PlanetMath (more info)
 Math for the people, by the people.
Encyclopedia | Requests | Forums | Docs | Wiki | Random | RSS  
Login
create new user
name:
pass:
forget your password?
Main Menu
Owner confidence rating: High Entry average rating: No information on entry rating
spherical derivative (Definition)

Let $ G \subset {\mathbb{C}}$ be a domain.

Definition 1   Let $ f \colon G \to {\mathbb{C}}$ be a meromorphic function, then the spherical derivative of $ f$, denoted $ f^\sharp$ is defined as
$\displaystyle f^\sharp(z) := \frac{2\lvert f'(z) \rvert}{1+\lvert f(z) \rvert^2}$    

for $ z$ where $ f(z) \not= \infty$ and when $ f(z) = \infty$ define
$\displaystyle f^\sharp(z) = \lim_{\zeta \to z} f^\sharp(\zeta) .$    

The second definition makes sense since a meromorphic functions has only isolated poles, and thus $ f^\sharp(\zeta)$ is defined by the first equation when we are close to $ z$. Some basic properties of the spherical derivative are as follows.

Proposition 1   If $ f \colon G \to {\mathbb{C}}$ is a meromorphic function then

Note that sometimes the spherical derivative is also denoted as $ \mu(f)(z)$ rather then $ f^\sharp(z)$.

Bibliography

1
John B. Conway. Functions of One Complex Variable I. Springer-Verlag, New York, New York, 1978.
2
Theodore B. Gamelin. Complex Analysis. Springer-Verlag, New York, New York, 2001.



"spherical derivative" is owned by jirka.
(view preamble | get metadata)

View style:

Log in to rate this entry.
(view current ratings)

Cross-references: continuous function, properties, equation, poles, isolated, function, meromorphic, domain
There is 1 reference to this entry.

This is version 4 of spherical derivative, born on 2004-04-16, modified 2006-06-19.
Object id is 5773, canonical name is SphericalDerivative.
Accessed 1812 times total.

Classification:
AMS MSC30D30 (Functions of a complex variable :: Entire and meromorphic functions, and related topics :: Meromorphic functions, general theory)

Pending Errata and Addenda
None.
[ View all 1 ]
Discussion
Style: Expand: Order:
forum policy

No messages.

Interact
post | correct | update request | add derivation | add example | add (any)