 of the partition function formula_14 to evaluate the expectation value of formula_15
 The time dependence of the states formula_16 is governed by the Schrödinger equation formula_17 which thus determines everything, corresponding of course to the Schrödinger picture.But since formula_18 is to be regarded as a small perturbation, it is convenient to now use instead the interaction picture representation, formula_19 in lowest nontrivial order.The time dependence in this representation is given by formula_20 where by definition for all t and formula_21 it is