First note that, given , we have
From there it follows that, given we have:
Now, let's go to the sums.
The LHS is equal to: . by the linearity of the integral:
And by the Binomial Theorem this equals:
But by the Binomial Theorem again: and now using the linearity of the integral we get and we are done
Comment: In general where is a contour enclosing 0
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