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spherical metric (Definition)

Suppose that $ \hat{\mathbb{C}} := {\mathbb{C}} \cup \{ \infty \}$ is the extended complex plane (the Riemann sphere).

Definition 1   Suppose $ \gamma \colon [0,1] \to \hat{\mathbb{C}}$ is a path in $ \hat{\mathbb{C}}$. The spherical length of $ \gamma$ is defined as
$\displaystyle \ell (\gamma) := 2 \int_\gamma \frac{\lvert dz \rvert}{1+\lvert z... ...} = 2 \int_0^1 \frac{\lvert \gamma'(t) \rvert}{1+\lvert \gamma(t) \rvert^2} dt.$    

Definition 2   Let $ z_1, z_2 \in \hat{\mathbb{C}}$, and let $ \Gamma$ be the set of all paths in $ \hat{\mathbb{C}}$ from $ z_1$ to $ z_2$, then the distance from $ z_1$ to $ z_2$ in the spherical metric is defined as
$\displaystyle \sigma(z_1,z_2) := \inf_{\gamma \in \Gamma} \ell(\gamma) .$    

More intuitivelly this is the shortest distance to travel from $ z_1$ to $ z_2$ if we think of these points as being on the Riemann sphere, and we can only travel on the Riemann sphere itself (we cannot “drill” a straight line from $ z_1$ to $ z_2$).

Bibliography

1
Theodore B. Gamelin. Complex Analysis. Springer-Verlag, New York, New York, 2001.



"spherical metric" is owned by jirka.
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Also defines:  spherical length
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Cross-references: line, straight, points, distance, path, Riemann sphere, extended complex plane
There are 3 references to this entry.

This is version 3 of spherical metric, born on 2004-04-16, modified 2005-03-07.
Object id is 5775, canonical name is SphericalMetric.
Accessed 4225 times total.

Classification:
AMS MSC30A99 (Functions of a complex variable :: General properties :: Miscellaneous)
 54-00 (General topology :: General reference works )

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