In mathematics, the Carathéodory metric is a metric defined on the open unit ball of a complex Banach space that has many similar properties to the Poincaré metric of hyperbolic geometry. It is named after the Greek mathematician Constantin Carathéodory.
Let (X, || ||) be a complex Banach space and let B be the open unit ball in X. Let Δ denote the open unit disc in the complex plane C, thought of as the Poincaré disc model for 2-dimensional real/1-dimensional complex hyperbolic geometry. Let the Poincaré metric ρ on Δ be given by
(thus fixing the curvature to be −4). Then the Carathéodory metric d on B is defined by
What it means for a function on a Banach space to be holomorphic is defined in the article on Infinite dimensional holomorphy.
- For any point x in B,
- d can also be given by the following formula, which Carathéodory attributed to Erhard Schmidt:
- For all a and b in B,
- with equality if and only if either a = b or there exists a bounded linear functional ℓ ∈ X∗ such that ||ℓ|| = 1, ℓ(a + b) = 0 and
- Moreover, any ℓ satisfying these three conditions has |ℓ(a − b)| = ||a − b||.
- Also, there is equality in (1) if ||a|| = ||b|| and ||a − b|| = ||a|| + ||b||. One way to do this is to take b = −a.
- If there exists a unit vector u in X that is not an extreme point of the closed unit ball in X, then there exist points a and b in B such that there is equality in (1) but b ≠ ±a.
Carathéodory length of a tangent vector
There is an associated notion of Carathéodory length for tangent vectors to the ball B. Let x be a point of B and let v be a tangent vector to B at x; since B is the open unit ball in the vector space X, the tangent space TxB can be identified with X in a natural way, and v can be thought of as an element of X. Then the Carathéodory length of v at x, denoted α(x, v), is defined by
One can show that α(x, v) ≥ ||v||, with equality when x = 0.
- Earle, Clifford J. and Harris, Lawrence A. and Hubbard, John H. and Mitra, Sudeb (2003). "Schwarz's lemma and the Kobayashi and Carathéodory pseudometrics on complex Banach manifolds". In Komori, Y.; Markovic, V.; Series, C. Kleinian groups and hyperbolic 3-manifolds (Warwick, 2001). London Math. Soc. Lecture Note Ser. 299. Cambridge: Cambridge Univ. Press. pp. 363–384.