# Scherk surface

2010 Mathematics Subject Classification: Primary: 53A10 Secondary: 49Q05 [MSN][ZBL]

A minimal surface discovered by H. Scherk in 1834. It is defined by the equation $z = \ln \frac{\cos y}{\cos x} = \ln \cos y - \ln \cos x$ and it is the only minimal surface that can be represented as a translation surface of the form $z = f(x) + g (y)$. Scherk's surface and its modifications are used for the construction of auxiliary functions that allow one to find examples of the unsolvability of the Dirichlet problem for the Euler–Lagrange equation for minimal surfaces over non-convex domains.

Scherk's surface possesses a number of interesting properties:

• it is a complete surface of infinite genus containing a countable number of straight lines;
• its universal covering surface presents an example of a complete minimal surface of conformally-hyperbolic type;
• its Gauss map omits exactly four points.

The last property of Scherk's surface becomes evident from its Weierstrass representation. More precisely if we consider the meromorphic function $w \mapsto f(w) = \frac{2}{1-w^4}$ and the holomorphic function $f (w) =w$, Scherk's surface can be parameterized as $\begin{array}{lll} x (r, \phi) &= \mathcal{Re}\, \int_0^\omega f (1-g^2)\, dw &= \mathcal{Re}\, (ln (1+ i \omega) + \ln (1- i \omega))\\ y (r, \phi) &= \mathcal{Re}\, \int_0^\omega i f (1+g^2)\, dw & = \mathcal{Re}\, i ( \ln (1+\omega) + \ln (1-\omega))\\ z (r, \phi) &= \mathcal{Re}\, \int_0^\omega 2 f g\, dw & = \mathcal{Re}\, \left[ \ln \frac{1-\omega}{1+\omega} + i \ln \frac{1+i\omega}{1-i\omega}\right] \, , \end{array}$ where $\omega = r e^{i\phi}$. The four points missing in the image of the Gauss map corresponds then to the four poles $\{\pm 1, \pm i\}$ of $f$.

By analogy with this representation one may construct the generalized Scherk surfaces using $f (w) = [(w-w_1) (w-w_2) (w-w_3) (w-w_4)]^{-1}$ with arbitrary distinct points $w_1, w_2, w_3, w_4$ in the complex plane. The Gauss map of this minimal surface omits precisely $4$ arbitrarily assigned points on $\mathbb S^2$.

The existence of such minimal surfaces motivated the conjecture that the only complete minimal surfaces in $\mathbb R^3$ whose (oriented) normals omit more than four directions are the planes. After several improvements of a classical result of Osserman, the conjecture was finally proved in [Fu]; see also [Xa].

Scherk's surface belongs to the family of so-called periodic minimal surfaces; a picture of it, and other interesting properties, can be found in [TF].

How to Cite This Entry:
Scherk surface. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Scherk_surface&oldid=32370
This article was adapted from an original article by I.Kh. Sabitov (originator), which appeared in Encyclopedia of Mathematics - ISBN 1402006098. See original article