# Sequence within a non-empty, bounded above set

• Jan 30th 2013, 12:35 PM
sakuraxkisu
Sequence within a non-empty, bounded above set
Let S be a non-empty subset of R (real numbers) that is bounded above. Show that there exists a sequence (xn, n is a natural number), contained in S (that is, xn is an element of S for all n in the set of natural numbers) and which is convergent with limit equal to sup S.

Any help would be greatly appreciated :)
• Jan 30th 2013, 12:57 PM
Plato
Re: Sequence within a non-empty, bounded above set
Quote:

Originally Posted by sakuraxkisu
Let S be a non-empty subset of R (real numbers) that is bounded above. Show that there exists a sequence (xn, n is a natural number), contained in S (that is, xn is an element of S for all n in the set of natural numbers) and which is convergent with limit equal to sup S.

Suppose that $\sigma=\sup(S)$. Two cases:
1) $\sigma\in S$, what is a constant sequence that works?

2) $\sigma \notin S$ and $\exists s_1\in S$ such that $s<\sigma$. You explain why!

If $n>1$ then $\exists s_n\in S$ such that $\sigma-\tfrac{1}{n}\le s_n<\sigma$. You explain why!