There is a further restriction — the solution must not grow at infinity, so that it has either a finite L -norm (if it is a bound state) or a slowly diverging norm (if it is part of a continuum): ‖ ‖ ψ ψ ‖ ‖ 2 = ∫ ∫ | ψ ψ ( x ) | 2 d x .