In general, the one-particle state could be described by a complete set of quantum numbers denoted by ν ν . For example, the three quantum numbers n , l , m associated to an electron in a coulomb potential, like the hydrogen atom, form a complete set (ignoring spin). Hence, the state is called | ν ν ⟩ ⟩ and is an eigenvector of the Hamiltonian operator. One can obtain a state function representation of the state using ψ ψ ν ν ( r ) = ⟨ ⟨ r | ν ν ⟩ ⟩ . All eigenvectors of a Hermitian operator form a complete basis, so one can construct any state | ψ ψ ⟩ ⟩ = ∑ ∑ ν ν | ν ν ⟩ ⟩ ⟨ ⟨ ν ν | ψ ψ ⟩ ⟩ obtaining the completeness relation: