Loewner differential equation

Subordinate univalent functions

Let f and g be holomorphic univalent functions on the unit disk D, |z| < 1, with f(0) = 0 = g(0).

f is said to be subordinate to g if and only if there is a univalent mapping φ of D into itself fixing 0 such that

for |z| < 1.

A necessary and sufficient condition for the existence of such a mapping φ is that

Necessity is immediate.

Conversely φ must be defined by

By definition φ is a univalent holomorphic self-mapping of D with φ(0) = 0.

Since such a map satisfies 0 < |φ'(0)| ≤ 1 and takes each disk D_r, |z| < r with 0 < r < 1, into itself, it follows that

and

Loewner chain

For 0 ≤ t ≤ ∞ let U(t) be a family of open connected and simply connected subsets of C containing 0, such that

if s < t,

and

Thus if s n ↑ t ,

in the sense of the Carathéodory kernel theorem.

If D denotes the unit disk in C, this theorem implies that the unique univalent maps f_t(z)

given by the Riemann mapping theorem are uniformly continuous on compact subsets of [ 0 , ∞ ) × D .

Moreover the function a ( t ) = f t ′ ( 0 ) is positive, continuous, strictly increasing and continuous.

By a reparametrization it can be assumed that

Hence

The univalent mappings f_t(z) are called a Loewner chain.

The Koebe distortion theorem shows that knowledge of the chain is equivalent to the properties of the open sets U(t).

Loewner semigroup

If f_t(z) is a Loewner chain, then

for s < t so that there is a unique univalent self mapping of the disk φ_s,t(z) fixing 0 such that

By uniqueness the mappings φ_s,t have the following semigroup property:

for s ≤ t ≤ r.

They constitute a Loewner semigroup.

The self-mappings depend continuously on s and t and satisfy

Loewner differential equation

The Loewner differential equation can be derived either for the Loewner semigroup or equivalently for the Loewner chain.

For the semigroup, let

then

with

for |z| < 1.

Then w(t)=φ_s,t(z) satisfies the ordinary differential equation

with initial condition w(s) = z.

To obtain the differential equation satisfied by the Loewner chain f_t(z) note that

so that f_t(z) satisfies the differential equation

with initial condition

The Picard–Lindelöf theorem for ordinary differential equations guarantees that these equations can be solved and that the solutions are holomorphic in z.

The Loewner chain can be recovered from the Loewner semigroup by passing to the limit:

Finally given any univalent self-mapping ψ(z) of D, fixing 0, it is possible to construct a Loewner semigroup φ_s,t(z) such that

Similarly given a univalent function g on D with g(0) =0, such that g(D) contains the closed unit disk, there is a Loewner chain f_t(z) such that

Results of this type are immediate if ψ or g extend continuously to ∂D. They follow in general by replacing mappings f(z) by approzimations f(rz)/r and then using a standard compactness argument.

Slit mappings

Holomorphic functions p(z) on D with positive real part and normalized so that p(0) = 1 are described by the Herglotz representation theorem:

where μ is probability measure on the circle. Taking a point measure singles out functions

with |κ(t)| = 1, which were the first to be considered by Loewner (1923).

Inequalities for univalent functions on the unit disk can be proved by using the density for uniform convergence on compact subsets of slit mappings. These are conformal maps of the unit disk onto the complex plane with a Jordan arc connecting a finite point to ∞ omitted. Density follows by applying the Carathéodory kernel theorem. In fact any univalent function f(z) is approximated by functions

which take the unit circle onto an analytic curve. A point on that curve can be connected to infinity by a Jordan arc. The regions obtained by omitting a small segment of the analytic curve to one side of the chosen point converge to g(D) so the corresponding univalent maps of D onto these regions converge to g uniformly on compact sets.

To apply the Loewner differential equation to a slit function f, the omitted Jordan arc c(t) from a finite point to ∞ can be parametrized by [0,∞) so that the map univalent map f_t of D onto C less c([t,∞)) has the form

with b_n continuous. In particular

For s ≤ t, let

with a_n continuous.

This gives a Loewner chain and Loewner semigroup with

where κ is a continuous map from [0,∞) to the unit circle.

To determine κ, note that φ_s,t maps the unit disk into the unit disk with a Jordan arc from an interior point to the boundary removed. The point where it touches the boundary is independent of s and defines a continuous function λ(t) from [0,∞) to the unit circle. κ(t) is the complex conjugate (or inverse) of λ(t):

Equivalently, by Carathéodory's theorem f_t admits a continuous extension to the closed unit disk and λ(t), sometimes called the driving function, is specified by

Not every continuous function κ comes from a slit mapping, but Kufarev showed this was true when κ has a continuous derivative.

Application to Bieberbach conjecture

Loewner (1923) used his differential equation for slit mappings to prove the Bieberbach conjecture

for the third coefficient of a univalent function

In this case, rotating if necessary, it can be assumed that a₃ is non-negative.

Then

with a_n continuous. They satisfy

If

the Loewner differential equation implies

and

So

which immediately implies Bieberbach's inequality

Similarly

Since a₃ is non-negative and |κ(t)| = 1,

using the Cauchy-Schwarz inequality.