In the two-state formalism of Yakir Aharonov, the weak expectation value of an operator A is ⟨χ|A|ψ⟩⟨χ|ψ⟩. This can have bizarre properties. If A is Hermitian, the weak expectation value can be complex. If A is a bounded operator with the absolute value of its eigenvalues all bounded by λ, the weak expectation can exceed λ.
If A=∑iλiPi where {Pi}i is a complete orthonormal set of projectors, the strong expectation value is ∑jλj|⟨χ|Pj|ψ⟩|2∑i|⟨χ|Pi|ψ⟩|2
which is also confusing as the act of measurement affects what is being postselected for.
More specifically, |⟨χ|ψ⟩|2=∑i,j⟨ψ|Pi|χ⟩⟨χ|Pj|ψ⟩≠∑i|⟨χ|Pi|ψ⟩|2
in general.
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