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?
|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\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\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