- #1
hages
- 2
- 0
Homework Statement
Notation:
|a_n| is the absolute value of a_n.
(s_n) signifies a sequences; s_n signifies the value of the sequence at a particular n.
Problem:
Let n, k be arbitrary elements in N.
Let (a_n) be a sequence such that lim inf |a_n| = 0. Prove that there is a subsequence (an_k) such that the series, SIGMA an_k from k=1 to +infinity converges.
Homework Equations
None.
The Attempt at a Solution
I am just at a complete loss for how to solve this problem.
Here's my outline:
Let s_n = SIGMA |a_k| from k = 1 to n. Since for all n, |a_n| => 0, (s_n) is nondecreasing.
Now if I can show that (s_n) is bounded, I'd know that (s_n) converges and thus would know there is a convergent subsequence (sn_j) where sn_j = SIGMA |an_k| from k=1 to j. Thus we'd say that SIGMA |an_k| from k=1 to j would be convergent. And thus we'd have that SIGMA an_k from k=1 to +infinity would be convergent because absolutely convergent series are convergent.
But following this route means I have to prove that (s_n) is bounded and that is where I happen to be stuck. Can anyone help me out?