Non-invertibility of certain almost Mathieu operators

It is shown that the almost Mathieu operators of the type $Te_n~=$ $e_{n-1}+\lambda sin(2nr)e_n+e_{n+1}$where $\lambda$ is real and $r$ is a rational multiple of $\pi$and $\{e_n:n=1,2,3,...\},$ an orthonormal basis for a Hilbert space, is not invertible.