Preamble

This paper, based on things already written somewhere with the addition of some details from lectures, contains nothing or little especially new. Even the main `result', summarized in Eq.(35) and that I hope will contribute to set down the questions raised by http://arxiv.org/abs/astro-ph/0508529astro-ph/0508529 [1], is just a simple extension of Eq. (8.33) of Ref. [2]. Therefore the debated question could be dismissed with a paper even shorter than http://arxiv.org/abs/astro-ph/0508529astro-ph/0508529. Nevertheless, I have taken the opportunity to reorganize old material for the benefit of my students, and I post these pages hoping they could be of some utility to those who wish to understand what there is behind formulas.