Suppose we have a quantum state, well described by its time-independent wave function Psi. And we have a well-defined Hermitian (self-adjoint) operator A. We successfully evaluate the expectation value of the operator A. Next we derive the general formula for the higher moments of A (i.e. the expectation value of An for n=2,3,4…).
In this situation, is it permitted to regard each of the An for n=1,2,3,… as a proper operator by itself?
For example, should every An have a positive variance and other statistical properties (as long as we restrict ourselves to the state Ψ)?
Can one make linear combinations of different powers to construct a new operator, e.g. B=A+A2?
Is it allowed to construct new operators acting on Ψ, that are defined in terms of their series expansion in An? For example, D=exp(CA) where C is a constant?
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