# How to determine whether to find an upper bound or lower bound

I have a rather general question about sequences:

How do you determine whether it is appropriate to find an upper or lower bound of a sequence? In particular, suppose $a_n = \frac{1}{2} + \frac{1}{3} + \cdots + \frac{1}{p_n}$ where $p_n$ denotes the $n^{th}$ prime. How do I know whether to bound the sequence from above or below?

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In order to do what? – Qiaochu Yuan Jul 2 '11 at 23:33
@Qiaochu Yuan: Just for the sake of seeing whether it is bounded above or below. – Damien Jul 2 '11 at 23:35
Isn't your question a tautology then? To prove that it's bounded above, bound it from above; to prove that it's not bounded above, bound it from below (by some sequence you know to not be bounded above). Similarly if you want to prove/disprove that it's bounded below. – mac Jul 2 '11 at 23:40