# When does the small semicircular contribution in contour integrals (not) matter?

Case $$1$$

In order to evaluate the Dirichlet integral $$I_1=\int\limits_{-\infty}^{+\infty}\frac{\sin x}{x}dx=\pi,$$ we can consider another integral $$I_1^\prime=\oint_C\frac{e^{iz}}{z}.$$ Now, we can choose a semicircular contour C of radius $$R$$ to close in the upper half-plane and make another small semicircular indentation around $$z=0$$ (of radius $$\varepsilon$$) to avoid the pole at $$z=0$$. In this problem, the little semicircle about $$z=0$$ does contribute and it cannot be neglected.

Case $$2$$

Now consider the integral $$I_2=\int\limits_{-\infty}^{+\infty}\frac{e^{ix}}{x-x_0}=2\pi i e^{ix_0}$$ with $$x_0>0$$. Let us make it a closed contour integral by regarding $$x$$ to be complex, choosing a large semicircle enclosed in the upper half-plane and a smaller semicircular indentation (of radius $$\varepsilon$$) in the lower half-plane about $$x=x_0$$. If we want to do this using residue theorem, picking up the residue at $$x=x_0$$, the small semicircular contribution does not matter.

Why does the smaller semicircular contribution matter in the first case but not in the second case?

• at a glance it is beacuse in $I_1$ there is no singularity at zero in the original integral, while the complex integral $I_1'$ has a singularity there, so the small circle compensates for that; $I_2$ already has a singularity, so the big circle already accoounts for it Jul 28 '20 at 2:08
• Thanks, but how could you know (without evaluating) that the small semicircular contribution around $x=x_0$ vanishes in the limit $\varepsilon\to 0$ for the second case? Don't you need to check that explicitly? Sorry if I am asking a blunt question. Jul 28 '20 at 2:16
• I thought a little more and I believe we are both wrong - first $I_1=\pi$ (the residue gives $2\pi$ and the small indentation takes $\pi$ away) but the second integral has to be interpreted in some way (the integrand is not absolutely convergent at $x_0$) and I think the Cauchy PV is actually $\pi i e^{ix_0}$ so indeed the small semicircle contributes too - assuming one interpretes $I_2$ that way of course - so it may be a matter of defintions if something else is meant by $I_2$ ($I_1$ being convergent there is no ambiguity) Jul 28 '20 at 3:04
• You are right! $I_1$ has value $\pi$. Will correct it. Jul 28 '20 at 3:10
• what is your meaning for $I_2$? Jul 28 '20 at 3:11

You shouldn't ever "neglect" a piece of the contour: what you may do is estimate its contribution and show that it goes to $$0$$.
• It is not $0$ in Case 2. Jul 28 '20 at 3:37