"Figure 1: a simple closed contour C in the complex z plane"
Here we follow standard texts, such as Spiegel (1964) or Levinson and Redheffer (1970). 
If is a closed contour (Figure 1.), and the complex valued function is an analytic function
of the complex variable inside the region bounded
by, and on then
If we substitute for and
By Green's theorem, for any two functions and such that
and exist in a two dimensional region bounded by a curve
If we apply Green's theorem to the real and complex terms of the integral above, we have (identifying the real and imaginary parts of
with and where appropriate, we have
Because is analytic inside its real and imaginary parts must satisfy the Cauchy-Riemann equations
Thus the real and imaginary parts vanish independently showing that
We note that the shape of is quite general. It may have any shape, as long as it does not cross itself, and may have any finite
number of corners, where the function describing the curve is continuous, but not differentiable.
The extension of Cauchy's theorem to a region with any finite number of holes is called the Cauchy-Goursat theorem.
Cauchy Goursat theorem
"Figure 2: f(z) is analytic in the shaded region"
If a complex valued function is analytic in a region of the complex plane bounded by a simple closed
curve , except possibly on any number of finite subdomains (holes) bounded by simple closed curves for then Cauchy's theorem holds in that region bounded by and all of the curves .
Consider a region bounded by a simple closed curve with a hole bounded by (See Figure 2.)
We may connect the two regions with a cut long the curve The integral over the full boundary of the
shaded region, where is analytic is given by
where the notation indicates that the integration path is in the clockwise (negative) direction in
the complex plane.
"Figure 3: equivalent contour integrals"
Reversing the direction of integration on the integral on the right hand side yields
Thus the integrals over the integration contours and are equivalent. Because
need not be analytic in the interior of these integrals are not necessarily zero.
We may have any finite number of holes in our domain, and the sum of the integrals over the curves bounding
these holes is equivalent to the integral over the bounding contour (See Figure 3.)
Another way of interpreting this result is that we may continuously deform the countour to any other closed
simple curve enclosing the same region. Again, there is no restriction on the shape of the contours,
only that they are connected, and that they have at most a finite number of corners.
- ↑ Spiegel, Murray R. "Theory and problems of complex variables, with an introduction to Conformal Mapping and its applications." Schaum's outline series (1964).
- ↑ Levinson, Norman, and Raymond M. Redheffer. "Complex variables." (1970), Holden-Day, New York.