Structured backward error in the WKB method

Robert Corless, Editor-in-Chief, Maple Transactions

Friday May 15th at 2:30am in K9509

The classical WKB method (also known as the WKBJ method, the LG method, or the phase integral method) for solving singularly perturbed linear differential equations has never, as far as we know, been looked at from the structured backward error (BEA) point of view. This is somewhat surprising, because a simple computation shows that for some important problems, the WKB method gives the exact solution of a problem of the same structure that can be expressed in finitely many terms. We show this is possible for George Green's canal equations (arguably where the WKB method was actually invented). This kind of analysis can be extremely useful in assessing the validity of a solution provided by the WKB method.
In this talk we show how to do this and explore some of the consequences, which include a new iterative algorithm to improve the quality of the WKB solution. We also explore a new hybrid method where the potential is approximated by Chebyshev polynomials, which can be implemented in a few lines of Chebfun.