The solutions of the Schrödinger equation take different forms for different values of x, depending on whether M (x) is positive or negative. When M (x) is constant and negative, 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 ) = − − k 2 Ψ Ψ ( x ) , where k 2 = − − 2 m ℏ ℏ 2 M .