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In complex analysis, the **Riemann mapping theorem** states that if *U* is a non-empty simply connected open subset of the complex number plane **C** which is not all of **C**, then there exists a biholomorphic mapping *f* (i.e. a bijective holomorphic mapping whose inverse is also holomorphic) from *U* onto the open unit disk

- History
- Importance
- Proof via normal families
- Simple connectivity
- Riemann mapping theorem
- Parallel slit mappings
- Sketch proof via Dirichlet problem
- Uniformization theorem
- Smooth Riemann mapping theorem
- Algorithms
- See also
- Notes
- References
- External links

This mapping is known as a **Riemann mapping**.^{ [1] }

Intuitively, the condition that *U* be simply connected means that *U* does not contain any “holes”. The fact that *f* is biholomorphic implies that it is a conformal map and therefore angle-preserving. Intuitively, such a map preserves the shape of any sufficiently small figure, while possibly rotating and scaling (but not reflecting) it.

Henri Poincaré proved that the map *f* is essentially unique: if *z*_{0} is an element of *U* and φ is an arbitrary angle, then there exists precisely one *f* as above such that *f*(*z*_{0}) = 0 and such that the argument of the derivative of *f* at the point *z*_{0} is equal to φ. This is an easy consequence of the Schwarz lemma.

As a corollary of the theorem, any two simply connected open subsets of the Riemann sphere which both lack at least two points of the sphere can be conformally mapped into each other.

The theorem was stated (under the assumption that the boundary of *U* is piecewise smooth) by Bernhard Riemann in 1851 in his PhD thesis. Lars Ahlfors wrote once, concerning the original formulation of the theorem, that it was “ultimately formulated in terms which would defy any attempt of proof, even with modern methods”.^{ [2] } Riemann's flawed proof depended on the Dirichlet principle (which was named by Riemann himself), which was considered sound at the time. However, Karl Weierstrass found that this principle was not universally valid. Later, David Hilbert was able to prove that, to a large extent, the Dirichlet principle is valid under the hypothesis that Riemann was working with. However, in order to be valid, the Dirichlet principle needs certain hypotheses concerning the boundary of *U* which are not valid for simply connected domains in general.

The first rigorous proof of the theorem was given by William Fogg Osgood in 1900. He proved the existence of Green's function on arbitrary simply connected domains other than **C** itself; this established the Riemann mapping theorem.^{ [3] }

Constantin Carathéodory gave another proof of the theorem in 1912, which was the first to rely purely on the methods of function theory rather than potential theory.^{ [4] } His proof used Montel's concept of normal families, which became the standard method of proof in textbooks.^{ [5] } Carathéodory continued in 1913 by resolving the additional question of whether the Riemann mapping between the domains can be extended to a homeomorphism of the boundaries (see Carathéodory's theorem).^{ [6] }

Carathéodory's proof used Riemann surfaces and it was simplified by Paul Koebe two years later in a way that did not require them. Another proof, due to Lipót Fejér and to Frigyes Riesz, was published in 1922 and it was rather shorter than the previous ones. In this proof, like in Riemann's proof, the desired mapping was obtained as the solution of an extremal problem. The Fejér–Riesz proof was further simplified by Alexander Ostrowski and by Carathéodory.^{[ citation needed ]}

The following points detail the uniqueness and power of the Riemann mapping theorem:

- Even relatively simple Riemann mappings (for example a map from the interior of a circle to the interior of a square) have no explicit formula using only elementary functions.
- Simply connected open sets in the plane can be highly complicated, for instance, the boundary can be a nowhere-differentiable fractal curve of infinite length, even if the set itself is bounded. The fact that such a set can be mapped in an
*angle-preserving*manner to the nice and regular unit disc seems counter-intuitive. - The analog of the Riemann mapping theorem for more complicated domains is not true. The next simplest case is of doubly connected domains (domains with a single hole). Any doubly connected domain except for the punctured disk and the punctured plane is conformally equivalent to some annulus {
*z*:*r*< |*z*| < 1} with 0 <*r*< 1, however there are no conformal maps between annuli except inversion and multiplication by constants so the annulus {*z*: 1 < |*z*| < 2} is not conformally equivalent to the annulus {*z*: 1 < |*z*| < 4} (as can be proven using extremal length). - The analogue of the Riemann mapping theorem in three or more real dimensions is not true. The family of conformal maps in three dimensions is very poor, and essentially contains only Möbius transformations (see Liouville's theorem).
- Even if arbitrary homeomorphisms in higher dimensions are permitted, contractible manifolds can be found that are not homeomorphic to the ball (e.g., the Whitehead continuum).
- The analogue of the Riemann mapping theorem in several complex variables is also not true. In (), the ball and polydisk are both simply connected, but there is no biholomorphic map between them.
^{ [7] }

**Theorem.** For an open domain *G* ⊂ ℂ the following conditions are equivalent:^{ [8] }

*G*is simply connected;- the integral of every holomorphic function
*f*around a closed piecewise smooth curve in*G*vanishes; - every holomorphic function in
*G*is the derivative of a holomorphic function; - every nowhere-vanishing holomorphic function
*f*on*G*has a holomorphic logarithm; - every nowhere-vanishing holomorphic function
*g*on*G*has a holomorphic square root; - for any
*w*not in*G*, the winding number of*w*for any piecewise smooth closed curve in*G*is 0; - the complement of
*G*in the extended complex plane ℂ ∪ {∞} is connected.

