- #1
GridironCPJ
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Homework Statement
Let S={1/k : k=1, 2, 3, ...} and furnish S with the usual real metric. Answer the following questions about this metric space:
(a) Which points are isolated in S?
(b) Which sets are open and closed in S?
(c) Which sets have a nonempty boundary?
(d) Which sets are dense in S?
(e) Which sets are nowhere dense in S?
Homework Equations
Use any variations of the definitions of the terms involved, no restrictions.
The Attempt at a Solution
I was able to show (a). I just took an open ball around each point 1/k in S s.t. the radius of each ball is 1/2k, so each of these balls contains only the point they center. Thus, all points of S are isolated. If you see a problem with this, please let me know.
For the rest, I'm confusing myself. Sets in S can only contain the points 1/k for k=1, 2, 3, ..., so no set can be open, right? If I take an open ball centered at a 1/k, then I cannot find an open ball inside of that open ball s.t. the smaller open ball is contained in S. However, none of these open balls are even sets in S since they have infinitely many points not in S, and S is our metric space, so..? It's amazing how elementary set theory can be so mind-twisting at times. I would appreciate help on as many of these as possible.