Compatible Observables occur when commutator is equal to 0.

[Â,B̂] = ÂB̂ - B̂Â

Constant of Motion
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    if d/dt Â = 0 and [Ĥ,Â] = 0, where Ĥ is the Hamiltonian operator.
    then Â is a constant of motion

    Thm: Two operators such that [Â,B̂] always have common Eigenstates