“…For these examples m = 2 and the most complete classification and structure results are known for the second order case. There is an extensive literature on the subject [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22], with a recent new burst of activity [23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44]. All such systems have been classified for real and complex Riemannian spaces with n = 2 and their associated quadratic algebras of symmetries computed [15,29,30,24,31].…”