- #1
Milky
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- 0
Homework Statement
If f(z) is analytic interior to and on a simple closed contour C, then all the derivatives [tex]f^k(z)[/tex] , k=1,2,3... exist in the domain D interior to C, and
[tex]f^k(z)=\frac{k!}{2i\pi}\int_{C}\frac{f(\zeta)d\zeta}{(\zeta-z)^{k+1}}[/tex]
Prove for second derivative.
The Attempt at a Solution
For the first derivative, I would start with
[tex]\frac{f(z+h)-f(z)}{h}=\frac{1}{2i\pi}\frac{1}{h}\int_{C}{f(\zeta)(\frac{1}{\zeta-(z+h)}-\frac{1}{\zeta-z})d\zeta[/tex]
=[tex]\frac{1}{2i\pi}}\int_{C}\frac{f(\zeta)d\zeta}{(\zeta-z)(\zeta-(z+h)}[/tex]
(HERE, I DON'T UNDERSTAND WHERE THE H DISAPPEARED TO)
=[tex]\frac{1}{2i\pi}}\int_{C}\frac{f(\zeta)d\zeta}{(\zeta-z)^2}+R[/tex]
where R= [tex]\frac{h}{2i\pi}\int_{C}\frac{f(\zeta)d\zeta}{(\zeta-z)^2(\zeta-z-h)}[/tex]
By the ML Theorem, we end up with the R term, and therefore the whole term going to zero, so that all that is left is:
[tex]\frac{1}{2i\pi}}\int_{C}\frac{f(\zeta)d\zeta}{(\zeta-z)^2}[/tex]
For the second derivative, I would start with
[tex]\frac{f'(z+h)-f'(z)}{h}=\frac{1}{2i\pi}}\int_{C}\frac{f(\zeta)d\zeta}{(\zeta-z)^2(\zeta-z-h)}[/tex]
Again, I get a term R that goes to zero, and am suppose to be left with
[tex]\frac{2}{2i\pi}}\int_{C}\frac{f(\zeta)d\zeta}{(\zeta-z)^3}[/tex]
Where does the two in the numerator come from? I'm confused by the calculations.