Cauchy's Integral Formula

Theorem (Cauchy's Integral Formula)

Letbe a simply connected region, letbe a simple closed contour inandbe a function analytic onThenfor any pointinside

Proof: Consider the integral

By the shrinking contour theorem we can replaceby any circleof radiusand centrelying insideto obtain(1)


using the parametrization


is continuous atso for eachthere issuch that

Now chooseto be any positive number such thatthen


Hencesinceis the length ofSinceis arbitrarily small

then from (1)

Cauchy's integral formula can be used in a variety of ways, of which more later.