or some other measure of predictability from the reduced density matrix of the system ρ ρ Ψ Ψ ( t ) (which is initially ρ ρ Ψ Ψ ( 0 ) = | Ψ Ψ ⟩ ⟩ ⟨ ⟨ Ψ Ψ | ). The entropy is a function of time and a functional of the initial state | Ψ Ψ ⟩ . Pointer states are obtained by minimizing H Ψ Ψ over | Ψ Ψ ⟩ and demanding that the answer be robust when varying the time t .