The "Bender and Orszag" analog for PDE There is a famous book written by Bender and Orszag named "Advanced Mathematical Methods for Scientists and Engineers: Asymptotic Methods and Perturbation Theory" which explains how to obtain approximated solutions for ordinary differential equation. Well, now I'm looking for something similar: a book of perturbation theory and asymptotic methods for PDE. There are many of course and I'd like something which focuses much more on application than mathematical rigor, but without being useless. Just like the "Bender-Orszag". Do you know anything? Ll.