# Show that a number belongs or does not belong to a Cantor set

I have trouble showing that $$\frac{10}{31}$$ and $$\frac{45}{69}$$ belong/do not belong to the Cantor set. I tried to do the following $$\sum_{n = x}^\infty \frac{2}{3^n} = \frac{2}{3^x} + \frac{2}{3^{(x + 1)}} + \frac{2}{3^{(x + 2)}} + \dots = \frac{2}{3^x} \left[ 1 + \frac{1}{3^1} + \frac{1}{3^2} + \dots \right] = \frac{2}{3^x} \sum_{k = 0}^\infty \frac{1}{x^k} = \frac{2}{3^x} \times \frac{1}{1- \frac{1}{3}} = \frac{1}{3^{x- 1}}$$ and then plug different values of $$x = 1, 2, \dots$$, but I could not seem to get these numbers. Does it mean they do not belong to the Cantor set? Please help if you can. Thank you so much.

• Well you could just do long division in base 3 to find their ternary expansions. Mar 21, 2021 at 21:55
• Cantor set is precisely the subset of $[0,1]$ where the elements have no 1's in their base 3 expansion.
– Ben
Mar 21, 2021 at 22:05
• A number is in the cantor set if in base $3$ it can be written with only the digits $0$ or $2.$ @Anon 1/3 is in the cantor set, even though it is $0.1$ base three because it can be written $0.022\dots$ in base $3.$ So you need to be careful about your statement. Mar 21, 2021 at 22:15
• $45/69$ is better known as $15/23$. Mar 21, 2021 at 22:18
• $1/3<15/23<2/3$,so $45/69$ gets kicked out immediately. Mar 21, 2021 at 22:27

Here is the start of a long division base $$3$$ for the first problem:

         0.0222010
------------
1011)101.0000000
20 22
-----
10 010
2 022
-----
2110
2022
----
1100
1011
----
120


At this point it is clear that $$\frac{10}{31}$$ is not in the Cantor set, since no later digits can change that $$1$$ into a $$2$$. One step earlier it wasn’t clear from the quotient digits alone, since $$(0.022201)_3$$ could conceivably have continued $$(0.02220122222\ldots)_3\,,$$ which is equal to $$(0.022202)_3$$, which is in the Cantor set. However, if you realize that the only numbers that have two ternary expansions are rational numbers that can be written with a denominator that is a power of $$3$$, you can stop as soon as you get that $$1$$.

What we have at this point says that $$(0.022201)_3<\frac{10}{31}<(0.022202)_3$$, so it is in one of the deleted open intervals.

The second one is even easier.

• There is no real need to do long division for the second fraction, as $1/3<15/23<2/3$, so it gets kicked out on the first step of the Cantor-set construction. Mar 21, 2021 at 22:24
• "It wasn't clear one step earlier", but it could have been clear one step earlier because for a fraction to have two base-$3$ expansions, its reduced form would have to have a power of $3$ in the denominator. Mar 22, 2021 at 0:30
• @AndreasBlass: Good point; I’ve revised the answer to include it. Thanks! Mar 22, 2021 at 0:34

Neither one is in the Cantor set. $$\dfrac{10}{31} = 0.02220101110012..._{3}$$, and $$\dfrac{45}{69} = \dfrac{15}{23} = 0.12212110220122..._{3}$$. Both has $$1$$ in their ternary presentation.

• This is correct, but I doubt if it is the point of the exercise. Mar 21, 2021 at 22:33
• Thank you. But could you show the contradiction proof? I really want to learn how to show it rigorously. Thanks! Mar 21, 2021 at 22:36
• @little_sky: Your numbers are specific and all you need is to express it in base $3$ with few minutes of calculations. If you have a general fraction with certain property, then I think a pure proof that you want might be worth the time as you can learn more about the Cantor set.
– user899577
Mar 21, 2021 at 22:42
• Thank you so much, @ODETutor Mar 21, 2021 at 22:48