A useful and convenient basis for a Fock space is the occupancy number basis. Given a basis { | ψ ψ i ⟩ ⟩ } i = 0 , 1 , 2 , … … of H , we can denote the state with n 0 particles in state | ψ ψ 0 ⟩ ⟩ , n 1 particles in state | ψ ψ 1 ⟩ ⟩ ,..., n k particles in state | ψ ψ k ⟩ ⟩ , and no particles in the remaining states, by defining