“…For instance, by considering the case in which all diagonal terms are equal to κ 11 , and respectively all offdiagonal terms are κ 12 , we find that the suitable conditions are κ 11 −κ 12 < 1, and κ 11 +2κ 12 < 1. On the other hand, by considering arbitrary diagonal terms, κ 11 , κ 22 , κ 33 , and all the off-diagonal terms vanishing except for κ 12 = κ 21 , we find a well behaved particle spectrum if κ 33 < 1, κ 11 + κ 22 < 1 and κ 11 +κ 22 +(κ 12 ) 2 −κ 11 κ 22 < 1. Also, tachyonic modes must appear whenever the argument in square root of ( 18) is negative.…”