“…In a linear setting, this is represented by a matrix, originally introduced by Dimarag-onas and co-workers [8,9,10,11,3,12,13,14,14,15,16]. As for later studies, without pretending to be exhaustive, we may quote [17,18,19,20,21,22]. We may refer the interested readers to the early review article by Doebling et al [1] and to the recent review by Hou and Xia [23].…”