Given our intuitions above, it would then seem natural to augment the discriminative objective function with a term to minimize the rank of M. In practice, the rank of an order-M tensor is computed using the nuclear norm X * which is defined as (Friedland and Lim, 2014), When M = 2, this reduces to the matrix nuclear norm (sum of singular values).
computing the rank of a tensor or its nuclear norm is NP-hard for tensors of order ≥ 3 (Friedland and Lim, 2014).