Neumann [21] which resolved an open problem in gravitational lensing.
Z
D
f2i∂∂|R|¯ 2=2i Z
D
∂(¯ f∂|R|2) (5.8)
=2i Z
D
∂(¯ fR∂R)¯ (5.9)
=2i Z
D
(∂+∂)(¯ fR∂R)¯ (5.10)
=2i Z
D
d(fR∂R)¯ (5.11)
=2i Z
∂DfR∂R¯ (5.12)
=2i Z
∂Df SD,R∂R (5.13)
= Ψ[f], (5.14)
where 5.8 follows since f is holomorphic, 5.9 follows since ¯Ris antiholomorphic onD, 5.10 follows since f R∂Ris a (1,0)-form and thus∂(f R∂R) is zero, 5.12 is Stoke’s theorem, and 5.13 follows since SD,R|∂D = R|∂D.
An important example of AQD’s for Rare given by the class of harmonic quadrature domains with data (2i∂∂|¯R|2,Ψ), where
Ψ≡
n
X
i=1
ciδai, (5.15)
where{ai}n
i=1,{ci}n
i=1 ⊂ C. We prove this below. Of particular importance is the case whereΨ = tδa. This corresponds to Laplacian growth with an empty initial condition.
Proposition 5.2.5. Let D be a bounded harmonic quadrature domain with piecewise C1boundary and data (2i∂∂|R|¯ 2,Ψ), whereΨis given by 5.15; then D is an AQD for R and
SD,R= 1
∂R∂[Z
D
Gδp(·,w)2i∂∂|R|¯ 2−
n
X
i=1
ciGδp(·,ai)+|R|2], (5.16)
where p is any point in Dc. Proof. Letp∈Dc. We claim that
SD,R= 1
∂R∂[Z
D
Gδp(·,w)2i∂∂|R|¯ 2−
n
X
i=1
ciGδp(·,ai)+|R|2], (5.17) is a Schwarz function for (D,R). LetS denote the right side of 5.17. We begin by showing
S|∂D=R|¯∂D.
Letz∈(D)c, thenGδp(z,·) is harmonic onD. By the quadrature identity forDwe have
Z
D
Gδp(z,w)2i∂∂|¯R|2(w)= Ψ[Gδp(z,·)] (5.18)
=
n
X
i=1
ciGδp(z,ai). (5.19)
It then follows that
∂Z
D
Gδp(z,w)2i∂∂|R|¯ 2(w)=∂
n
X
i=1
ciGδp(z,ai), (5.20)
wherez∈(D)cand∂acts on the first variable. By continuity and by hypothesis on the boundary it follows that 5.20 continues to hold forz∈∂D. It then follows that
S|∂D= 1
∂R∂|R|2|∂D
= 1
∂RR∂R|¯ ∂D
=R¯|∂D.
We now show thatS is meromorphic onD. Since∂Ris a holomorphic one-form onD, from 5.17 it suffices to show that
∂[Z
D
Gδp(·,w)2i∂∂|R|¯ 2(w)−
n
X
i=1
ciGδp(·,ai)+|R|2]
is a meromorphic one-form onD. To show this it suffices to show that Z
D
Gδp(·,w)2i∂∂|R|¯ 2(w)−
n
X
i=1
ciGδp(·,ai)+|R|2 (5.21)
is harmonic onDexcept at a discrete set of points where it has logarithmic poles. Since
n
X
i=1
ciGδp(·,ai)
is harmonic onDexcept at{ai}where it has logarithmic poles, it suffices to show that
Z
D
Gδp(·,w)2i∂∂|R|¯ 2(w)+|R|2 (5.22)
is harmonic onD. Let ˜R∈C2(M) be an extension ofR|D. Then
Z
D
Gδp(·,w)2i∂∂|R|¯ 2(w)|D+|R|2|D=Z
D
Gδp(·,w)2i∂∂|¯R|˜2(w)|D+|R|˜2|D (5.23)
=Z
M
Gδp(·,w)2i∂∂|¯R|˜2(w)|D− Z
Dc
Gδp(·,w)2i∂∂|¯R|˜2(w)|D+|R|˜2|D (5.24)
=(|R|˜2(p)− |R|˜2)|D− Z
Dc
Gδp(·,w)2i∂∂|¯R|˜2(w)|D+|R|˜2|D (5.25)
=|R|˜2(p)− Z
Dc
Gδp(·,w)2i∂∂|¯R|˜2(w)|D, (5.26) where we used the distributional property ofGδpin 5.25. Since forw∈Dc,Gδp(·,w) is harmonic onD, 5.26 is easily seen to be harmonic onD. This completes the proof.
The following lemma allows us to generate AQDs from classical quadrature domains.
Lemma 5.2.6. LetΩ ⊂Cbe a classical analytic quadrature domain with linear functionalΨ. Let R be a meromorphic function on a compact Riemann surface M. Let D be a connected component of R−1(Ω), then D is an AQD for R.
Proof. SinceΩis a classical analytic quadrature domain, the domainΩhas a Schwarz function,SΩ. We claim thatSD,R =SΩ◦R. First observe thatSΩ◦Ris meromorphic, so it suffices to show that
SΩ◦R|∂D=R|∂D, (5.27)
Letz∗∈∂D, thenz∗ <Dand thusR(z∗)<Ω. Also,z∗ ∈D, and thusR(z∗)∈R(D) by the continuity ofRon D. SoR(z∗)∈D\D≡∂D. Identity 5.27 then follows immediately:
SΩ◦R(z∗)=SΩ(R(z∗))=R(z∗). (5.28) The importance of the previous lemma is that it provides an easy way to construct examples of AQDs since classical quadrature domains have been well studied. However, most AQDs cannot be generated in this fash- ion and the subject seems much richer than the classical setting.
