Suppose we are given a Hilbert space and a Hermitian operator over it called the Hamiltonian H . Ignoring complications about continuous spectra, we consider the discrete spectrum of H and a basis of eigenvectors { | ψ ψ λ λ ⟩ ⟩ } (see spectral theorem for Hermitian operators for the mathematical background): ⟨ ψ ψ λ λ 1 | ψ ψ λ λ 2 ⟩ = δ δ λ λ 1 λ λ 2 , where δ δ i j is the Kronecker delta δ δ i j = { 0 if i ≠ ≠ j , 1 if i = j , and the { | ψ ψ λ λ ⟩ ⟩ } satisfy the eigenvalue equation H | ψ ψ λ λ ⟩ = λ λ | ψ ψ λ λ ⟩ .