Equation (9.1.14) seems to contradict (9.1.4), since it has a minus sign. Equation (9.1.14) is an equation for the density matrix elements, however, while (9.1.4) is for the time-dependent operator. Show that these two equations both give the same result for the time evolution of the element ρmn = c^∗_mc_n by explicit substitution, using |ψ_m〉 = b^†_m|0〉 and

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Η = ΣEbbi |v) = Σqb|0). (9.1.18)