Entrywise matrix L1 norm ($\Vert M{\Vert }_{1,\mathrm{entry}}={\sum }_{r,s}|M_{rs}|$)

For a real matrix $M$, the entrywise L1 norm is ${\sum }_{r,s}|M_{rs}|$, the L1 norm of its vector of entries. It is the usual penalty in the all-entry convention for the Graphical Lasso.