Proving a sequence is a Cauchy sequence from other theorems

Question

I would like to know if this is an acceptable proof. I have the following statement

Show directly that a bounded, monotone increasing sequence is a Cauchy sequence.

Let $\{x_n\}$ be a sequence that is bounded and monotone increasing. Then, by the Monotone Convergence Theorem, $\{x_n\}$ converges. Furthermore, by the Cauchy Criterion, we know that every convergent sequence is a Cauchy sequence and so we are done. QED

Does this work?

In general, when asked to prove “directly” it means using the basic definitions. — Anurag A, Nov 23 '17 at 20:35
$L=\sup_n x_n$ is an upper bound, and you can find $n$ such that $L-x_n$ is as small as you like. — copper.hat, Nov 23 '17 at 20:36
Thank you very much! But does my argument make sense if they were not asking me to ''show directly''? — Michelle Drolet, Nov 23 '17 at 22:21
For a direct argument, see https://math.stackexchange.com/questions/566635/show-that-every-monotonic-increasing-and-bounded-sequence-is-cauchy?rq=1 — Arnaud D., Nov 24 '17 at 09:10

score 1 · Accepted Answer · answered Nov 23 '17 at 20:41

Let $L=\sup_{n}x_{n}$, for $\epsilon>0$, find some $N$ such that $L-\epsilon<x_{N}\leq L$. As $\{x_{n}\}$ is increasing, $L-\epsilon<x_{N}\leq x_{n}\leq L$ for all $n\geq N$. In particular, $|x_{n+p}-x_{n}|=x_{n+p}-x_{n}=x_{n+p}-L+L-x_{n}\leq L-x_{n}<\epsilon$ for all $n\geq N$ and $p=1,2,...$

score 0 · Answer 2 · answered Nov 23 '17 at 20:37

0

Start here. Put $s = \sup_n x_n$; show that $x_n \to s$.

answered Nov 23 '17 at 20:37

ncmathsadist

49,383

This does not provide an answer to the question. To critique or request clarification from an author, leave a comment below their post. - From Review – Arnaud D. Nov 23 '17 at 21:44
He is looking for a hint, not for someone to give away the shop. – ncmathsadist Nov 24 '17 at 01:12
But the question says "Prove directly (...)". I think this means "without using the fact that it is convergent." – Arnaud D. Nov 24 '17 at 09:06

Proving a sequence is a Cauchy sequence from other theorems

2 Answers2