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May 23[edit]

Help for an estimate[edit]

Hello, I would like to estimate several derivatives wrt the variable s of the integral function

I(s,a)=\int _{a}^{1}{\sqrt {a^{2}s+z^{2}}}dz

I am looking for estimates of the form

|{\frac {\partial ^{k}I}{\partial s}}(s,a)|\leq C

where C does not depend on a. In principle one could estimate every single derivative but it would take a very long time and so I am looking for a general scheme.

Any idea?--Pokipsy76 (talk) 16:34, 23 May 2010 (UTC)[reply]

As a firs step, I'd change variable and write

z:=a|s|^{1/2}u.

Not clear what's the range of a and s, btw.--pm a 17:06, 23 May 2010 (UTC)[reply]

Thank you for your help. Your suggestion is very smart (I don't think I could ever think it) indeed the integral becomes

a^{2}s\int _{s^{-1/2}}^{s^{-1/2}a^{-1}}{\sqrt {1+u^{2}}}du

so when I derive it wrt s I obtain a simple algebraic expression which can be estimated easily. It was not obvious to me that having the variable only on the integration domain would allow to eliminate the integral after a derivative.--Pokipsy76 (talk) 17:38, 23 May 2010 (UTC)[reply]

Isometries of the Poincare Disc[edit]

Hi everyone,

Could anyone direct me to a proof of the fact that orientation-preserving isometries of the hyperbolic disc are of the form

$\psi \to w{\frac {\psi -\lambda }{{\bar {\lambda }}\psi -1}}$

where $|\lambda |<1$ and $|w|=1$ ?

I have the result but I can't seem to find a proof anywhere online. Of course, if you could provide a short proof instead that would be greatly useful too, but I'm very much capable of understanding a proof on my own if it'll save you time linking be to one rather than writing it out yourself :-)

Thanks very much,

82.133.94.184 (talk) 23:05, 23 May 2010 (UTC)[reply]

The characterization of the holomorphic automorphisms of the disk is in almost every book on complex variable, e.g Rudin's Real and complex analysis. Here there is something in Möbius transformation. --pm a 04:12, 24 May 2010 (UTC)[reply]