“…Systems (1) satisfying (2) and (3) have been studied extensively, see for instance [7,9,10,14,15,17,21]. For the symmetrical cases, that is a = 0, k 1 = k 3 and l 1 = l 3 , see [9] for the bifurcation sets and existence of the limit cycles, see [10] for the amplitude and period of the limit cycles, see [17] for the global phase portraits and bifurcation diagrams.…”