Exactness of Second Order Ordinary Differential Equations and   Integrating Factors
  A new problem is studied, the concept of exactness of a second order nonlinear ordinary differential equations is established. A method is constructed to reduce this class into a first order equations. If the second order equation is not exact we introduce, under certain conditions, an integrating factor that transform it to an exact one. 
R. AlAhmad, M. Al-Jararha and H. Almefleh
math.CA