The range of problems capable of being addressed using this theory was expanded by Lighthill's and Langer's methods. The Lindstedt-Poincar?method yields approximate solutions to nonlinear differential equations using this theory. M?ller and Plesset applied it to obtain the correlation error produced by Hartree-Fock theory. Prandtl applied this theory to solve boundary layer problems dealing with the flow of low velocity fluids over solid objects. Like Rayleigh-Ritz and WKB, it can be used to approximate eigenenergies. For 10 points, name this theory that, when applied to quantum mechanical systems, starts from solutions to the Schr?dinger's equation and approximates 
The range of problems capable of being addressed using this theory was expanded by Lighthill's and Langer's methods. The Lindstedt-Poincar?method yields approximate solutions to nonlinear differential equations using this theory. M?ller and Plesset applied it to obtain the correlation error produced by Hartree-Fock theory. Prandtl applied this theory to solve boundary layer problems dealing with the flow of low velocity fluids over solid objects. Like Rayleigh-Ritz and WKB, it can be used to approximate eigenenergies. For 10 points, name this theory that, when applied to quantum mechanical systems, starts from solutions to the Schr?dinger's equation and approximates the effect of small additional forces on the eigenenergies and wavefunction.
The range of problems capable of being addressed using this theory was expanded by Lighthill's and Langer's methods.
perturbation theory