Cauchy Integral formulas

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The Cauchy Integral formulas for a complex valued function which is analytic inside and on a simple closed curve in some region of the complex plane, for a complex number inside

Proof by induction:

Proof of the case

We consider the case of . We begin by using the Cauchy integral formula to formally define the derivative of



Multiplying and dividing by and adding and subtracting <mmath> h </math> in the numerator allows us to write


We may estimate the second integral noting that and considering deforming the contour of integration to a circle of radius such that and from the maximum modulus theorem that

proving the theorem for the case .

It is possible to prove a finite number of cases using a similar construction.

Proof going from to the -th case

For the -th case we can consider beginning with the case and formally constructing the derivative

In the limit,


Thus,

We note that it is also prove the general case by differentiating each side of the Cauchy integral formula times with respect to , where the -th partial derivative with respect to is brought inside the integral

This is the complex integral form of the Leibniz rule for differentiating under the integral sign.

Return to Complex Analysis.