# Residue Calculus evaluation of definite integral

Find the integral of $$\int_0^1 \frac{dx}{\sqrt{x^2 - 1}}$$ by considering a dumbbell contour and finding the residue of the branch of $$\frac{1}{\sqrt{z^2 - 1}}$$ at $\infty$.

Now the dumbbell contour consists of a small circle about $z=-1$ (oriented anticlockwise), a small circle about $z=1$ (oriented anticlockwise) and two line segments joining the circles: one above the real axis (directed from $1$ to $-1$) and one below (directed from $-1$ to $1$).

Apparently, the answer is $\dfrac{-\pi \cdot i}{2}$ or $\dfrac{\pi \cdot i}{2}$ depending on what branch is used.

I have been struggling with this integral for some time so if someone could provide a fairly detailed answer, I would be very grateful.

Let's pick a branch cut on $\Re(z)\in [-1,1]$ such that $\arg(z-1) \in (-\pi,\pi]$ and $\arg(z+1)\in (-\pi,\pi]$. Under this branch cut, we have a continuous function for $x <-1$ $$f(z) = (|z+1|e^{\pm i\pi}|z-1|e^{\pm i\pi})^{-1/2} = \frac{-1}{\sqrt{|z+1||z-1|}}$$

With that in mind, the contour consists of two half-circles and two line segments \begin{align} &C_1: z = -1 + re^{i\theta_1}, \ \theta_1 \in (\pi/2,\pi) \cup (-\pi, -\pi/2] \\ &C_2: z = x + ri, \ x \in (-1,1) \\ &C_3: z = 1 + re^{i\theta_2}, \ \theta_2 \in (-\pi/2,\pi/2) \\ &C_4: z = x - ri, \ x \in (-1,1) \end{align}

in the limit $r\to 0$. The integral looks like $$\int_C = \int_{C_1} + \int_{C_2} + \int_{C_3} - \int_{C_4}$$

Using the estimation lemma, you can prove that both half-circles go to $0$ $$\left|\int_{C_1,\ C_3} \frac{1}{\sqrt{z^2-1}} dz\right| \le \frac{\pi r}{\sqrt{r^2+1}} \to 0$$

For the two line segments, observe that $$f(C_2) \to (|z+1|e^{i0}|z-1|e^{i\pi})^{-1/2} = \frac{-i}{\sqrt{|z+1||z-1|}}$$ $$f(C_4) \to (|z+1|e^{i0}|z-1|e^{-i\pi})^{-1/2} = \frac{i}{\sqrt{|z+1||z-1|}}$$

Thus $$\int_C f(z)\ dz \to -2i\int_{-1}^1 \frac{1}{\sqrt{|x^2-1|}}\ dx = -4 \int_0^1 \frac{1}{\sqrt{x^2-1}}\ dx$$

To finish off the integral, we find the residue of $f$ at infinity, which is the same as finding the residue at $0$ of $$g(w) = -\frac{1}{w^2}f\left(\frac{1}{w}\right) = -\frac{1}{w\sqrt{1-w^2}}$$

You can tell this is a single pole, so $$\operatorname*{Res}_{w=0} g(w) = \lim_{w\to 0} \big(w g(w)\big) = -1$$

Altogether this gives a result of $$\int_0^1 \frac{1}{x^2-1}\ dx = -\frac{1}{4}(-2\pi i) = \frac{\pi i}{2}$$

for this branch

For the other branch, the branch cut is on $\Re(z) \in [1,\infty) \cup (-\infty,-1]$, such that $\arg(z+1)\in (-\pi,\pi]$ and $\arg(z-1) \in [0,2\pi)$. Of course this forces you to invert the contour entirely so the two line segments are parallel to the branch cut, so it may not be as straightforward

• Hi Dylan, thank you so much for your response, but could you please explain why the argument for $|z+1|$ is 0 for both line segments yet the argument for $|z-1|$ changes from $-\pi$ to $\pi$ – Ditherer Dec 14 '17 at 9:28
• I think I'm getting it now, you could also have the argument for $|z-1|$ is 0 for both line segments yet the argument for $|z+1|$ changes from $\pi$ to $-\pi$ – Ditherer Dec 14 '17 at 9:44
• Okay, I'm nearly there now... just explain to me a couple of things. The first is the numerator of the bound, $\sqrt{r^2 - 1}$. The second thing is how $-2i\int_{-1}^1 \frac{dx}{\sqrt{|x^2 - 1|}}$ becomes $-4\int_0^1 \frac{dx}{\sqrt{x^2 - 1}}$. I think the first integral is due to it being the sum of $C_1$, $C_2$, $C_3$, $C_4$. As for the second, the integral from -1 to 1 is twice that from 0 to 1, but how does $i$ cancel? – Ditherer Dec 14 '17 at 9:59
• Because $x^2 - 1 < 0$ in the interval so $|x^2-1| = -(x^2-1)$. Also the function is even – Dylan Dec 15 '17 at 12:27
• For the other question, imagine that $\arg(z-1)$ and $\arg(z+1)$ each have individual branch cuts in their negative real parts. However, the overlap cuts "cancel" each other out to make their product continuous on $x < -1$. As a consequence of this $\arg(z+1)$ is continuous on $x > -1$ but has a jump on $x < -1$ – Dylan Dec 15 '17 at 12:32

Let $$f(z)=\frac{1}{\sqrt{z^2-1}}, \quad z\in\mathbb C\setminus[-1,1].$$ First choose the branch with $f(2)=\frac{1}{\sqrt{3}}$. (You can choose the opposite one too.)

Next observe (not trivial) that $f$ is odd and we have the Laurent series $$f(z)=\frac{1}{z}+{\mathcal O}(z^{-3}),$$ and hence $$\int_C f(z)\,dz=2\pi i,$$ where $C$ is the dumbell contour (or any closed simple curve containing $[-1,1]$ in its interior).

In particular, $$\int_C\frac{dz}{\sqrt{z^2-1}}=\lim_{t\searrow0}\left(\int_{-1}^1\frac{dx}{\sqrt{(x-it)^2-1}}-\int_{-1}^1\frac{dx}{\sqrt{(x+it)^2-1}}\right) \\ \overset{\text{f odd}}{=}2\lim_{t\searrow0}\int_{-1}^1\frac{dx}{\sqrt{(x-it)^2-1}} =4\lim_{t\searrow0}\int_{0}^1\frac{dx}{\sqrt{(x-it)^2-1}}.$$

Finally, $$\int_0^1\frac{dx}{\sqrt{x^2-1}}=\frac{1}{4}\int_C f(z)\,dz=\frac{\pi i}{2}.$$

• Sorry, could you provide more details about the Laurent series as well as the very last step? – Ditherer Dec 13 '17 at 11:41