Quartic anharmonic oscillator with potential V(x)= x(2) + g(2)x4 was the first non-exactly-solvable problem tackled by the newly-written Schroedinger equation in 1926. Since that time thousands of articles have been published on the subject, mostly about the domain of small g(2) (weak coupling regime), although physics corresponds to g(2) ~ 1, and they were mostly about energies.This book is focused on studying eigenfunctions as a primary object for any g(2). Perturbation theory in g(2) for the logarithm of the wavefunction is matched to the true semiclassical expansion in powers of : it leads to locally-highly-accurate, uniform approximation valid for any g(2) [0, ) for eigenfunctions and even more accurate results for eigenvalues. This method of matching can be easily extended to the general anharmonic oscillator as well as to the radial oscillators. Quartic, sextic and cubic (for radial case) oscillators are considered in detail as well as quartic double-well potential.

Today Lie group theoretical approach to differential equations has been extended to new situations and has become applicable to the majority of equations that frequently occur in applied sciences. Newly developed theoretical and computational methods are awaiting application. Students and applied scientists are expected to understand these methods. Volume 3 and the accompanying software allow readers to extend their knowledge of computational algebra.
Written by the world's leading experts in the field, this up-to-date sourcebook covers topics such as Lie-BAcklund, conditional and non-classical symmetries, approximate symmetry groups for equations with a small parameter, group analysis of differential equations with distributions, integro-differential equations, recursions, and symbolic software packages. The text provides an ideal introduction to modern group analysis and addresses issues to both beginners and experienced researchers in the application of Lie group methods.