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I want to prove $\frac{\mathrm{d} }{\mathrm{d} x}\Theta =\delta (x)$ using this representation of the delta function: $\delta(x)= \frac{1}{2\pi} \int_{-\infty}^{\infty} e^{ikx}dk $ This should be easy. I just need to integrate $\frac{1}{2\pi} \int_{-\infty}^{\infty} e^{ikx}dk $ with respect to x to get $ \Theta(x) $

This is the first step:

$$\Theta(x) = \int_{-\infty}^\infty \frac{-i}{2 \pi k} e^{ikx} \, dk $$

This doesn't look like the step function to me. So I already suspect that I'm going down the wrong road here.

But, unless I'm being naïve, theoretically this should work.

I tried to do integration by parts, but I think that I'm going down the wrong road because I just have a more complicated mess to deal with. Have I made an error? Is there another way to evaluate this integral?

The Step Function:

$\theta(x) = 1$ if $x>0$

$\theta(x) = 0$ if $x \leq 0$

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  • $\begingroup$ Welcome to Math.SE! Could you provide us with the definition of the stepfunction $\Theta$? Apart from the missing constant $2\pi$, everything looks fine, although I don't know whether this is the easiest way of proving this. $\endgroup$ – Hrodelbert Apr 27 '15 at 10:16
  • $\begingroup$ @Hrodelbert Yes. I thought this would be simple until I started applying integration by parts and got a big mess. $\endgroup$ – mathmath12 Apr 27 '15 at 10:19
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Informally: $$\int_{-1}^{1} f(x)\Theta'(x) \, dx = \left[\vphantom{\frac11} f(x) \Theta(x) \right]_{-1}^{+1} - \int_{-1}^1 f'(x)\Theta(x)\,dx = f(1) - \left[f(x) \vphantom{\frac11} \right]_0^1 = f(0)$$

which is precisely what you want from $\delta(x)$. So $\delta(x)=\Theta'(x)$.

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What you're trying to do technically needs to be understood in the context of distribution theory. But since we can hardly go about introducing the intricate details of distribution theory here, I'll just offer a quick suggestion: show that $$ \int_{-\infty}^\infty \delta(x)\psi(x)~dx = -\int_{-\infty}^\infty \Phi(x)\frac{d}{dx}\psi(x)~dx $$ for all smooth functions $\psi$ with compact support (i.e. vanishes outside of a finite closed interval). Then integrate the right-hand side by parts to see that what you have is $$ \int_{-\infty}^\infty \delta(x)\psi(x)~dx = \int_{-\infty}^\infty \frac{d}{dx}\Phi(x)\psi(x)~dx $$ for all smooth $\psi$ of compact support. Therefore "$\delta(x) = \frac{d}{dx}\Phi(x)$."

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