On the thin boundary of the fat attractor

For, $0<\lambda<1$, consider the transformation $T(x) = d x $ (mod 1) on the circle $S^1$, a $C^1$ function $A:S^1 \to \mathbb{R}$, and, the map $F(x,s) = ( T(x) , \lambda \, s + A(x))$, $(x,s)\in S^1 \times \mathbb{R}$. We denote $\mathcal{B}= \mathcal{B}_\lambda$ the upper boundary of the attractor (known as fat attractor). We are interested in the regularity of $\mathcal{B}_\lambda$, and, also in what happens in the limit when $\lambda\to 1$. We also address the analysis of following conjecture which were proposed by R. Bam\'on, J. Kiwi, J. Rivera-Letelier and R. Urz\'ua: for any fixed $\lambda$, generically $C^1$ on the potential $A$, the upper boundary $\mathcal{B}_\lambda$ is formed by a finite number of pieces of smooth unstable manifolds of periodic orbits for $F$. We show the proof of the conjecture for the class of $C^2$ potentials $A(x)$ satisfying the twist condition. We do not need the generic hypothesis. When $\lambda$ is close to $1$ and the potential $A$ is generic a more precise description can be done. In this case the pieces of $C^1$ curves on the boundary have some special properties. Finally, we present the general analysis of the case where $A$ is Lipschitz and its relation with Ergodic Transport.