# Cantor Space is Nowhere Dense

## Theorem

Let $T = \left({\mathcal C, \tau_d}\right)$ be the Cantor space.

Then $T$ is nowhere dense in $\left[{0 \,.\,.\, 1}\right]$.

## Proof 1

From Cantor Set is Closed in Real Number Space, $\CC$ is closed.

So from Closed Set equals its Closure:

- $\CC^- = \CC$

where $\CC^-$ denotes the closure of $\CC$.

Let $0 \le a < b \le 1$.

Then $I = \openint a b$ is an open interval of $\closedint 0 1$.

Let $\epsilon = b - a$.

Clearly $\epsilon > 0$.

Let $n \in \N$ such that $3^{-n} < \epsilon$.

So there exists an open interval of $\closedint 0 1$ which has been deleted from $\closedint 0 1$ during the process of creating $\CC$.

Thus no open interval of $\closedint 0 1$ is disjoint from *all* the open intervals deleted from $\closedint 0 1$.

So an open interval of $\closedint 0 1$ can not be a subset of $\CC = \CC^-$.

Hence the result, by definition of nowhere dense.

$\blacksquare$

## Proof 2

Let $S_n$ and $C_n$ be as in the definition of the Cantor set as a limit of a decreasing sequence.

Then the length of every interval in $S_n$ is seen to be $\dfrac 1 {3^n} = 3^{-n}$.

Let $0 \le a < b \le 1$.

Then $\left({a \,.\,.\, b}\right) \subseteq \left[{0 \,.\,.\, 1}\right]$ is an open interval.

Let $n \in \N$ such that $3^{-n} < b - a$, so that the length of every interval in $S_n$ is $3^{-n} < b - a$.

Therefore, as the intervals in $S_n$ do not overlap, no interval of length $b - a$ is contained in $C_n = \displaystyle \bigcup S_n$.

Consequently, as $\mathcal C \subseteq C_n$, no interval of length $b - a$ is contained in $\mathcal C$.

Since the interval $\left({a \,.\,.\, b}\right)$ was of arbitrary length, there do not exist any open intervals in $\mathcal C$.

Hence the result, by definition of nowhere dense.

$\blacksquare$

## Sources

- 2005: René L. Schilling:
*Measures, Integrals and Martingales*... (previous) ... (next): $\S 7$: Problem $10 \ \text{(v)}$