Consider the complex integral (it's a probability density)

<br />
\int^{\infty}_{-\infty} \frac{e^{-i x a}}{(1+i x <br />
b)^{k_{1}}(1-i x \frac{c}{N})^{N k_{2}}}dx<br />
for a \geq 0

I need to find the residue at the pole x_{0}=i/b
(of order k_{1}).
According to the theory, that residue is:

 Res_{x_{0}}=\frac{1}{(k_{1}-1)!} \frac{d^{k_{1}-1}}{dx^{k_{1}-1}} \, [ f(x) (1+i x b)^{k_{1}}]_{x=x_{0}}

I found the residue. Now, the integral should equal to

2 \pi i Res_{x_{0}}

Please could you say where I did a mistake? Because the result I got is not a probability density...
Thank you!