In complex analysis, a Schwarz–Christoffel mapping is a conformal map of the upper half-plane or the complex unit disk onto the interior of a simple polygon. Such a map is guaranteed to exist by the Riemann mapping theorem (stated by Bernhard Riemann in 1851); the Schwarz–Christoffel formula provides an explicit construction. They were introduced independently by Elwin Christoffel in 1867 and Hermann Schwarz in 1869.
to the interior of the polygon. The function f maps the real axis to the edges of the polygon. If the polygon has interior angles , then this mapping is given by
where is a constant, and are the values, along the real axis of the plane, of points corresponding to the vertices of the polygon in the plane. A transformation of this form is called a Schwarz–Christoffel mapping.
The integral can be simplified by mapping the point at infinity of the plane to one of the vertices of the plane polygon. By doing this, the first factor in the formula becomes constant and so can be absorbed into the constant . Conventionally, the point at infinity would be mapped to the vertex with angle .
In practice, to find a mapping to a specific polygon one needs to find the values which generate the correct polygon side lengths. This requires solving a set of nonlinear equations, and in most cases can only be done numerically.
Consider a semi-infinite strip in the z plane. This may be regarded as a limiting form of a triangle with vertices P = 0, Q = π i, and R (with R real), as R tends to infinity. Now α = 0 and β = γ = π⁄2 in the limit. Suppose we are looking for the mapping f with f(−1) = Q, f(1) = P, and f(∞) = R. Then f is given by
Evaluation of this integral yields
where C is a (complex) constant of integration. Requiring that f(−1) = Q and f(1) = P gives C = 0 and K = 1. Hence the Schwarz–Christoffel mapping is given by
This transformation is sketched below.
Other simple mappings
A mapping to a plane triangle with interior angles and is given by
which can be expressed in terms of hypergeometric functions.
The upper half-plane is mapped to the square by
where F is the incomplete elliptic integral of the first kind.
The upper half-plane is mapped to a triangle with circular arcs for edges by the Schwarz triangle map.
- The Schwarzian derivative appears in the theory of Schwarz–Christoffel mappings.
- Christoffel, Elwin Bruno (1867). "Sul problema delle temperature stazionarie e la rappresentazione di una data superficie" [On the problem of stationary temperatures and the representation of a given surface]. Annali di Matematica Pura ed Applicata (in Italian). 1: 89–103. doi:10.1007/BF02419161.
- Driscoll, Tobin A.; Trefethen, Lloyd N. (2002). Schwarz–Christoffel Mapping. Cambridge University Press. doi:10.1017/CBO9780511546808.
- Schwarz, Hermann Amandus (1869). "Ueber einige Abbildungsaufgaben" [About some mapping problems]. Crelle's Journal (in German). 70: 105–120. doi:10.1515/crll.1869.70.105.
- Forsyth, Andrew Russell (1918) [1st ed. 1893]. Theory of Functions of a Complex Variable. Cambridge. §§267–270, pp. 665–677.
- Nehari, Zeev (1982) , Conformal mapping, New York: Dover Publications, ISBN 978-0-486-61137-2, MR 0045823
- The Conformal Hyperbolic Square and Its Ilk Chamberlain Fong, Bridges Finland Conference Proceedings, 2016
An analogue of SC mapping that works also for multiply-connected is presented in: Case, James (2008), "Breakthrough in Conformal Mapping" (PDF), SIAM News, 41 (1).