“…Bienstock () (see also Shaw et al., ) shows that problem (1) with cardinality constraint is NP‐hard, even when . Several authors (see, e.g., Chang et al., ; Fernández and Gómez, ; Soleimani et al., ; Deng and Lin, ; Woodside‐Oriakhi et al., ) consider an equality version for cardinality constraint and propose mainly heuristic methods to solve the corresponding problem: Moreover, finding the assets that should be included in the optimal portfolio is, in general, an NP‐hard problem (Moral‐Escudero et al., ).…”