The problems of modern society are complex, interdisciplinary and
nonlin- ear. onlinear problems are therefore abundant in several diverse
disciplines. Since explicit analytic solutions of nonlinear problems in
terms of familiar, well- trained functions of analysis are rarely
possible, one needs to exploit various approximate methods. There do
exist a number of powerful procedures for ob- taining approximate
solutions of nonlinear problems such as, Newton-Raphson method,
Galerkins method, expansion methods, dynamic programming, itera- tive
techniques, truncation methods, method of upper and lower bounds and
Chapligin method, to name a few. Let us turn to the fruitful idea of
Chapligin, see [27] (vol I), for obtaining approximate solutions of a
nonlinear differential equation u' = f(t, u), u(O) = uo. Let fl' h be
such that the solutions of 1t' = h (t, u), u(O) = uo, and u' = h(t, u),
u(O) = uo are comparatively simple to solve, such as linear equations,
and lower order equations. Suppose that we have h(t, u) s f(t, u) s h(t,
u), for all (t, u).