help about the Nonequilibrium Green's Function
in H. Haug and A.-P. Jauho Book
Quantum kinetics in transport and Optics of Semiconductors
Eq.(4.31)
C^r(t,t')=A^<(t,t')B^r(t,t')+A^r(t,t')B^<(t,t')+A^r(t,t')B^r(t,t')
I can not derive this equation, my result has a extra term \theta(t-t')
i.e...