Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

I'm currently learning about the renormalization group (RG) in condensed matter physics and just want to clarify a couple of things:

When doing the RG transformation, there's a flow to a fixed point. A coupling constant is a relevant operator (or relevant coupling, depending on which book you look at) if it gets larger as the transformation continues, and flows towards the fixed point.

Have I understood that correctly?

If doing perturbative renormalization, is it right that if you have a relevant operator it is not possible to use that operator in a perturbative expansion because it's large once the transformations are done (even if it was small to start with - before the RG transformations were done)?

share|cite|improve this question
Yes and yes. Though quite often people ignore convergence issues. For real problems, it's best to let nature/experiment decide whether a series converges rather than trying to prove it one way or the other (semi-tongue in cheek). – genneth Jan 17 '12 at 14:50
@genneth Thank you. Any chance you could look at my other question on renormalization? - It's linked to this one, so I thought you might know the answer. – Space boy Jan 17 '12 at 15:41
The fixed point can be at small coupling, like large N. – Ron Maimon Jan 17 '12 at 16:51
@Ron Thank you, but I'm a beginner at this, and don't understand how your answer relates to my question - could you clarify what you mean? – Space boy Jan 17 '12 at 17:29
@Space boy: be patient, real answers will come. The comments are just off the cuff. – Ron Maimon Jan 17 '12 at 21:53

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Browse other questions tagged or ask your own question.