, n}) can be done by taking the gradients of α i with respect to the training objective and directly plugging it into a gradientbased optimization framework, such as the limited memory variable metric (LMVM) used by Smith et al.
unlike its more popular sibling 1 2 norm (used in group lasso), which seeks feature sparsity at the group-level, 2 1 norm encourages sparsity within feature groups.