I'm currently reading the article "A Generalization of a Poincaré–Bendixson Theorem to Closed Two-Dimensional Manifolds" by Arthur Shwartz. The paper first establishes a result for minimal sets, which are closed, non-empty subsets of $M$ which are invariant under the flow of some $C^2$ vector field, and contain no such proper subsets. In particular, the result can be summarized as follows:
Suppose that $M$ is a compact, connected, two-dimensional manifold and $\Sigma \subset M$ is a minimal set. Then $\Sigma$ is either:
- a fixed point,
- a periodic orbit (homeomorphic to $S^1$), or
- $M$ itself (which must be homeomorphic to $T^2$).
I was able to follow the proof of this theorem, but fail to understand the subsequent corollary, which may be stated as:
Suppose that $M$ is a compact, connected, orientable, two-dimensional manifold such that $M$ is not a minimal set and for some $x \in M$, the $\omega$-limit set of $x$—$\omega(x)$—contains no fixed points. Then $\omega(x)$ is homeomorphic to $S^1$ and the flow of $x$—$\varphi_{t}(x)$—winds towards $\omega(x)$ as $t \to \infty$.
The argument presented is that $\omega(x)$ must contain a minimal set (say $\Sigma$) which is homeomorphic to $S^1$ by the previous theorem. Now since $M$ is orientable, there exists a neighborhood $N$ of $\Sigma$ which is homeomorphic to $(-1, 1)\times S^1$ such that $\Sigma$ is mapped onto $\{0\}\times S^1$ and the line segments $(-1, 1)\times \{y\}$ are transversal arcs for $y \in S^1$. The setup is clear to me, but I don't follow the remaining steps of the proof: "It follows from the continuity of the flow, and the fact that $\Sigma \subset \omega(x)$, that there exist $0 < t_1 < t_2$, $q \in \omega(x)$, and a transversal $I_q$ through $q$ such that $\varphi_{t_2}(x)$ is between $\varphi_{t_1}(x)$ and $q$ on $I_q$. Thenceforth $\varphi_{t}(x)$ is "trapped" between the closed curve formed by $\varphi_{[t_1, t_2]}(x)$ and the portion of $I_q$ between $\varphi_{t_1}(x)$ and $\varphi_{t_2}(x)$ and $\Sigma$. Thus we find the next intersection of $\varphi_{t}(x)$ with $I_q$ between $\varphi_{t_2}(x)$ and $q$. Repetition of this argument yields the conclusion of the corollary."
I believe that we're aiming to show that $\omega(x) = \Sigma$ (in particular by a monotonicity-type argument on the surface of the cylinder analogous to that used in the proof of the Poincaré–Bendixson theorem in $\mathbb{R}^2$). But how can we be sure that $q \in N$, or that the flow remains within $N$ (and hence its image under the homeomorphism remains on the surface of the cylinder, which I understand to be what "traps" it)? Even if so, how would this ultimately show that $q \in \Sigma$?