2 ξ ξ p ξ ξ 2 − − m k − − m E ξ ξ = − − Γ Γ , 2 η η p η η 2 − − m k − − m E η η = Γ Γ , where Γ is a constant of motion. Subtraction and re-expression in terms of the Cartesian momenta p x and p y shows that Γ is equivalent to the LRL vector