“…Finding the optimal solution of Equation (2) is an NP-hard [43], i.e., various subsets of the endmembers that are possibly present must be verified for each mixed pixel from a given spectral library. As a remedy, several efficient linear sparse techniques are proposed for the unmixing process, e.g., [4,5,17,18,26]. Minimizing the 1 -norm as approximation instead of the 0 -norm is one of the earliest methods proposed to avoid an exhaustive search for Equation (2) (e.g., see [44,45] and the references therein; see also [17,22,26,35,41,46,47] for unmixing techniques), as follows:…”