A popular regularization penalty is ∑ j β j 2 {\displaystyle \sum _{j}\beta _{j}^{2}} , which is the squared Euclidean norm of the weights, also known as the L 2 {\displaystyle L_{2}} norm. Other norms include the L 1 {\displaystyle L_{1}} norm, ∑ j | β j | {\displaystyle \sum _{j}|\beta _{j}|} , and the L 0 {\displaystyle L_{0}} norm, which is the number of non-zero β j {\displaystyle \beta _{j}} s. The penalty will be denoted by C ( g ) {\displaystyle C(g)} .