(1) ⇒ (2) because any continuous closed curve, with base point *a* in *G*, can be continuously deformed to the constant curve *a*. So the line integral of *f**dz* over the curve is 0.

(2) ⇒ (3) because the integral over any piecewise smooth path γ from *a* to *z* can be used to define a primitive.

(3) ⇒ (4) by integrating *f*^{−1}*df*/*dz* along γ from *a* to *x* to give a branch of the logarithm.

(4) ⇒ (5) by taking the square root as *g* (*z*) = exp *f*(*z*)/2 where *f* is a holomorphic choice of logarithm.

(5) ⇒ (6) because if γ is a piecewise closed curve and *f*_{n} are successive square roots of *z* − *w* for *w* outside *G*, then the winding number of *f*_{n} ∘ γ about *w* is 2^{n} times the winding number of γ about 0. Hence the winding number of γ about *w* must be divisible by 2^{n} for all *n*, so must equal 0.

(6) ⇒ (7) for otherwise the extended plane ℂ ∪ {∞} \ *G* can be written as the disjoint union of two open and closed sets *A* and *B* with ∞ in *B* and *A* bounded. Let δ > 0 be the shortest euclidean distance been *A* and *B* and build a square grid on ℂ with length δ/4 with a point *a* of *A* at the centre of a square. Let *C* be the compact set of the union of all squares with distance ≤ δ/4 from *A*. Then *C* ∩ *B* = ∅ and ∂*C* does not meet *A* or *B*: it consists of finitely many horizontal and vertical segments in *G* forming a finite number of closed rectangular paths γ_{j} in *G* . Taking *C*_{i} to be all the squares covering *A*, the (2 π)^{−1} ∫_{∂C} d arg(*z* − *a*) equals the sum of the winding numbers of *C*_{i} over *a*, so gives 1. On the other hand the sum of the winding numbers of γ_{j} about *a* equals 1. Hence the winding number of at least one of the γ_{j} about *a* is non-zero.

(7) ⇒ (1) This is a purely topological argument. Let γ be a piecewise smooth closed curve based at *z*_{0} in *G*. By approximation γ is in the same homotopy class as a rectangular path on the square grid of length δ > 0 based at *z*_{0}; such a rectangular path is determined by a succession of *N* consecutive directed vertical and horizontal sides. By induction on *N*, such a path can be deformed to a constant path at a corner of the grid. If the path intersects at a point *z*_{1}, then it breaks up into two rectangular paths of length < *N*, so can be deformed to the constant path at *z*_{1} by the induction hypothesis and elementary properties of the fundamental group. The reasoning follows a "northeast argument":^{ [9] }^{ [10] } in the non self-intersecting path there will be a corner *z*_{0} with largest real part (easterly) and then amongst those one with largest imaginary part (northerly). Reversing direction if need be, the path go from *z*_{0} − δ to *z*_{0} and then to *w*_{0} = *z*_{0} − *i**n* δ for *n* ≥ 1 and then goes leftwards to *w*_{0} − δ. Let *R* be the open rectangle with these vertices. The winding number of the path is 0 for points to the right of the vertical segment from *z*_{0} to *w*_{0} and −1 for points to the right; and hence inside *R*. Since the winding number is 0 off *G*, *R* lies in *G*. If *z* is a point of the path, it must lie in *G*; if *z* is on ∂*R* but not on the path, by continuity the winding number of the path about *z* is −1, so *z* must also lie in *G*. Hence *R* ∪ ∂*R* ⊂ *G*. But in this case the path can be deformed by replacing the three sides of the rectangle by the fourth, resulting in 2 less sides. (Self-intersections are permitted.)

**Weierstrass's convergence theorem.**The uniform limit on compacta of a sequence of holomorphic functions is holomorphic; similarly for derivatives.

- This is an immediate consequence of Morera's theorem for the first statement. Cauchy's integral formula gives a formula for the derivatives which can be used to check that the derivatives also converge uniformly on compacta.
^{ [11] }

- This is an immediate consequence of Morera's theorem for the first statement. Cauchy's integral formula gives a formula for the derivatives which can be used to check that the derivatives also converge uniformly on compacta.

**Hurwitz's theorem.**If a sequence of nowhere-vanishing holomorphic functions on an open domain has a uniform limit on compacta, then either the limit is identically zero or the limit is nowhere-vanishing. If a sequence of univalent holomorphic functions on an open domain has a uniform limit on compacta, then either the limit is constant or the limit is univalent.

