Thread: subsequence help.

Let {sn} be a sequence and let E be the set containing members x such that x is a limit of a subsequence of sn. Let S*=sup(E).
Prove S* is a member of E.

I do not understand the first part of the proof (from a textbook). It goes if S* = infinity then E is unbounded (fine so far). Then {sn} is unbounded, hence there is a subsequence -> infinity.

This raised a few questions.

If {sn} is unbounded, does sn -> + or - infinity?
The proof also seems to imply {sn} -> infinity iff xn -> infinity, where xn is a subsequence. Correct?

Thanks

I guess is a sequence of real numbers. Well, if is unbounded, then the sequence is either unbounded below or unbounded above. In the first case, we can find a subsequence which converges to , and in the second case we can find a subsequence which converges to . Here, since in the first part, then is not bounded above.
If we have for a subsequence of , it doesn't mean that . For example, take (the limit doesn't exist).

So you are saying {sn} is unbounded iff it has a subsequence which is unbounded?

If so, since sn is unbounded it tends to infinity, hence it has a subsequence tending to infinity. Correct?

Originally Posted by Duke

Let {sn} be a sequence and let E be the set containing members x such that x is a limit of a subsequence of sn. Let S*=sup(E). Prove S* is a member of E.
The proof (from a textbook). It goes if S* = infinity then E is unbounded (fine so far). Then {sn} is unbounded, hence there is a subsequence -> infinity.

In these cases it is best to say what textbook you are using.
Not knowing where that proof is going makes it really hard to comment.

Here is the way I know the proof works.
Suppose that and that .

Construct a sequence so that each
A bounded montone sequence has a limit: .
It is easy to see . But that is a contradiction.
Thus

I'm using Rudin's analysis. I was just struggling to see how he goes from (1) if S* = infinity then E is unbounded to (2) Then {sn} is unbounded, and then finally (3) hence there is a subsequence -> infinity. Can you tell me the results he is using to get from (1) to (2) and then (2) to (3) ? Thanks for your alternative proof as well.