatrix}}+\sin(\alpha ){\begin{bmatrix}A\sinh \left({\frac {\operatorname {Re} (\omega )}{A}}\right)\sin \left({\frac {\operatorname {Im} (\omega )}{A}}\right)\\A\sinh \left({\frac {\operatorname {Re} (\omega )}{A}}\right)\cos \left({\frac {\operatorname {Im} (\omega )}{A}}\right)\\\operatorname {Im} (\omega )\\\end{bmatrix}}}$

Choosing the parameters of the surface as ${\displaystyle \omega =s+i(A\phi )}$:

${\displaystyle \mathbf {X} (s,\phi )=\cos(\alpha ){\begin{bmatrix}A\cosh \left({\frac {s}{A}}\right)\cos \left(\phi \right)\\-A\cosh \left({\frac {s}{A}}\right)\sin \left(\phi \right)\\s\\\end{bmatrix}}+\sin(\alpha ){\begin{bmatrix}A\sinh \left({\frac {s}{A}}\right)\sin \left(\phi \right)\\A\sinh \left({\frac {s}{A}}\right)\cos \left(\phi \right)\\A\phi \\\end{bmatrix}}}$

At the extremes, the surface is a catenoid ${\displaystyle (\alpha =0)}$ or a helicoid ${\displaystyle (\alpha =\pi /2)}$. Otherwise, ${\displaystyle \alpha }$ represents a mixing angle. The resulting surface, with domain chosen to prevent self-intersection, is a catenary rotated around the ${\displaystyle \mathbf {X} _{3}}$ axis in a helical fashion.

## Lines of curvature

One can rewrite each element of second fundamental matrix as a function of ${\displaystyle f}$ and ${\displaystyle g}$, for example

${\displaystyle \mathbf {X_{uu}} \cdot \mathbf {\hat {n}} ={\frac {1}{|g|^{2}+1}}{\begin{bmatrix}\operatorname {Re} \left((1-g^{2})f'-2gfg'\right)\\\operatorname {Re} \left((1+g^{2})f'i+2gfg'i\right)\\\operatorname {Re} \left(2gf'+2fg'\right)\\\end{bmatrix}}\cdot {\begin{bmatrix}\operatorname {Re} \left(2g\right)\\\operatorname {Re} \left(-2gi\right)\\\operatorname {Re} \left(|g|^{2}-1\right)\\\end{bmatrix}}=-2\operatorname {Re} (fg')}$

And consequently the second fundamental form matrix can be simplified as

${\displaystyle {\begin{bmatrix}-\operatorname {Re} fg'&\;\;\operatorname {Im} fg'\\\operatorname {Im} fg'&\;\;\operatorname {Re} fg'\end{bmatrix}}}$

One of its eigenvectors is

${\displaystyle {\overline {\sqrt {fg'}}}}$
which represents the principal direction in the complex domain.[6] Therefore, the two principal directions in the ${\displaystyle uv}$ space turn out to be
${\displaystyle \phi =-{\frac {1}{2}}\operatorname {Arg} (fg')\pm k\pi /2}$