Exploring the Construction of the Cantor Set in Base 3

  • Thread starter Dragonfall
  • Start date
  • Tags
    Cantor Set
In summary, the conversation discusses finding a closed form expression for the endpoints used in the construction of the Cantor set and proving their denseness in the set. The closed form expression is k/3n for all n, and for all 0 < k < 3n, but it is shown that this is not always the case. The conversation also explores the concept of base 3 and how it relates to the Cantor set, ultimately proving that the set is dense using this fact.
  • #1
Dragonfall
1,030
4
How can I write down a closed form expression for the endpoints used in the construction of the Cantor set? i.e., 0, 1, 1/3, 2/3, 1/9, 2/9, 7/9, 8/9, etc.
 
Physics news on Phys.org
  • #2
You don't see any patterns there? Try writing down more terms. Or is it that you're trying to prove some expression is correct?
 
  • #3
I have to show that these numbers are dense in the Cantor set. I found another way without using the closed form.
 
  • #4
The closed form is k/3n for all n, and for all 0 < k < 3n.
 
  • #5
AKG said:
The closed form is k/3n for all n, and for all 0 < k < 3n.
No, it isn't. when n= 1, the first "cut", the enpoints are 1/3 and 2/3 so that the intervals left are [0, 1/3], [2/3, 1] but when you remove the middle third of those, the endpoint are 0, 1/9, 2/9, 1/3= 3/9, 2/3= 6/9, 7/9, 8/9, 1. In your notation, k/3n, with n= 2, k is not 4 or 5. It gets worse with higher values of n.

Dragonfall, think in terms of base 3. Writing a number between 0 and 1 in base 3, the first "digit" (trigit?) may be 0, 1, or 2. If 0, the number is between 0 and 1/3; if 1, between 1/3 and 2/3; if 2, between 2/3 and 1. When you remove the middle third you remove all numbers have a "1" as first digit. Now, all numbers between 0 and 1/3 must have .00, .01, or .02 as first two digits, all numbers between 2/3 and 1 must have .20, .21, or .22 as first two digits. When you remove the middle third of each you remove all numbers that have 1 as the second digit. Do you see what happens in the limit?
 
  • #6
Sorry, just wasn't thinking.
 
  • #7
HallsofIvy said:
Dragonfall, think in terms of base 3. Writing a number between 0 and 1 in base 3, the first "digit" (trigit?) may be 0, 1, or 2. If 0, the number is between 0 and 1/3; if 1, between 1/3 and 2/3; if 2, between 2/3 and 1. When you remove the middle third you remove all numbers have a "1" as first digit. Now, all numbers between 0 and 1/3 must have .00, .01, or .02 as first two digits, all numbers between 2/3 and 1 must have .20, .21, or .22 as first two digits. When you remove the middle third of each you remove all numbers that have 1 as the second digit. Do you see what happens in the limit?

Yes, I was able to prove that the Cantor set contains those and only those numbers whose triadic expansion contains only 0 or 2. I did not use this fact for the denseness; however, but I did use it to prove that any number in [0,2] can be written as a sum of 2 'Cantor numbers'.

For denseness, I simply showed that since anything between two endpoints is either entirely in or out of the Cantor set, and if there exists a non-endpoint such that some closed ball of radius e about it contains no endpoint, then the Cantor set has at least length 2e, which is impossible.
 
Last edited:

What is a Cantor Set?

A Cantor Set is a fractal set constructed by repeatedly removing the middle third of a line segment. It is named after Georg Cantor, a mathematician who first described it in the late 19th century.

How many endpoints does a Cantor Set have?

A Cantor Set has an infinite number of endpoints, as the process of removing the middle third of a line segment continues infinitely.

Are the endpoints of a Cantor Set countably infinite or uncountably infinite?

The endpoints of a Cantor Set are uncountably infinite, meaning that they cannot be put into a one-to-one correspondence with the counting numbers. This is because the endpoints are not discrete points, but rather form a continuous line.

Do the endpoints of a Cantor Set have a specific pattern or structure?

The endpoints of a Cantor Set do not follow a specific pattern or structure, as they are created through a recursive process of removing the middle third of a line segment. However, they do exhibit self-similarity, meaning that each section of the set is similar to the whole set.

Can the endpoints of a Cantor Set be represented as a decimal or binary number?

No, the endpoints of a Cantor Set cannot be represented as a decimal or binary number. This is because the endpoints are not discrete points, but rather form a continuous line, making it impossible to assign a specific numerical value to each endpoint.

Similar threads

  • Calculus and Beyond Homework Help
Replies
14
Views
1K
  • Calculus and Beyond Homework Help
Replies
8
Views
326
Replies
4
Views
1K
  • Calculus and Beyond Homework Help
Replies
2
Views
635
  • Calculus and Beyond Homework Help
Replies
3
Views
1K
  • Calculus and Beyond Homework Help
Replies
1
Views
702
  • Calculus and Beyond Homework Help
Replies
3
Views
1K
  • Calculus and Beyond Homework Help
Replies
2
Views
1K
  • Calculus and Beyond Homework Help
Replies
2
Views
2K
Replies
5
Views
1K
Back
Top