A sensible sparsity constraint is the L 0 {\displaystyle L_{0}} norm ‖ w ‖ 0 {\displaystyle \|w\|_{0}} , defined as the number of non-zero elements in w {\displaystyle w} . Solving a L 0 {\displaystyle L_{0}} regularized learning problem, however, has been demonstrated to be NP-hard.[3]