# Questions re Cantor Set

1. Sep 6, 2008

### Bob3141592

The Cantor Set is making me very confused. I can understand that since only open sets are removed, the Cantor Set if a collection of closed sets. I believe I understand that the Cantor Set has measure zero, and therefore contains only intervals of zero measure. I can see that the endpoints of the segments left behind are never removed, and that there are a (countably) infinite number of them. What I didn't realize until recently is that not every point in the Cantor Set is an endpoint, but it also contains interior points, like 1/4, which is never an endpoint. That makes the Cantor Set uncountable infinite. What confuses me are the endpoints associated with that interior point. Since all intervals in the set must have 0 measure, I think there cannot be an explicit point other than 1/4 to be the boundary of the interval. But if the boundary point cannot be definitively named, in what sense can we call that interval closed? This is where I get all confused. I can only imagine the endpoints associated with 1/4 to be in the neighborhood of 1/4, and so it seems like the definition of an open ball about that point. But it has to be closed, and I can't get a handle on this.

Any help in seeing how this works would be greatly appreciated.

2. Sep 7, 2008

### gel

The cantor set doesn't contain any intervals (except for its individual points), and there's no "endpoints" associated with arbitrary points in the set.

3. Sep 8, 2008

### boombaby

well, I'm not sure if it is okay to find a interval of measure zero, since interval is defined(imo) as [a,b] where a<b, a,b is in R, and therefore its measure is not zero.
What I am sure is that 1/4 is not an interior point since any neighborhood of it contains points not in Cantor Set.

4. Sep 8, 2008

### boombaby

oh, could anyone remind me how to determine if 1/4 is in Cantor Set or not?...I got stuck on this....
Thanks a lot

5. Sep 8, 2008

### Bob3141592

Perhaps the Wikipedia page on the Cantor Set is wrong. That was the basis of my information, and it didn't quite seem right to me, but I presumed it was my understanding that was wrong and not the page itself.

The Mathematics dictionary by James and James says all points in the Cantor Set are "frontier" points but I'm hazy on the distinctions between endpoints, boundary points and frontier points, if any.

6. Sep 8, 2008

### gel

the base 3 expansion of 1/4 is 0.02020202..., which doesn't contain any 1s

7. Sep 9, 2008

### boombaby

well, I think I get it, thanks a lot