# Trying to learn Spinf Foams and a lot of questions

1. Dec 8, 2009

### zwicky

Hi everybody!!

To point, I'm studying the paper of Perez (http://arxiv.org/PS_cache/gr-qc/pdf/0402/0402110v3.pdf) in at the end of page 12 can be read something like
$$\sum_{j_p}(2j_{j_p}+1)<\prod_p\chi_{j_p}(U_p)s,s'>=<\prod_{j_p}\sum_{j_p}(2j_{j_p}+1)\chi_{j_p}(U_p)s,s'>$$

My question is, which is the trick to interchange the prod by the sum? I know that the the prod on the r.h.s runs in j_p instead of p, but still I'm not sure about it.