Last semester in my course QM 2 we discussed the Rayleigh-Schrödinger perturbation theory. A very elegant theory, based on a general principle: when terms are depending on a certain factor to the nth order, where the factor is very small if not infinitesimally small, you can collect the terms that have the same nth order dependence, and that state that: the terms of the 0th order are the most prominent the terms of the 1th order are second most prominent etc. (I'm a bit clumsy at wording this in English, my apologies.) Now, this semester I'm working on a individual project about thermoacoustics, and I'm using a similar method of perturbation theory - the principle of order division is the same, only the context is different. I've been using this trick without ever questioning where it came from - however, it's time to write a report about it, so I feel I really should know where it came from. However, I can't seem to find a good site about this, and considering I don't even know the name of such a theory, it's not easy to look for it either. So, my question is: what is the name of the abstract theory that describes this method of succesive approximation? It's always nice if you can provide a lucid link as well, but providing me a name would be a great help already. Thanks.