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I know this is a basic question, but I could not find a question on the site about exactly this question, even by using https://approach0.xyz/search/ Maybe I searched wrongly ?

Show that $\lim_{n \to \infty} x_n = a \;\implies\; \lim_{n \to \infty} |x_n| = |a|$.

Let $(x_n)_{n \in \mathbb{N}}$ be a real sequence and $a \in \mathbb{R}$.

Show that $$ \lim_{n \to \infty} x_n = a \;\implies\; \lim_{n \to \infty} |x_n| = |a| $$

$\lim_{n \to \infty} x_n = a$ means that for each $\epsilon > 0$ there exists $N \in \mathbb{N}$ such that $\forall n > N$ there is $|x_n - a| < \epsilon$

Using the triangle inequality: $$ \epsilon > |x_n - a| \geq \bigl| |x_n| - |a| \bigr| $$

But $\epsilon > \bigl| |x_n| - |a| \bigr|$ is the definition of $\lim_{n \to \infty} |x_n| = |a|$

This completes the proof.


Note that the reverse claim $$ \lim_{n \to \infty} x_n = a \;\Longleftarrow\; \lim_{n \to \infty} |x_n| = |a| $$

does not hold in general. For example if $x_n = (-1)^n$ then $$\lim_{n \to \infty} |x_n| = 1$$ but $$\lim_{n \to \infty} x_n \neq 1$$

The exception being for $a = 0$, because in this case \begin{align} \bigl| |x_n| - 0 \bigr| < \epsilon \; &\Longleftrightarrow \; |x_n - 0| < \epsilon \\[4pt] \lim_{n \to \infty} |x_n| = |0| \; &\Longleftrightarrow \; \lim_{n \to \infty} x_n = 0 \end{align}

Is my attempt correct?

Sammy Black
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wengen
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    Yes, that's the exact way to do it, with the reverse triangle inequality. – Martin Argerami Oct 11 '23 at 22:28
  • @MartinArgerami Thank you for your quick reply. Feel free to post your comment as answer if you want, so I can accept it. I'm honestly surprised I did not find this question on the site, but again maybe I used the wrong search terms – wengen Oct 11 '23 at 22:30
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    Your algebra is fine but I would very much recommend you pay more attention to writing up your proof and making sure that the logic is clear. For example, the definition of $\lim_{n \to \infty} |x_n|=|a|$ is not "$\epsilon>||x_n|-|a||$", it is "for every $\epsilon>0$, there exists (etc etc)". – David Oct 11 '23 at 22:37
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    An alternative approach would be to split into two cases: $a\geq 0$ and $a<0.$ – Adam Rubinson Oct 11 '23 at 22:38
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    Notation: use |x| for absolute value $|x|$, as \mid has built in horizontal whitespace for building sets: ${x \in A \mid x \notin B}$ – Sammy Black Oct 11 '23 at 22:42
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    @SammyBlack The recommended way to typeset absolute values is \lvert x \rvert. For example, |\sin x| results in the incorrect spacing $|\sin x|$, whereas \lvert \sin x \rvert gives the correct spacing $\lvert \sin x \rvert$. – L. F. Oct 11 '23 at 22:52
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    Moreover, since the absolute value is a continuous function the result follows directly. – Deif Oct 11 '23 at 22:53
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    @L.F. You're right, of course. I was suggesting just the bar, as it looks a lot better than \mid (which formats with \mathrel class I believe) without needing separate symbols with \mathopen and \mathclose classes. For the OP, some of this is explained briefly here. – Sammy Black Oct 11 '23 at 23:01
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    Well done for searching! When I look for "limit of absolute value", I find this and this, which are fairly similar. Much discussion here is under "continuity of the absolute value/modulus function", eg here, here and here (although the last one is more topological). It's even a special case of this. – Izaak van Dongen Oct 11 '23 at 23:37

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