Thus d d t A ( t ) = i ℏ ℏ [ H , A ( t ) ] + e i H t / ℏ ℏ ( ∂ ∂ A ∂ ∂ t ) e − − i H t / ℏ ℏ , whence the above Heisenberg equation of motion emerges, since the convective functional dependence on x (0) and p (0) converts to the same dependence on x (t), p (t), so that the last term converts to ∂ A (t)/∂ t . [ X, Y ] is the commutator of two operators and is defined as [ X, Y ] := XY − YX.