“…Antonacci [3,4] gave conditions for existence of solutions with stronger constraints on the potential but without the homogeneity condition, and without the negative definite condition on the matrix. Generalizations of the above results are given by Antonacci and Magrone [2], Barletta and Livrea [6], Guo and Xu [16], Li and Zou [24], Faraci and Livrea [15], Bonanno and Livrea [7,8], Jiang [21,22], Shilgba [39,40], Faraci and Iannizzotto [14] and Tang and Xiao [53]. Some authors considered the second order system (1) where the potential function V (t, x) is quadratically bounded as |x| → ∞.…”