  The method of variation of parameter (VOP) for solving linear ordinary
differential equation is revisited in this article. Historically, Lagrange and
Euler explained the method of variation of parameter in the context of
perturbation method. In this article, we explain the construction of particular
solutions of a linear ordinary differential equation in the light of linearly
independent functions in a more systematic way. In addition, we have shown that
if the time variation of the constants contribute substantially to the velocity
then also the solution remains invariant. VOP method for system of n linear ODE
is discussed. Duhamels principle has also been studied in reference to a system
of n linear ODE for completeness of this review. Finally, applications of VOP
method for constructing Green's function is reported.