IfDpossesses a Schwarz function, it actually possesses an infinite dimensional family of Schwarz functions as the following simple lemma shows.
Lemma 5.2.7. Let D be an AQD for R and let T be a rational function with poles offR(D). Then D is an AQD for T◦R where the Schwarz function is given by
SD,T◦R=T◦SD,R. Proof. The functionT◦SD,Ris clearly meromorphic, and
T◦SD,R|∂D=T ◦R|¯∂D=T◦R|∂D. Therefore,
SD,T◦R=T◦SD,R.
In the next proposition we provide a correspondence between AQDs and a special type of meromorphic functions on compact Riemann surfaces with real structure. The benefit of this correspondence is that there is no direct reference to the quadrature domainD—one can find AQDs of a connectivitycsimply by finding such a meromorphic function on a Riemann surfaceNof genusc−1 with real structure.
Proposition 5.2.8. Let N be a compact Riemann surface with real structure given by an antiholomorphic involutionτ. LetΓdenote the set of fixed points ofτ. Assume thatΓdivides N into two components and let E be a connected component of N\Γ. Suppose there exists a meromorphic functionψon N satisfying
ψ|E =R◦φ|E,
whereφ: E → M is continuous, injective, and univalent on E and R is a meromorphic function on M with poles offE. Thenφ(E)is an AQD with for R. Conversely, if D is an AQD for R then there exists a compact Riemann surface N satisfying the hypotheses above and a meromorphic functionψon N satisfying
ψ|E =R◦φ|E, whereφ:E→D is continuous, injective, and univalent on E.
Proof. Suppose we are given such aψ, then the function
S ≡ψ◦τ◦φ−1,
is meromorphic onφ(E). Ifz∗∈∂φ(E), then by the hypotheses onφ,φ−1(z∗)∈∂E, and so
S(z∗)=ψ◦τ◦φ−1(z∗)
=ψ◦φ−1(z∗)
=(R◦φ)◦φ−1(z∗)
=R(z¯ ∗).
SoS|∂φ(E)=R|¯∂φ(E).ThereforeS is a Schwarz function forφ(E), and soφ(E) is an AQD forR.
We now prove the converse. SupposeDis an AQD forR. From the definition of anAQD, there exists a univalent mapκ : D → E, whereE is a finite bordered Riemann surface which extends to a continuous injection fromD→ E. Denote the inverse ofκbyφ. Let ˜Ebe a copy ofEand letN ≡Et∂EtE˜be the Schottky double, thenNsatisfies the hypotheses in the proposition and
ψ(z)≡
R◦φ(z) :z∈E SD,R◦φ(z) :z∈E˜ is a meromorphic function onNwhich satisfies
ψ|E =R◦φ,
andφ:E→Dis continuous, injective, and is univalent onD.
Proposition 5.2.9. Let D ⊂ M be an AQD for R. Let c denote the connectivity of D and let g denote the
genus of M. Then there exists a compact Riemann surface N of genus gc and a meromorphic function T on N such that
T ◦φ=SD,R, whereφ:D→N is continuous, injective and univalent on D.
Proof. SinceDis anAQD, there exists a finite bordered Riemann surface,E, and a continuous and injective mapφ:D→ E, which is univalent onD. The connectivity ofEis alsoc. Let{Bi}c
i=1, denote the connected components of M\E. Consider the collection of open Riemann surfaces{M\Bi}with opposite conformal structure. LetNbe the Riemann surface obtained by gluing eachM\BitoEalong the boundary∂Ei. LetN be the resulting compact surface with genusgc. Since the boundary ofEis analytic, the complex structure of Eextends in a neighborhood of the boundary which is consistent with the complex structures on each{M\Bi}.
This induces a complex structure onN. Let
T(z)≡
SD,R◦φ−1 :z∈E
R◦φ−1 :z∈M\Bi, 1≤i≤c.
ThenT is meromorphic andT◦φ=SD,R.
Corollary 5.2.10. Let D⊂Cˆ be an AQD for R. Then there exists a rational function T such that T ◦φ=SD,R,
whereφ:D→Cˆ is univalent.
We use proposition 5.2.8 to characterize the boundaries of AQDs.
Proposition 5.2.11. Let D be an AQD for R. Then there exists an irreducible polynomial P∈ C[x,y], such that
∂D⊂ {P(R(z),R(z))=0}
and thus R(∂D)is a subset of an algebraic curve inC.
SinceDis anAQD, there exists a univalent mapκ : D→ E, whereEis a finite bordered Riemann surface which extends to a continuous injection fromD→E. Denote the inverse ofκbyφ. Let ˜Ebe a copy ofEand letN≡Et∂EtE˜be the Schottky double. OnNwe have the following functions:
Proof.
ψ(z)≡
R◦φ(z) :z∈E SD,R◦φ(z) :z∈E˜
and
ψ(z)˜ ≡
SD,R◦φ(z) :z∈E R¯◦φ(z) :z∈E.˜
We saw in prop 5.2.8 thatψis meromorphic onN. An analogous argument shows that ˜ψis meromorphic.
Recall that the transcendence degree of the field of meromorphic functions on a compact Riemann Surface is one. It follows that there exists a polynomialPsuch thatP(ψ,ψ)˜ =0. In particular, forz∈∂D=φ(∂E), we haveP(R(z),R(z))¯ =0. It then follows thatR(∂D)⊂ {P(z,z)¯ =0}and thusR(∂D) is a subset of an algebraic curve inC.