The most physically interesting systems are not exactly solvable in quantum
mechanics. For one-dimensional bound systems without exact solutions, we a
nalytically and numerically find that the Rayleigh-Schrodinger perturbed se
ries sensitively depends on an unsolvable integration, which leads to numer
ical instability in quantum mechanics. By using an exact formal solution of
the non-homogeneous Schrodinger equation, we demonstrate the existence of
analytically bound states and propose a simple scheme to truncate infinity
so that the instability difficulty is avoided.