- If the limit function is non-zero, then its zeros have to be isolated. Zeros with multiplicities can be counted by the winding number (2
*i*π)^{−1}∫_{C}*g*(*z*)^{−1}*g*‘(*z*)*dz*for a holomorphic function*g*. Hence winding numbers are continuous under uniform limits, so that if each function in the sequence has no zeros nor can the limit. For the second statement suppose that*f*(*a*) =*f*(*b*) and set*g*_{n}(*z*) =*f*_{n}(*z*) −*f*_{n}(*a*). These are nowhere-vanishing on a disk but*g*(*z*) =*f*(*z*) −*f*(*b*) vanishes at*a*, so*g*must vanish identically.^{ [12] }

- If the limit function is non-zero, then its zeros have to be isolated. Zeros with multiplicities can be counted by the winding number (2

**Definitions.** A family of holomorphic functions on an open domain is said to be *normal* if any sequence of functions in has a subsequence that converges to a holomorphic function uniformly on compacta. A family is *compact* if whenever a sequence *f*_{n} lies in and converges uniformly to *f* on compacta, then *f* also lies in . A family is said to be *locally bounded* if their functions are uniformly bounded on each compact disk. Differentiating the Cauchy integral formula, it follows that the derivatives of a locally bounded family are also locally bounded.^{ [13] }^{ [14] }

**Montel's theorem.**Every locally bounded family of holomorphic functions in a domain*G*is normal.

- Let
*f*_{n}be a totally bounded sequence and chose a countable dense subset*w*_{m}of*G*. By locally boundedness and a "diagonal argument", a subsequence can be chosen so that*g*_{n}is convergent at each point*w*_{m}. It must be verified that this sequence of holomorphic functions converges on*G*uniformly on each compactum*K*. Take*E*open with*K*⊂*E*such that the closure of*E*is compact and contains*G*. Since the sequence (*g*_{n}′) is locally bounded, |*g*_{n}′| ≤*M*on*E*. By compactness, if δ > 0 is taken small enough, finitely many open disks*D*_{k}of radius δ > 0 are required to cover*K*while remaining in*E*. Since- ,

- || ≤
*M*|*a*−*b*| ≤ 2 δ*M*. Now for each*k*choose some*w*_{i}in*D*_{k}where*g*_{n}(*w*_{i}) converges, taking*n*and*m*so large to be within δ of its limit. Then for*z*in*D*_{k}, - Hence the sequence (
*g*_{n}) forms a Cauchy sequence in the uniform norm on*K*as required.^{ [15] }^{ [16] }

- Let

**Riemann mapping theorem.**If*G*is a simply connected domain ≠ ℂ and*a*lies in*G*, there is a unique conformal mapping*f*of*G*onto the unit disk*D*normalized such that*f*(*a*) = 0 and*f*′(*a*) > 0.

- Uniqueness follows because of
*f*and*g*satisfied the same conditions*h*=*f*∘*g*^{−1}would be a univalent holomorphic map of the unit disk with*h*(0) = 0 and*h*‘(0) >0. But by the Schwarz lemma, the univalent holomorphic maps of the unit disk onto itself are given by the Möbius transformations*k*(*z*) =*e*^{iθ}(*z*− α)/(1 − α**z*) with |α| < 1. So*h*must be the identity map and*f*=*g*. - To prove existence, take to be the family of holomorphic univalent mappings
*f*of*G*into the open unit disk*D*with*f*(*a*) = 0 and*f*‘(*a*) > 0. It is a normal family by Montel's theorem. By the characterization of simple-connectivity, for*b*in ℂ \*G*there is a holomorphic branch of the square root in*G*. It is univalent and*h*(*z*_{1}) ≠ −*h*(*z*_{2}) for*z*_{1}and*z*_{2}in*G*. Since*G*must contain a closed disk Δ with centre*h*(*a*) and radius*r*> 0, no points of −Δ can lie in*G*. Let*F*be the unique Möbius transformation taking ℂ \ −Δ onto*D*with the normalization*F*(*h*(*a*)) = 0 and*F*′(*h*(*a*)) > 0. By construction*F*∘*h*is in , so that is*non-empty*. The method of Koebe is to use an*extremal function*to produce a conformal mapping solving the problem: in this situation it is often called the*Ahlfors function*of*G*, after Ahlfors.^{ [17] }Let 0 <*M*≤ ∞ be the supremum of*f*′(*a*) for*f*in . Pick*f*_{n}in with*f*_{n}′(*a*) tending to*M*. By Montel's theorem, passing to a subsequence if necessary,*f*_{n}tends to a holomorphic function*f*uniformly on compacta. By Hurwitz's theorem,*f*is either univalent or constant. But*f*has*f*(*a*) = 0 and*f*′(*a*) > 0. So*M*is finite, equal to*f*′(*a*) > 0 and*f*lies in . It remains to check that the conformal mapping*f*takes*G**onto**D*. If not, take*c*≠ 0 in*D*\*f*(*G*) and let*H*be a holomorphic square root of (*f*(*z*) −*c*)/(1 −*c***f*(*z*)) on*G*. The function*H*is univalent and maps*G*into*D*. Let*F*(*z*) =*e*^{iθ}(*H*(*z*) −*H*(*a*))/(1 −*H*(*a*)**H*(*z*)) where*H*′(*a*)/|*H*′(*a*)| =*e*^{−iθ}. Then*F*lies in and a routine computation shows that*F*′(*a*) =*H*′(*a*) / (1 − |*H*(*a*)|^{2}) =*f*′(*a*) (√|*c*| +√|*c*|^{−1})/2 >*f*′(*a*) =*M*. This contradicts the maximality of*M*, so that*f*must take all values in*D*.^{ [18] }^{ [19] }^{ [20] }

