Schwarz–Ahlfors–Pick theorem

Schwarz–Ahlfors–Pick theorem

In mathematics, the Schwarz–Ahlfors–Pick theorem is an extension of the Schwarz lemma for hyperbolic geometry, such as the Poincaré half-plane model. It states that the Poincaré metric is distance-decreasing on harmonic functions.

The theorem states that every holomorphic automorphism of the unit disk "U", or the upper half-plane "H", with distances defined by the Poincaré metric, is a contraction mapping. That is, every such analytic mapping will not increase the distance between points. Stated more precisely: Theorem: (SchwarzAhlforsPick) For all holomorphic automorphisms f:U ightarrow U, one has ho(f(z_1),f(z_2)) leq ho(z_1,z_2) for points z_1,z_2 in U and Poincaré distance ho.

For any tangent vector "T", the hyperbolic length of the tangent vector does not increase: |f^*(T)| leq |T|.


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