Many problems in his books stress this property of perturbation theory.

On the more advanced side, mathematical perturbation theory is also sometimes used.

It is possible to treat the equation using perturbation theory.

Techniques from perturbation theory find ample application in such areas.

Under some circumstances, perturbation theory is an invalid approach to take.

In a similar way as for small perturbations, it is possible to develop a strong perturbation theory.

In practice, some kind of approximation (perturbation theory) is generally required.

In that case the degenerate perturbation theory must be applied.

This makes it possible to use perturbation theory to obtain results that would otherwise be extremely difficult to find.

A short introduction to the perturbation theory of linear operators.