Can Cantor set constructed on non compact or disconnected set, such as $(0,1)$ or $[0,1]$\ $\mathbb{Q}$

general-topologymeasure-theoryreal-analysis

The construction of Cantor set is always start from a compact set. I wonder what if we have $[0,1)$ at first, and then construct Cantor set?

Edit:My original thought is we must construct Cantor set from compact set, so the complement of Cantor set will give an open interval in $[0,1)$. Which doesn't sound right now. What about $(0,1)$ instead? What is the case of $[0,1]$\ $\mathbb{Q}$?Can we still construct Cantor set from them?

Best Answer

I think I know what you are after.

The first iteration $D_1 = [0,1)$.

The second iteration $D_2 = [0,\frac13) \cup [\frac23,1)$.

The second iteration $D_3 = [0,\frac19) \cup [\frac29,\frac13) \cup [\frac23,\frac79) \cup [\frac89,1)$.

The $k$th iteration $D_k = \frac13 C_{k-1} \cup (\frac23 + \frac13 C_{k-1})$.

Then $D = \bigcap_k D_k$.

We can express $D$ using base 3 expansions. The Cantor Set is $$ C = \{0.a_1a_2a_3\dots (\text{base $3$}):a_i=0\text{ or }2\} ,$$ and $D$ is those elements of the Cantor set whose base 3 expansion does not end in recurring 2s.

If you start with $(0,1)$, you will end up with those elements of the Cantor set whose base 3 expansion neither terminate, nor end up in recurring 2s.

If you start with $[0,1] \setminus \mathbb Q$, you will simply end up with $C \setminus \mathbb Q$.

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