- Uniqueness follows because of

**Remark.** As a consequence of the Riemann mapping theorem, every simply connected domain in the plane is homeomorphic to the unit disk. If points are omitted, this follows from the theorem. For the whole plane, the homeomorphism φ(*z*) = *z*/(1 + |*z*|) gives a homeomorphism of ℂ onto *D*.

Koebe's uniformization theorem for normal families also generalizes to yield uniformizers *f* for multiply-connected domains to finite **parallel slit domains**, where the slits have angle θ to the *x*-axis. Thus if *G* is a domain in ℂ ∪ {∞} containing ∞ and bounded by finitely many Jordan contours, there is a unique univalent function *f* on *G* with *f*(*z*) = *z*^{−1} + *a*_{1}*z* + *a*_{2}*z*^{2} ⋅⋅⋅ near ∞, maximizing Re *e*^{ −2i θ}*a*_{1} and having image *f*(*G*) a parallel slit domain with angle θ to the *x*-axis.^{ [21] }^{ [22] }^{ [23] }

The first proof that parallel slit domains were canonical domains for in the multiply connected case was given by David Hilbert in 1909. Jenkins (1958), on his book on univalent functions and conformal mappings, gave a treatment based on the work of Herbert Grötzsch and René de Possel from the early 1930s; it was the precursor of quasiconformal mappings and quadratic differentials, later developed as the technique of extremal metric due to Oswald Teichmüller.^{ [24] } Menahem Schiffer gave a treatment based on very general variational principles, summarised in addresses he gave to the International Congress of Mathematicians in 1950 and 1958. In a theorem on "boundary variation" (to distinguish it from "interior variation"), he derived a differential equation and inequality, that relied on a measure-theoretic characterisation of straight-line segments due to Ughtred Shuttleworth Haslam-Jones from 1936. Haslam-Jones' proof was regarded as difficult and was only given a satisfactory proof in the mid-1970s by Schober and Campbell–Lamoureux.^{ [25] }^{ [26] }^{ [27] }

Schiff (1993) gave a proof of uniformization for parallel slit domains which was similar to the Riemann mapping theorem. To simplify notation, horizontal slits will be taken. Firstly, by Bieberbach's inequality, any univalent function *g*(*z*) = *z* + *c**z*^{2} + ··· with *z* in the open unit disk must satisfy |*c*| ≤ 2. As a consequence, if *f*(*z*) = *z* + *a*_{0} + *a*_{1}*z*^{–1} + ··· is univalent in | *z* | > *R*, then | *f*(*z*) – *a*_{0} | ≤ 2 | *z* |: take *S* > *R*, set *g*(*z*) = *S* [*f*(*S*/*z*) – *b*]^{–1} for *z* in the unit disk, choosing *b* so the denominator is nowhere-vanishing, and apply the Schwarz lemma. Next the function *f*_{R}(*z*) = *z* + *R*^{2}/*z* is characterized by an "extremal condition" as the unique univalent function in *z* > *R* of the form *z* + *a*_{1}*z*^{–1} + ··· that maximises Re *a*_{1}: this is an immediate consequence of Grönwall's area theorem, applied to the family of univalent functions *f*(*z**R*) / *R* in *z* > 1.^{ [28] }^{ [29] }

