“…Yang [74] and E.W. Mielke [46] who showed, respectively, that Einstein spaces satisfy equations (3) and (2).There is a substantial bibliography devoted to the study of the system (2), (3) in the special case (4) and one can get an idea of the historical development of the Yang-Mielke theory of gravity from [13,19,20,50,55,64,66,67,73].…”