Non-existence of invariant circles

The dynamical system associated to the difference equation has been studied numerically by several authors. On the basis of numerical evidence, they conclude that there exists a numberk0≈ 0.97 such that there are homotopically non-trivial invariant circles for |k|≤k0and there are none for |k|>k0. In this note, we give a simple rigorous proof that there are none for |k|> .