- #1
UVCatastrophe
- 37
- 8
For a research project, I have to take multiple derivatives of a Yukawa potential, e.g.
## \partial_i \partial_j ( \frac{e^{-m r}}{r} ) ##
or another example is
## \partial_i \partial_j \partial_k \partial_\ell ( e^{-mr} ) ##
I know that, at least in the first example above, there will be a Dirac delta function somewhere in the answer,
## \partial_i \partial_j \{ \ldots \} \supset - 4\pi \delta^{i j} \delta^{(3)} (\mathbf{x} ) ##
If you do this by naively applying the chain rule and product rule to the functions, unit vectors, and their derivatives, you will miss it. The way to see that the answer contains a delta function is to apply the divergence theorem, realize there's a contradiction, and then add the 3D Dirac delta ad hoc. (Refer to Chapter 1.5 of Griffiths Electrodynamics for instance.)
This is a subtle point: easy to miss, and worrisome that I have to put some of the answer in by hand. Does anyone know if there are other functions, or combinations of functions and unit vectors, that straight up chain/product rule will give incomplete answers? Or can someone assure me that the delta function is the only case? Thanks!
## \partial_i \partial_j ( \frac{e^{-m r}}{r} ) ##
or another example is
## \partial_i \partial_j \partial_k \partial_\ell ( e^{-mr} ) ##
I know that, at least in the first example above, there will be a Dirac delta function somewhere in the answer,
## \partial_i \partial_j \{ \ldots \} \supset - 4\pi \delta^{i j} \delta^{(3)} (\mathbf{x} ) ##
If you do this by naively applying the chain rule and product rule to the functions, unit vectors, and their derivatives, you will miss it. The way to see that the answer contains a delta function is to apply the divergence theorem, realize there's a contradiction, and then add the 3D Dirac delta ad hoc. (Refer to Chapter 1.5 of Griffiths Electrodynamics for instance.)
This is a subtle point: easy to miss, and worrisome that I have to put some of the answer in by hand. Does anyone know if there are other functions, or combinations of functions and unit vectors, that straight up chain/product rule will give incomplete answers? Or can someone assure me that the delta function is the only case? Thanks!