“…Authors have studied the case where the ntuple is comprised of real numbers [Bor95,Cia68,Fri78,Kel71,Per53,Sal72,Sou83,Sul49], the case where the matrices under consideration are symmetric [Fie74,JLL96], and the general problem [Joh81,LM99,LL79,Rea94,Rea96,Wuw97]. Various necessary conditions and sufficient conditions have been provided, but a complete characterization is known for real n-tuples only for n ≤ 4 [ Kel71,Sul49] and for complex n-tuples only for n ≤ 3 [LL79].…”