Consider a 2D sigma model with a symmetry group G and whose generators obey [TA,TB]=fCABTC
From this it is straightforward to construct a conserved quantity QA=∫∞−∞j0A(x,t)dx
In integrable systems, we can generate an infinite amount of additional, non-local conserved charges if the current satisfies the flatness condition ∂μjν−∂νjμ+[jμ,jν]=0 ,
QA (1)=fABC∫∞−∞dx∫∞xdyj0B(x,t)j0C(y,t)−2∫∞−∞dxjA1(x,t) .
I don't understand how this result is obtained from the flatness condition, and I can't find a derivation anywhere (the result is just stated, e.g. in section 2 of http://arxiv.org/abs/hep-th/0308089 and equations (11-12) of http://arxiv.org/abs/hep-th/0404003).
Any help would be appreciated.
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