To prove now that the multiply connected domain *G* ⊂ ℂ ∪ {∞} can be uniformized by a horizontal parallel slit conformal mapping *f*(*z*) = *z* + *a*_{1}*z*^{–1} + ···, take *R* large enough that ∂*G* lies in the open disk |*z*| < *R*. For *S* > *R*, univalency and the estimate | *f*(*z*) | ≤ 2 |*z*| imply that, if *z* lies in *G* with | *z* | ≤ *S*, then | *f*(*z*) | ≤ 2*S*. Since the family of univalent *f* are locally bounded in *G* \ {∞}, by Montel's theorem they form a normal family. Furthermore if *f*_{n} is in the family and tends to *f* uniformly on compacta, then *f* is also in the family and each coefficient of the Laurent expansion at ∞ of the *f*_{n} tends to the corresponding coefficient of *f*. This applies in particular to the coefficient: so by compactness there is a univalent *f* which maximizes Re *a*_{1}. To check that *f*(*z*) = *z* + *a*_{1} + ⋅⋅⋅ is the required parallel slit transformation, suppose *reductio ad absurdum* that *f*(*G*) = *G*_{1} has a compact and connected component *K* of its boundary which is not a horizontal slit. Then the complement *G*_{2} of *K* in ℂ ∪ {∞} is simply connected with *G*_{2} ⊃ *G*_{1}. By the Riemann mapping theorem there is a conformal mapping *h*(*w*) = *w* + *b*_{1}*w*^{−1} + ⋅⋅⋅ such that *h*(*G*_{2}) is ℂ with a horizontal slit removed. So *h*(*f*(*z*)) = *z* + (*a*_{1} + *b*_{1})*z*^{−1} + ⋅⋅⋅ and hence Re (*a*_{1} + *b*_{1}) ≤ Re *a*_{1} by the extremality of *f*. Thus Re *b*_{1} ≤ 0. On the other hand by the Riemann mapping theorem there is a conformal mapping *k*(*w*) = *w* + *c*_{0} + *c*_{1}*w*^{−1} + ⋅⋅⋅ from |*w*| > *S* onto *G*_{2}. Then *f*(*k*(*w*)) − *c*_{0} = *w* + (*a*_{1} + *c*_{1}) *w*^{−1} + ⋅⋅⋅. By the strict maximality for the slit mapping in the previous paragraph Re *c*_{1} < Re (*b*_{1} + *c*_{1}), so that Re *b*_{1} > 0. The two inequalities for Re *b*_{1} are contradictory.^{ [30] }^{ [31] }^{ [32] }

The proof of the uniqueness of the conformal parallel slit transformation is given in Goluzin (1969) and Grunsky (1978). Applying the inverse of the Joukowsky transform *h* to the horizontal slit domain, it can be assumed that *G* is a domain bounded by the unit circle *C*_{0} and contains analytic arcs *C*_{i} and isolated points (the images of other the inverse of the Joukowsky transform under the other parallel horizontal slits). Thus, taking a fixed *a* in *G*, there is a univalent mapping *F*_{0}(*w*) = *h* ∘ *f* (*w*) = (*w* - *a*)^{−1} + *a*_{1} (*w* − *a*) + *a*_{2}(*w* − *a*)^{2} + ⋅⋅⋅ with image a horizontal slit domain. Suppose that *F*_{1}(*w*) is another uniformizer with *F*_{1}(*w*) = (*w* - *a*)^{−1}+ *b*_{1} (*w* − *a*) + *b*_{2}(*w* − *a*)^{2} + ⋅⋅⋅. The images under *F*_{0} or *F*_{1} of each *C*_{i} have a fixed *y*-coordinate so are horizontal segments. On the other hand *F*_{2}(*w*) = *F*_{0}(*w*) − *F*_{1}(*w*) is holomorphic in *G*. If it is constant, then it must be identically zero since *F*_{2}(*a*) = 0. Suppose *F*_{2} is non-constant. Then by assumption *F*_{2}(*C*_{i}) are all horizontal lines. If *t* is not in one of these lines, Cauchy's argument principle shows that the number of solutions of *F*_{2}(*w*) = *t* in *G* is zero (any *t* will eventually be encircled by contours in *G* close to the *C*_{i}'s). This contradicts the fact that the non-constant holomorphic function *F*_{2} is an open mapping.^{ [33] }

Given *U* and a point *z*_{0} in *U*, we want to construct a function *f* which maps *U* to the unit disk and *z*_{0} to 0. For this sketch, we will assume that *U* is bounded and its boundary is smooth, much like Riemann did. Write

where *g* = *u* + *iv* is some (to be determined) holomorphic function with real part *u* and imaginary part *v*. It is then clear that *z*_{0} is the only zero of *f*. We require |*f*(*z*)| = 1 for *z* ∈ ∂*U*, so we need

on the boundary. Since *u* is the real part of a holomorphic function, we know that *u* is necessarily a harmonic function; i.e., it satisfies Laplace's equation.

The question then becomes: does a real-valued harmonic function *u* exist that is defined on all of *U* and has the given boundary condition? The positive answer is provided by the Dirichlet principle. Once the existence of *u* has been established, the Cauchy–Riemann equations for the holomorphic function *g* allow us to find *v* (this argument depends on the assumption that *U* be simply connected). Once *u* and *v* have been constructed, one has to check that the resulting function *f* does indeed have all the required properties.^{ [34] }

The Riemann mapping theorem can be generalized to the context of Riemann surfaces: If *U* is a non-empty simply-connected open subset of a Riemann surface, then *U* is biholomorphic to one of the following: the Riemann sphere, ** C ** or * D *. This is known as the uniformization theorem.

