Can a bounded subsequence have infinitely many convergent subsequences?

1. Nov 29, 2011

Szichedelic

I'm not sure if I am confusing myself or not, but a friend and I were trying to figure this out. Basically, I know that if a sequence is bounded, we are guaranteed at least one convergent subsequences. However, is it possible for a bounded sequence to have infinitely many of such subsequences?

2. Nov 29, 2011

mathman

Yes: 1, 1, 1/2, 1, 1/2, 1/3, 1, 1/2, 1/3, 1/4, 1, 1/2, 1/3, 1/4, 1/5, .......

1/n is a convergent subsequence limit for every n.

3. Nov 29, 2011

jgens

A bounded sequence can actually have uncountably many ($2^{\aleph_0}$) convergent subsequences. In particular, for each i in N, let {xi,n} be an enumeration of the rational numbers in [0,1]. Then consider the sequence:

x1,1, x1,2, x2,1, x1,3, x2,2, x3,1, ...

That is, enumerate all the xi,n such that i+n = 2, then all elements such that i+n = 3, then all elements such that i+n = 4, and so on. Since the rationals are dense in R, this gives us a convergent subsequence for every real number in [0,1].

Last edited: Nov 29, 2011