Many problems in science and engineering are related to the analytic continuation and in particular infinitesimal analytic continuation to the upper or lower complex plane, i.e., a generic *complex* function $f(\omega)$ with real $\omega$ is changed to 
$$F(\omega)=\lim_{\eta\rightarrow0^+}f(\omega\pm i\eta).$$ 
In many applications, only the part introduced by $\eta$ is necessary, which is 
$$A(\omega)=\lim_{\eta\rightarrow0^+}[f(\omega+i\eta)-f(\omega-i\eta)].$$ 

**Question**: How to symbolically obtain $A(\omega)$?  

A related [question][1] here deals with the simplest case (the first formula below), but I am asking about more general cases when symbolic Fourier transform or integral does not work. 

Simple `Limit` does not work, because the [Sokhotski–Plemelj][2] formula
$$\lim_{\eta\to0^{+}} \frac{1}{x\pm i\eta}= \mp i\pi\delta(x) + {\mathcal{P}} {\Big(\frac{1}{x}\Big)}$$
plays a role in the calculation. Here $\delta$ denotes Dirac delta function (`DiracDelta`), $\mathcal{P}$ denotes the Cauchy principal value. A more general version of this formula is
$$\lim_{\eta\to0^{+}} \frac{1}{(x\pm i\eta)^{n+1}}= \mp i\pi(-1)^n\frac{\delta^{(n)}(x)}{n!} + {\mathcal{P}} {\Big(\frac{1}{x^{n+1}}\Big)}$$
where $\delta^{(n)}$ denotes the $n$-th derivative of $\delta$-function. But note that all these will just appear as convenient symbols, *not* to be evaluated anywhere.


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Two examples are given below, where all parameters are real. Therefore only the imaginary part is purely generated by $\eta$.
For $f_1(\omega)=\frac{a\omega+\sqrt{c+\omega^2}}{e}$, we have 
\begin{equation}
\begin{split}
A(\omega)&=2i\lim_{\eta\rightarrow0^+}\Im f(\omega+i\eta)\\
&=2i\lim_{\eta\rightarrow0^+}\Im\frac{a(\omega+i\eta)+\sqrt{c+(\omega+i\eta)^2}}{e}\\
&\approx2i\lim_{\eta\rightarrow0^+}\Im\frac{a(\omega+i\eta)+c+\omega^2+i\eta\frac{\omega}{c+\omega^2}}{e}\\
&=2i\lim_{\eta\rightarrow0^+}\frac{ai\eta+i\eta\frac{\omega}{c+\omega^2}}{e}\\
&=0.
\end{split}
\end{equation}
For $f_2(\omega)=\frac{a+b\omega}{\omega-c}$ we have 
\begin{equation}
\begin{split}
A(\omega)&=\lim_{\eta\rightarrow0^+} f(\omega+i\eta)-f(\omega-i\eta)\\
&=\lim_{\eta\rightarrow0^+}(a + b c)(\frac{1}{\omega-c+i\eta}-\frac{1}{\omega-c-i\eta})\\
&=[-i\pi\delta(\omega-c)+\mathcal{P}(\frac{1}{\omega-c})]-[+i\pi\delta(\omega-c)+\mathcal{P}(\frac{1}{\omega-c})]\\
&=-2i\pi(a+bc)\delta(\omega-c).
\end{split}
\end{equation}
**Here, simple `Limit` does not work, which merely gives 0 as shown below.**

    f[ω_] := (a + b ω)/(ω - c);
    Limit[f[ω + I η] - f[ω - I η], η -> 0, Direction -> "FromAbove"]



I thought about this for some time, but due to my very limited MMA skill, I don't see how to realize such a calculation.


  [1]: https://mathematica.stackexchange.com/q/148734
  [2]: https://en.wikipedia.org/wiki/Sokhotski%E2%80%93Plemelj_theorem#Version_for_the_real_line