I have a problem concerning the product of two tempered distributions.(adsbygoogle = window.adsbygoogle || []).push({});

We have the temp. distr.

[tex] F(x) = \dfrac{1}{4 \pi} \mathrm{sgn}(x^{0}) \delta(x^{2}) - \dfrac{i}{4 \pi} \mathcal{P}\dfrac{1}{x^{2}} [/tex],

where x is a vector in Minkowski space and [tex]\mathcal{P}[/tex] is the principal value (in the sense of distributions).

The Fourier transformed expression is:

[tex] \hat{F}(p) = \dfrac{i}{2 \pi} \theta(p^{0}) \delta(p^{2}) [/tex].

Now, my problem is that I don't know how to define the product [tex] F(x)^{2} [/tex]. I read in a book that this expression is well-definied, because if we consider the Fourier transform of this product, which will lead to a convolution

[tex] (2 \pi)^{-2} \int d^{4} q \hat{F}(p-q) \hat{F}(q) [/tex]

,we obviously see that the intersection of the two [tex] \hat{F} [/tex] is compact.

I don't "see" that this intersection is really compact. Do you know, why it is compact and why this "intersection-condition" is sufficient to prove that the product in coordinate-space [tex] F(x)^{2} [/tex] resp. the convolution is well-definied?

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# Product of distributions

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