# How to express $\frac{r}{r^3}$

Is it available write \frac{r}{||r||^3} in cvx. I’ve tried several ways, but I failed.
r is a vector.

In one dimension, this equals -1/r^2 for r < 0 and 1/r^2 for r > 0. So this is neither convex nor concave. Things don;t get any better in higher dimensions.