For time-independent quantum mechanical systems with discrete energy eigenstates, a similar theorem holds. For every ε ε > 0 and T 0 > 0 there exists a time T larger than T 0 , such that | | ψ ψ ( T ) ⟩ ⟩ − − | ψ ψ ( 0 ) ⟩ ⟩ | < ε ε , where | ψ ψ ( t ) ⟩ ⟩ denotes the state vector of the system at time t.