I am studying a flow in discrete time over a cylinder $ X= \mathbb{R} \times \mathbb{S}^1$ of the form:
$$\begin{cases}y_{n+1}= \frac{1}{2}y_n
\\ \theta_{n+1}= \theta_n + by_n + \frac{1}{2} \end{cases} $$
where the last identity is intended modulo 1, i.e. identifying $$\mathbb{S}^1= \mathbb{R}/\mathbb{Z}.$$
This system is very easy because I have that y contracts and I have unique invariant limit cycle in $y=0$ which is asymptotically stable and is an normally hyperbolic invariant manifold, since I can split the tangent space to the curve $y=0$ in $X$ as the direct sum of the tangent space in such curve "plus" the contractive direction.
Chosen smooth function $ f=f(\theta)\, \colon\, \mathbb{S}^1 \to \mathbb{R}$ and taken $\sigma $ small I want to perturbate the $y$ cooordinate such that I consider the system:
$$\begin{cases}y_{n+1}= \frac{1}{2}y_n + \sigma f(\theta_n)
\\ \theta_{n+1}= \theta_n + by_n + \frac{1}{2} \end{cases} $$
It is well known that a normally hyperbolic invariant manifold $M \subset X$ is preserved under small perturbation, in the sense that for $\sigma$ small enough there exists $M_{\sigma}$ sucht that it is normally hyperbolic and invariant for the perturbed system and is diffeomorphic to $M$. If such exists,
it is easy -I think- to see that trajectories converge exponentially to it, because:
$T_{M_{\sigma}}X= TM_{\sigma} \oplus M_{\sigma}^{s}$, where the latter is the perturbed stable direction and a trajectory starting in $(y,\theta) \in X$ has no other choice that getting to $M_{\sigma}$ by following the contractive direction (since I have no unstable manifold).
First question is : Is this argument true? Because I feel like I am missing something;
Second question: The proof of persistence of invariant manifold relies on the fact that stable and unstable manifolds persist and I take the intersection. In such case I have no unstable manifold in the non perturbed system, hence I do not understand how this manifold "persists", in the sense explained above. Basically the problem is, I perturb a system which has exponential converge to a set and I want to prove there still has exponential convergence to some set which is homeomorphic to the first set using persistence of normally hyperbolic invariant manifolds, but I am not understanding exactly everything. I would appreciate any sort of help.