In the case of a simply connected bounded domain with smooth boundary, the Riemann mapping function and all its derivatives extend by continuity to the closure of the domain. This can be proved using regularity properties of solutions of the Dirichlet boundary value problem, which follow either from the theory of Sobolev spaces for planar domains or from classical potential theory. Other methods for proving the smooth Riemann mapping theorem include the theory of kernel functions^{ [35] } or the Beltrami equation.

Computational conformal mapping is prominently featured in problems of applied analysis and mathematical physics, as well as in engineering disciplines, such as image processing.

In the early 1980s an elementary algorithm for computing conformal maps was discovered. Given points in the plane, the algorithm computes an explicit conformal map of the unit disk onto a region bounded by a Jordan curve with This algorithm converges for Jordan regions^{ [36] } in the sense of uniformly close boundaries. There are corresponding uniform estimates on the closed region and the closed disc for the mapping functions and their inverses. Improved estimates are obtained if the data points lie on a curve or a K-quasicircle. The algorithm was discovered as an approximate method for conformal welding; however, it can also be viewed as a discretization of the Loewner differential equation.^{ [37] }

The following is known about numerically approximating the conformal mapping between two planar domains.^{ [38] }

Positive results:

- There is an algorithm A that computes the uniformizing map in the following sense. Let be a bounded simply-connected domain, and ∂Ω is provided to A by an oracle representing it in a pixelated sense (i.e., if the screen is divided to pixels, the oracle can say whether each pixel belongs to the boundary or not). Then A computes the absolute values of the uniformizing map with precision in space bounded by and time , where C depends only on the diameter of and Furthermore, the algorithm computes the value of φ(w) with precision as long as Moreover, A queries ∂Ω with precision of at most In particular, if ∂Ω is polynomial space computable in space for some constant and time then A can be used to compute the uniformizing map in space and time

- There is an algorithm A′ that computes the uniformizing map in the following sense. Let be a bounded simply-connected domain, and Suppose that for some ∂Ω is given to A′ with precision by pixels. Then A′ computes the absolute values of the uniformizing map within an error of in randomized space bounded by and time polynomial in (that is, by a BPL(
*n*)-machine). Furthermore, the algorithm computes the value of with precision as long as

Negative results:

- Suppose there is an algorithm A that given a simply-connected domain with a linear-time computable boundary and an inner radius > 1/2 and a number computes the first digits of the conformal radius then we can use one call to A to solve any instance of a #SAT(
*n*) with a linear time overhead. In other words, #P is poly-time reducible to computing the conformal radius of a set.

- Consider the problem of computing the conformal radius of a simply-connected domain where the boundary of is given with precision by an explicit collection of pixels. Denote the problem of computing the conformal radius with precision by Then, is AC0 reducible to for any

- Measurable Riemann mapping theorem
- Schwarz–Christoffel mapping – a conformal transformation of the upper half-plane onto the interior of a simple polygon.
- Conformal radius

- ↑ The existence of f is equivalent to the existence of a Green’s function.
- ↑ Ahlfors, Lars (1953), L. Ahlfors; E. Calabi; M. Morse; L. Sario; D. Spencer (eds.), "Developments of the Theory of Conformal Mapping and Riemann Surfaces Through a Century",
*Contributions to the Theory of Riemann Surfaces*: 3–4 - ↑ For the original paper, see Osgood 1900. For accounts of the history, see Walsh 1973 , pp. 270–271; Gray 1994 , pp. 64–65; Greene & Kim 2017 , p. 4. Also see Carathéodory 1912 , p. 108, footnote ** (acknowledging that Osgood 1900 had already proven the Riemann mapping theorem).
- ↑ Gray 1994 , pp. 78–80, citing Carathéodory 1912
- ↑ Greene & Kim 2017 , p. 1
- ↑ Gray 1994 , pp. 80–83
- ↑ Remmert 1998, section 8.3, p. 187
- ↑ See
- ↑ Gamelin 2001 , pp. 256–257, elementary proof
- ↑ Berenstein & Gay 1991 , pp. 86–87
- ↑ Gamelin 2001
- ↑ Gamelin 2001
- ↑ Duren 1983
- ↑ Jänich 1993
- ↑ Duren 1983
- ↑ Jänich 1993
- ↑ Gamelin 2001 , p. 309
- ↑ Duren 1983
- ↑ Jänich 1993
- ↑ Ahlfors 1978
- ↑ Jenkins 1958 , pp. 77–78
- ↑ Duren 1980
- ↑ Schiff 1993 , pp. 162–166
- ↑ Jenkins 1958 , pp. 77–78
- ↑ Schober 1975
- ↑ Duren 1980
- ↑ Duren 1983
- ↑ Schiff 1993
- ↑ Goluzin 1969 , pp. 210–216
- ↑ Schiff 1993
- ↑ Goluzin 1969 , pp. 210–216
- ↑ Nehari 1952 , pp. 351–358
- ↑ Goluzin 1969 , pp. 214−215
- ↑ Gamelin 2001 , pp. 390–407
- ↑ Bell 1992
- ↑ A Jordan region is the interior of a Jordan curve.
- ↑ Marshall, Donald E.; Rohde, Steffen (2007). "Convergence of a Variant of the Zipper Algorithm for Conformal Mapping".
*SIAM Journal on Numerical Analysis*.**45**(6): 2577. CiteSeerX 10.1.1.100.2423 . doi:10.1137/060659119. - ↑ Binder, Ilia; Braverman, Mark; Yampolsky, Michael (2007). "On the computational complexity of the Riemann mapping".
*Arkiv för Matematik*.**45**(2): 221. arXiv: math/0505617 . Bibcode:2007ArM....45..221B. doi:10.1007/s11512-007-0045-x.

