“…In [5], Hartman obtained the following inequality: Over the past few decades, there have been many new proofs and generalizations of the inequality (2). It has been generalized to nonlinear second order equations Yong-In Kim [3,10,11], to delay differential equations [2], to higher order differential equations [9,13,14], to discrete linear Hamiltonian systems [4], and so on [6,7,8,12].…”