silly little joke

#

The dyadic Green’s tensor

$$ \bar{\bar{G}} (\mathbf{r}, \mathbf{r}') = \left[\mathbb{I} + \frac{1}{k^2} \nabla \nabla\right] \frac{e^{\mathrm{i} k |\mathbf{r} - \mathbf{r}'|}}{4\pi |\mathbf{r} - \mathbf{r}'|} $$

should not be confused with the mysterious forest-dwelling dryadic Green’s tensor.