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[tex](\textbf{q}\bullet\nabla)(\textbf{p}\bullet\nabla)\frac{1}{r} = -(\textbf{p}\bullet\textbf{q})\frac{1}{r^{3}} + 3(\textbf{p}\bullet\textbf{r})(\textbf{q}\bullet\textbf{r})\frac{1}{r^{5}}[/tex]

r is distance between field point and dipole source, [tex]\textbf{p}[/tex] is dipole moment, and I believe [tex]\textbf{q}[/tex] may be quadrupole moment tensor (what is that anyways?),

How is the above equation derived ? and exactly how is it related to the more physically and mathematically lucid dipole potential below:

[tex](\textbf{p}\bullet\nabla)\frac{1}{r} = -(\textbf{p}\bullet\textbf{r})\frac{1}{r^{3}}[/tex]

r is distance between field point and dipole source, [tex]\textbf{p}[/tex] is dipole moment, and I believe [tex]\textbf{q}[/tex] may be quadrupole moment tensor (what is that anyways?),

How is the above equation derived ? and exactly how is it related to the more physically and mathematically lucid dipole potential below:

[tex](\textbf{p}\bullet\nabla)\frac{1}{r} = -(\textbf{p}\bullet\textbf{r})\frac{1}{r^{3}}[/tex]

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