I saw this in a book:
can be simplified to , if one is interested only in the time scale slower than ,
I don't know how this is got, could someone help me with it? Many thanks!
I'm not sure now, ...
The author of that book is coming to visit the day after tomorrow, I'll take that chance and consult him directly. After that I will post the what he says to me.
I strongly suspect he had some typo or something! That solution is wrong.