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**Complex analysis**, traditionally known as the **theory of functions of a complex variable**, is the branch of mathematical analysis that investigates functions of complex numbers. It is useful in many branches of mathematics, including algebraic geometry, number theory, analytic combinatorics, applied mathematics; as well as in physics, including the branches of hydrodynamics, thermodynamics, and particularly quantum mechanics. By extension, use of complex analysis also has applications in engineering fields such as nuclear, aerospace, mechanical and electrical engineering.

In the field of complex analysis in mathematics, the **Cauchy–Riemann equations**, named after Augustin Cauchy and Bernhard Riemann, consist of a system of two partial differential equations which, together with certain continuity and differentiability criteria, form a necessary and sufficient condition for a complex function to be complex differentiable, that is, holomorphic. This system of equations first appeared in the work of Jean le Rond d'Alembert. Later, Leonhard Euler connected this system to the analytic functions. Cauchy then used these equations to construct his theory of functions. Riemann's dissertation on the theory of functions appeared in 1851.

In mathematics, mathematical physics and the theory of stochastic processes, a **harmonic function** is a twice continuously differentiable function *f* : *U* → **R**, where *U* is an open subset of **R**^{n}, that satisfies Laplace's equation, that is,

In mathematics, particularly in complex analysis, a **Riemann surface** is a one-dimensional complex manifold. These surfaces were first studied by and are named after Bernhard Riemann. Riemann surfaces can be thought of as deformed versions of the complex plane: locally near every point they look like patches of the complex plane, but the global topology can be quite different. For example, they can look like a sphere or a torus or several sheets glued together.

In mathematics, the **uniformization theorem** says that every simply connected Riemann surface is conformally equivalent to one of three Riemann surfaces: the open unit disk, the complex plane, or the Riemann sphere. In particular it implies that every Riemann surface admits a Riemannian metric of constant curvature. For compact Riemann surfaces, those with universal cover the unit disk are precisely the hyperbolic surfaces of genus greater than 1, all with non-abelian fundamental group; those with universal cover the complex plane are the Riemann surfaces of genus 1, namely the complex tori or elliptic curves with fundamental group **Z**^{2}; and those with universal cover the Riemann sphere are those of genus zero, namely the Riemann sphere itself, with trivial fundamental group.

The theory of **functions of several complex variables** is the branch of mathematics dealing with complex-valued functions. The function is n-tuples of complex numbers, classically considered on the complex coordinate space .

In complex analysis, a branch of mathematics, **Morera's theorem**, named after Giacinto Morera, gives an important criterion for proving that a function is holomorphic.

In mathematics, the **Schwarz lemma**, named after Hermann Amandus Schwarz, is a result in complex analysis about holomorphic functions from the open unit disk to itself. The lemma is less celebrated than deeper theorems, such as the Riemann mapping theorem, which it helps to prove. It is, however, one of the simplest results capturing the rigidity of holomorphic functions.

In mathematics, **Carathéodory's theorem** is a theorem in complex analysis, named after Constantin Carathéodory, which extends the Riemann mapping theorem. The theorem, first proved in 1913, states that the conformal mapping sending the unit disk to the region in the complex plane bounded by a Jordan curve extends continuously to a homeomorphism from the unit circle onto the Jordan curve. The result is one of Carathéodory's results on prime ends and the boundary behaviour of univalent holomorphic functions.

**Geometric function theory** is the study of geometric properties of analytic functions. A fundamental result in the theory is the Riemann mapping theorem.

In mathematics and in particular the field of complex analysis, **Hurwitz's theorem** is a theorem associating the zeroes of a sequence of holomorphic, compact locally uniformly convergent functions with that of their corresponding limit. The theorem is named after Adolf Hurwitz.

In mathematics, the **conformal radius** is a way to measure the size of a simply connected planar domain *D* viewed from a point *z* in it. As opposed to notions using Euclidean distance, this notion is well-suited to use in complex analysis, in particular in conformal maps and conformal geometry.

