Abstract
For an observable which can be represented by a self-adjoint operator belonging to a rather restrictive class, it is shown that the problem of calculating the expectation value up to the nth order is equivalent to solving n commutator equations. In the first order the theory yields naturally the interchange theorem for any observable. For the calculation of the first-order corrections, the theory leads to a differential equation which is solved explicitly for a wide class of multiplication operators in the configuration space.

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