# Evaluating $\int_{-\infty}^\infty \frac{\sin x}{x-i} dx$

I would like to evaluate the integral $$\int_{-\infty}^\infty \frac{\sin x}{x-i} dx,$$ which I believe should be equal to $\frac{\pi}{e}$. However, I cannot reproduce this result by hand. My work is as follows: first, we evaluate the indefinite integral.

\begin{align*} \int \frac{\sin x}{x-i} dx &= \int \frac{\sin(u+i)}{u} du \text{ where }u=x-i \\ &= \int \frac{\sin u \cos i + \cos u \sin i}{u} du \\ &= \mathrm{Si}(u) \cos i + \mathrm{Ci}(u) \sin i \\ &= \mathrm{Si}(x-i) \cosh 1 + i\mathrm{Ci}(x-i) \sinh 1 \\ \end{align*}

Then, we insert the bounds.

\begin{align*} \int_{-\infty}^\infty \frac{\sin x}{x-i} dx &= \mathrm{Si}(\infty-i) \cosh 1 + i\mathrm{Ci}(\infty-i) \sinh 1 \\ &\phantom{=}-\mathrm{Si}(-\infty-i) \cosh 1 - i\mathrm{Ci}(-\infty-i) \sinh 1 \\ &= \frac{\pi}{2}\cosh 1 + 0 +\frac{\pi}{2} \cosh 1 + \pi \sinh 1 \\ &= \pi (\cosh 1 + \sinh 1) \\ &= \pi e \end{align*}

I assume I have made some mistake in manipulating the complex sine and cosine integrals, since the result would be correct if $\mathrm{Ci}(-\infty-i)$ were evaluated to $-i \pi$. However, I cannot pinpoint the error.

• $u=x-i$ but you have sustituted it by $x+i$. I think that's the problem. – A. A. Jul 25 '14 at 7:07
• @Adolfo Thanks, I've fixed the signs on $i$. However, this doesn't seem to change anything. – David Zhang Jul 25 '14 at 7:11
• @Adolfo No, I take that back--$\mathrm{Ci}$ appears to have a branch cut across the negative real axis, which is the source of the issue. Thanks! – David Zhang Jul 25 '14 at 7:18
• Try to use contour integration and see what you get. – Mhenni Benghorbal Jul 25 '14 at 7:44

The mistake is simple--$\mathrm{Ci}$ has a branch cut across the negative real axis, so $\mathrm{Ci}(-\infty - i)$ should indeed evaluate to $-i \pi$ rather than $i \pi$.
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