The solutions of this equation represent travelling waves, with phase-constant + k or − k. Alternatively, if M (x) is constant and positive, then the Schrödinger equation can be written in the form d 2 d x 2 Ψ Ψ ( x ) = 2 m ℏ ℏ 2 M ( x ) Ψ Ψ ( x ) = κ κ 2 Ψ Ψ ( x ) , where κ κ 2 = 2 m ℏ ℏ 2 M .