Blaschke product

From Wikipedia, the free encyclopedia
Jump to: navigation, search

In complex analysis, the Blaschke product is a bounded analytic function in the open unit disc constructed to have zeros at a (finite or infinite) sequence of prescribed complex numbers

a0, a1, ...

inside the unit disc.

Blaschke products were introduced by Wilhelm Blaschke (1915). They are related to Hardy spaces.

Definition[edit]

A sequence of points (a_n) inside the unit disk is said to satisfy the Blaschke condition when

\sum_n (1-|a_n|) <\infty.

Given a sequence obeying the Blaschke condition, the Blaschke product is defined as

B(z)=\prod_n B(a_n,z)

with factors

B(a,z)=\frac{|a|}{a}\;\frac{a-z}{1 - \overline{a}z}

provided a ≠ 0. Here \overline{a} is the complex conjugate of a. When a = 0 take B(0,z) = z.

The Blaschke product B(z) defines a function analytic in the open unit disc, and zero exactly at the an (with multiplicity counted): furthermore it is in the Hardy class H^\infty.[1]

The sequence of an satisfying the convergence criterion above is sometimes called a Blaschke sequence.

Szegő theorem[edit]

A theorem of Gábor Szegő states that if f is in H^1, the Hardy space with integrable norm, and if f is not identically zero, then the zeroes of f (certainly countable in number) satisfy the Blaschke condition.

Finite Blaschke products[edit]

Finite Blaschke products can be characterized (as analytic functions on the unit disc) in the following way: Assume that f is an analytic function on the open unit disc such that f can be extended to a continuous function on the closed unit disc

\overline{\Delta}= \{z \in \mathbb{C}\,|\, |z|\le 1\}

which maps the unit circle to itself. Then ƒ is equal to a finite Blaschke product

 B(z)=\zeta\prod_{i=1}^n\left({{z-a_i}\over {1-\overline{a_i}z}}\right)^{m_i}

where ζ lies on the unit circle and mi is the multiplicity of the zero ai, |ai| < 1. In particular, if ƒ satisfies the condition above and has no zeros inside the unit circle then ƒ is constant (this fact is also a consequence of the maximum principle for harmonic functions, applied to the harmonic function log(|ƒ(z)|)).

See also[edit]

References[edit]

  1. ^ Conway (1996) 274