Proving every Cauchy sequence in $\mathbb{R}$ is convergent. Does the following proof work?

Question

I tried to prove this but my proof did not match with my book's. So I want to verify whether my proof is correct or not.

Theorem: Every Cauchy sequence in $\mathbb R$ has a limit.

Let us assume the contrary that there is a sequence $(a_n)$ which is Cauchy but not convergent.

1.Since the sequence is not convergent,for all real $a$, there must be an $\epsilon$ such that for all $n \in \mathbb N$, $\exists n_0 \geq n$ such that $|a_{n_0}-a|\geq \epsilon$.

2.Since $(a_n)$ is Cauchy, we can show that that particular $\epsilon$ we talked above,there is a natural $N$ such that for all $n,m\geq N$, $|a_n-a_m|<\epsilon$.

3.Go and look $(1)$. I can thus find $m_0 \geq N$ such that $|a_{m_0}-a| \geq \epsilon >|a_{m_0}-a_n|$ for all $n \geq N$. Since $a$ is arbitrary, putting $a=a_n$,we get contradiction.Thus, the proof.

Answer 1

In step 1, you're fixing $n_0$ which has the property $|a_{n_0} - a| \geq \epsilon$, but in step 3, you're using that $|a_{m_0} - a| \geq \epsilon$ for your $m$ large enough.

You can't first fix $n_0$ and then replace it. That's already assuming that your sequence $a_n$ converges to $a$, which is what you're trying to prove.

The standard argument constructs (using Bolzano-Weierstrass) a convergent subsequence of $a_n$. It can then be shown that any Cauchy sequence with a convergent subsequence is convergent.

A quicker way to see that your argument doesn't work is the following: it doesn't rely on the "completeness" of $\mathbb{R}$. You will learn that - in more general settings - convergent sequences are always Cauchy, but not all Cauchy sequences converge. For this you really need the completeness (which is used in the proof of Bolzano Weierstrass).

Answer 2

From other answers, I became completely sure that my proof must be wrong because if I imitate my proof for a Cauchy sequence in $\mathbb Q$,the same result would follow. But here is what went wrong in my proof:
I claimed to replace $a_n$ in my proof in (3) and then,tried to get a contradiction. But to replace a particular $a_n$, I must first know $N$ in the above proof.
Note that $\epsilon$ is dependent upon $a$,the real number. Now,when I do choose $a=a_n$ in my proof, I get that it must give a particular $\epsilon$ which in turn would create a new value of $N$ and I can not make sure that the value of $a_n$ I have choosen is such that $n \geq N$.