Statement in complex analysis
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.
Statement
Let be the open
unit disk in the
complex plane centered at the
origin, and let be a
holomorphic map such that and on .
Then for all , and .
Moreover, if for some non-zero or , then for some with .
[1]
Proof
The proof is a straightforward application of the
maximum modulus principle on the function
which is holomorphic on the whole of , including at the origin (because is differentiable at the origin and fixes zero). Now if denotes the closed disk of radius centered at the origin, then the maximum modulus principle implies that, for , given any , there exists on the boundary of such that
As we get .
Moreover, suppose that for some non-zero , or . Then, at some point of . So by the maximum modulus principle, is equal to a constant such that . Therefore, , as desired.
Schwarz–Pick theorem
A variant of the Schwarz lemma, known as the Schwarz–Pick theorem (after
Georg Pick), characterizes the analytic automorphisms of the unit disc, i.e.
bijective
holomorphic mappings of the unit disc to itself:
Let be holomorphic. Then, for all ,
and, for all ,
The expression
is the distance of the points , in the
Poincaré metric, i.e. the metric in the Poincaré disc model for
hyperbolic geometry in dimension two. The Schwarz–Pick theorem then essentially states that a holomorphic map of the unit disk into itself decreases the distance of points in the Poincaré metric. If equality holds throughout in one of the two inequalities above (which is equivalent to saying that the holomorphic map preserves the distance in the Poincaré metric), then must be an analytic automorphism of the unit disc, given by a
Möbius transformation mapping the unit disc to itself.
An analogous statement on the
upper half-plane can be made as follows:
Let be holomorphic. Then, for all ,
This is an easy consequence of the Schwarz–Pick theorem mentioned above: One just needs to remember that the
Cayley transform
maps the upper half-plane conformally onto the unit disc . Then, the map is a holomorphic map from onto . Using the Schwarz–Pick theorem on this map, and finally simplifying the results by using the formula for , we get the desired result. Also, for all ,
If equality holds for either the one or the other expressions, then must be a
Möbius transformation with real coefficients. That is, if equality holds, then
with and .
Proof of Schwarz–Pick theorem
The proof of the Schwarz–Pick theorem follows from Schwarz's lemma and the fact that a
Möbius transformation of the form
maps the unit circle to itself. Fix and define the Möbius transformations
Since and the Möbius transformation is invertible, the composition maps to and the unit disk is mapped into itself. Thus we can apply Schwarz's lemma, which is to say
Now calling (which will still be in the unit disk) yields the desired conclusion
To prove the second part of the theorem, we rearrange the left-hand side into the difference quotient and let tend to .
Further generalizations and related results
The
Schwarz–Ahlfors–Pick theorem provides an analogous theorem for hyperbolic manifolds.
De Branges' theorem, formerly known as the Bieberbach Conjecture, is an important extension of the lemma, giving restrictions on the higher derivatives of at in case is
injective; that is,
univalent.
The
Koebe 1/4 theorem provides a related estimate in the case that is univalent.
See also
References
-
^ Theorem 5.34 in Rodriguez, Jane P. Gilman, Irwin Kra, Rubi E. (2007). Complex analysis : in the spirit of Lipman Bers ([Online] ed.). New York: Springer. p. 95.
ISBN
978-0-387-74714-9.
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cite book}}
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link)
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