In mathematics, the **Beltrami equation**, named after Eugenio Beltrami, is the partial differential equation

In complex analysis and geometric function theory, the **Grunsky matrices**, or **Grunsky operators**, are infinite matrices introduced in 1939 by Helmut Grunsky. The matrices correspond to either a single holomorphic function on the unit disk or a pair of holomorphic functions on the unit disk and its complement. The **Grunsky inequalities** express boundedness properties of these matrices, which in general are contraction operators or in important special cases unitary operators. As Grunsky showed, these inequalities hold if and only if the holomorphic function is univalent. The inequalities are equivalent to the inequalities of Goluzin, discovered in 1947. Roughly speaking, the Grunsky inequalities give information on the coefficients of the logarithm of a univalent function; later generalizations by Milin, starting from the Lebedev–Milin inequality, succeeded in exponentiating the inequalities to obtain inequalities for the coefficients of the univalent function itself. The Grunsky matrix and its associated inequalities were originally formulated in a more general setting of univalent functions between a region bounded by finitely many sufficiently smooth Jordan curves and its complement: the results of Grunsky, Goluzin and Milin generalize to that case.

In mathematics, the **Loewner differential equation**, or **Loewner equation**, is an ordinary differential equation discovered by Charles Loewner in 1923 in complex analysis and geometric function theory. Originally introduced for studying slit mappings, Loewner's method was later developed in 1943 by the Russian mathematician Pavel Parfenevich Kufarev (1909–1968). Any family of domains in the complex plane that expands continuously in the sense of Carathéodory to the whole plane leads to a one parameter family of conformal mappings, called a **Loewner chain**, as well as a two parameter family of holomorphic univalent self-mappings of the unit disk, called a **Loewner semigroup**. This semigroup corresponds to a time dependent holomorphic vector field on the disk given by a one parameter family of holomorphic functions on the disk with positive real part. The Loewner semigroup generalizes the notion of a univalent semigroup.

In mathematics, the **Carathéodory kernel theorem** is a result in complex analysis and geometric function theory established by the Greek mathematician Constantin Carathéodory in 1912. The uniform convergence on compact sets of a sequence of holomorphic univalent functions, defined on the unit disk in the complex plane and fixing 0, can be formulated purely geometrically in terms of the limiting behaviour of the images of the functions. The kernel theorem has wide application in the theory of univalent functions and in particular provides the geometric basis for the Loewner differential equation.

In mathematics, a **quasicircle** is a Jordan curve in the complex plane that is the image of a circle under a quasiconformal mapping of the plane onto itself. Originally introduced independently by Pfluger (1961) and Tienari (1962), in the older literature they were referred to as **quasiconformal curves**, a terminology which also applied to arcs. In complex analysis and geometric function theory, quasicircles play a fundamental role in the description of the universal Teichmüller space, through quasisymmetric homeomorphisms of the circle. Quasicircles also play an important role in complex dynamical systems.

In mathematics, **Sobolev spaces for planar domains** are one of the principal techniques used in the theory of partial differential equations for solving the Dirichlet and Neumann boundary value problems for the Laplacian in a bounded domain in the plane with smooth boundary. The methods use the theory of bounded operators on Hilbert space. They can be used to deduce regularity properties of solutions and to solve the corresponding eigenvalue problems.

In mathematics, a **planar Riemann surface** is a Riemann surface sharing the topological properties of a connected open subset of the Riemann sphere. They are characterized by the topological property that the complement of every closed Jordan curve in the Riemann surface has two connected components. An equivalent characterization is the differential geometric property that every closed differential 1-form of compact support is exact. Every simply connected Riemann surface is planar. The class of planar Riemann surfaces was studied by Koebe who proved in 1910 as a generalization of the uniformization theorem that every such surface is conformally equivalent to either the Riemann sphere or the complex plane with slits parallel to the real axis removed.

In mathematics, **differential forms on a Riemann surface** are an important special case of the general theory of differential forms on smooth manifolds, distinguished by the fact that the conformal structure on the Riemann surface intrinsically defines a Hodge star operator on 1-forms without specifying a Riemannian metric. This allows the use of Hilbert space techniques for studying function theory on the Riemann surface and in particular for the construction of harmonic and holomorphic differentials with prescribed singularities. These methods were first used by Hilbert (1909) in his variational approach to the Dirichlet principle, making rigorous the arguments proposed by Riemann. Later Weyl (1940) found a direct approach using his method of orthogonal projection, a precursor of the modern theory of elliptic differential operators and Sobolev spaces. These techniques were originally applied to prove the uniformization theorem and its generalization to planar Riemann surfaces. Later they supplied the analytic foundations for the harmonic integrals of Hodge (1940). This article covers general results on differential forms on a Riemann surface that do not rely on any choice of Riemannian structure.

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