“…Problem (1) encompasses a broad range of the regression models. It includes as special cases: ridge regression [22], when λ > 0, µ = 0 and k ≥ p; lasso [34], when λ = 0, µ ≥ 0 and k ≥ p; elastic net [44] when λ, µ > 0 and k ≥ p; best subset selection [29], when λ = µ = 0 and k < p. Additionally, Bertsimas and Van Parys [7] propose to solve (1) with λ > 0, µ = 0 and k < p for high-dimensional regression problems, while Mazumder et al [28] study (1) with λ = 0, µ > 0 and k < p for problems with low Signal-to-Noise Ratios (SNR). The results in this paper cover all versions of (1) with k < p; moreover, they can be extended to problems with non-separable regularizations of the form λ Aβ 2 2 + µ Cβ 1 , resulting in sparse variants of the fused lasso [35], generalized lasso [36] and smooth lasso [21], among others.…”