Fourier Transform of $\displaystyle \frac{x}{x+ic}$ $$\int_{-\infty} ^\infty \frac{x}{x+ic}e^{ikx}dx$$ where $c$ is some positive real constant. 
I've tried substituton $u=x+ic$ giving:
$$\int_{-\infty}^\infty \frac{u-ic}{u}e^{ik(u-ic)}du \\
= \int_{-\infty} ^\infty \frac{u-ic}{u}e^{iku}e^{kc}du \\
= e^{kc}\left(\int_{-\infty} ^\infty e^{iku}du-ic\int_{-\infty} ^\infty\frac{e^{iku}}{u}du\right) \\
= e^{kc}\left(\left.\frac{e^{iku}}{ik}\right|_{-\infty} ^\infty -ic\int_{-\infty} ^\infty\frac{e^{iku}}{u}du\right) $$
I don't know how to evaluate the second integral, and I'm not sure if there's just an easier way to do the whole calculation that I'm missing.
 A: Using $\frac{x}{x+ic} = 1 - \frac{ic}{x+ic}$, we have
\begin{align}
I(k,c)
&= \int_{-\infty}^{+\infty}dx\, \frac{x}{x+ic} e^{i k x}\\
&= \int_{-\infty}^{+\infty}dx\,e^{i k x} \;-\; ic \int_{-\infty}^{+\infty}dx\,\frac{e^{i k x}}{x+ic}\\
&=2\pi\,\delta(k)\;-\; ic \int_{-\infty}^{+\infty}dx\,\frac{e^{i k x}}{x+ic}\, ,\qquad\qquad\qquad (1)
\end{align}
where we have used the integral representation of the Dirac delta function,
$$
\delta(k) = \frac{1}{2\pi}\int_{-\infty}^{+\infty}dx\,e^{i k x}\, .
$$
The latter integral in equation (1) above can be evaluated by contour integration using a semicircular contour. Since $c>0$, the pole at $-ic$ is in the lower half-plane. When $k>0$ we must close the contour in the upper-half-plane, so that the $e^{ikx}$ term will decay properly. Since there are no poles in the upper-half-plane, this yields $0$.
When $k<0$, we close the contour in the lower-half-plane, enclosing the pole. Thus:
$$
-\; ic \int_{-\infty}^{+\infty}dx\,\frac{e^{i k x}}{x+ic}
\;=\;
-ic \cdot (-2\pi i) \cdot e^{i k (-ic)}
\;=\;
-2\pi\, c\, e^{-c |k|} \qquad (k < 0)
$$
When $k = 0$, the integral in Eqn. (1) becomes
\begin{align}
-ic \int_{-\infty}^{+\infty}dx\,\frac{1}{x+ic}
&= -ic \int_{-\infty}^{+\infty}dx\,\frac{x-ic}{x^2+c^2}\\
&= -ic \int_{-\infty}^{+\infty}dx\,\frac{x}{x^2+c^2} \;-\; c^2 \int_{-\infty}^{+\infty}dx\,\frac{1}{x^2+c^2}\\
&= -ic \times 0 \;-\; \pi c\, ,
\end{align}
where the first integral becomes zero by symmetry.
Putting all of this together, we have
\begin{equation}
I(k,c)
\;=\;
2\pi\,\delta(k)\;-\;\pi\, c\, e^{-c |k|}\times
\begin{cases}
0 & k >0\\
1 & k = 0\\
2 & k < 0
\end{cases}\, .
\end{equation}
