# How can we expand product of two currents into series?

by ndung200790
Tags: currents, expand, product, series
 P: 520 Please teach me this: In QFT book of Peskin&Schroeder write: J$_{\mu}$(x)J$_{\nu}$(0)~C$_{\mu\nu}$$^{1}$(x).1+C$_{\mu\nu}$$^{q^{-}q}$q$^{-}$q(0)+C$_{\mu\nu}$$^{F^{2}}$(x)(F$_{\alpha\beta}$$^{a}$)$^{2}$(0)... I do not know how to expand the product into this series. Thank you very much for your kind helping.