We know that $\mathbf{r} \cdot \mathbf{r} = x^2 + y^2 + z^2$, so we have:
$$(\mathbf{r} \cdot \mathbf{r}) \mathbf{a} = (x^2 + y^2 + z^2) \mathbf{a} = x^2 \mathbf{a} + y^2 \mathbf{a} + z^2 \mathbf{a}$$
Now, we need to find the divergence of this expression.
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