Connected components of $\mathbb{R}^2 \setminus \gamma$, where $\gamma \subset \mathbb{R}^2$ is a closed path

connectednesscurvesgeneral-topologyplane-curves

Let $\gamma$ be a closed path in $\mathbb{R}^2$ ($\gamma : [0,1] \to \mathbb{R}^2$ is continuous and satisfys $\gamma(0)=\gamma(1)$). Are the following statements true?

  1. $\gamma$ divides the plane into (at most countably infinite, since every component would contain a rational point) connected components $Q_1, Q_2, Q_3,…$:

$$\mathbb{R}^2 \setminus \gamma = \bigcup_{i=1}^{\infty} Q_i$$

  1. There is exactly one unbounded component (which we call) $Q_1$. $Q_i$ is bounded for every $i\geq 2$.

  2. The boundary of the union of all bounded components is equal to $\gamma$:
    $$\partial \bigcup_{i=2}^{\infty} Q_i = \gamma$$
    If 3 doesn't hold, do we at least know that the boundary is a subset of $\gamma$?

Essentially I am asking if a generalization of the Jordan curve theorem for not necessarily simply connected paths is true.

Best Answer

  1. This is true, for the reason you specified (and since the components partition the space $\mathbb{R}^2 \setminus \gamma$)
  2. This is true. The function $|\gamma| : [0,1] \to \mathbb{R}$ is a continuous function with a compact domain, so it has a maximum $R$. Draw a circle centered at $(0,0)$ with radius $R+1$. All the points outside this circle must lie in the same component.
  3. This is false. For instance, if $\gamma([0,1]) = [0,1]^2$ is a spacefilling curve, then you'd have $\mathbb{R}^2 \setminus \gamma = \mathbb{R}^2 \setminus [0,1]^2$ is connected, so $\partial(\bigcup_{i=2}^\infty Q_i) = \partial(\emptyset) = \emptyset \neq [0,1]^2$. It will, however, be the case that $\partial\bigcup_{i=2}^\infty Q_i \subseteq \gamma$, which follows from the fact each $Q_i \subset \mathbb{R}^2$ is open.
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