[Math] Contour integration of trigonometric functions

complex-analysis

I'm trying to show that:

$$ \int_0 ^{\infty} \frac{\sin(px)\sin(qx)}{x^2} dx = \frac{\pi}{2}\min{(p,q)}$$

With $p,q \ge 0$

So far I have considered the function $f(z) = \frac{e^{itz}}{z^2}$ and integrated this around a semi-circular contour, radius R, suitably indented at the origin. The residue at $z=0$, coming from a double pole I get as $it$. Taking the limit as $R \to \infty$ and then looking at the real part I obtain:

$$\int_0 ^{\infty} \frac{\cos(tx)}{x^2} = \frac{-\pi}{2}$$

Now writing $t = p+q$ I can expand the $cos$ however I am having difficulty seeing how $\min{(p,q)}$ could possibly come into this problem.

Any help is appreciated,
Thanks

Best Answer

Hint: You are not looking for $\min(p,q)$ but for a term that looks like:

$$|p+q|-|p-q|=2\min(p,q)$$

There is a similar formula for $\max(p,q)$

$$|p+q|+|p-q|=2\max(p,q)$$

Also consider that we can write

$$\sin(px)\sin(qx) = \dfrac{\cos((p-q)x)-\cos((p+q)x)}{2}$$

I think you can see a clear resemblance in the formulae.

It is another important note that $$\int_0^{\infty} \dfrac{\cos(tx)}{x^2} dx$$ does not converge because of issues around zero. Instead consider $$\int_0^{\infty} \dfrac{\cos(ax)-\cos(bx)}{x^2} dx$$