# What is the operation of a number in between left double bar of norm symbol, $\vert x \vert y\vert\vert$?

I am reading the paper about group lasso. The equation (3) in the paper has a number, $\sqrt p_{l}$, between left double bar of norm sign as: $\vert\sqrt p_{l}\vert\beta^{(l)}\vert\vert_{2}$ (where $\vert\vert \cdot \vert\vert$ is norm symbol). What is the operation here?

Excerpt from paper