The http://en.wikipedia.org/wiki/Fine_structure#Darwin_term" contains a (3D-)delta function as a result of taking the Laplacian of the Coulomb potential. I'm trying to find out why. I've been searching, and I've so far come across different views of the Laplacian of 1/r at the origin. Either it's considered zero, or(adsbygoogle = window.adsbygoogle || []).push({});

[tex]

\nabla^2\frac 1 r = -\,\frac{\delta(r)}{r^2},

[/tex]

but I can't find any reference that says it's [itex]\delta^3(r)[/itex]. It has the right units, but that's about all I can say about it. Could someone clarify this?

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# Laplacian of 1/r in Darwin term

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