Limit of the absolute value of a function

Solution 1:

Yes.

Proof. Suppose $\lim\limits_{x \to c} |f(x)| = 0$. Then $\lim\limits_{x \to c} -|f(x)| = 0$ also. For any $x$, we have $$ -|f(x)| \le f(x) \le |f(x)| $$ implying $\lim\limits_{x \to c} f(x) = 0$ by the squeeze theorem.

Note that $\lim_{x \to c} |f(x)| = L$ does not imply the existence of $\lim_{x \to c} f(x)$ when $L \ne 0$.

Solution 2:

Yes. Note that $\lim_{x\to c}f(x)=0$ translates to: For all $\epsilon>0$ there exists a $\delta>0$ such that for all $x$ with $0<|x-c|<\delta$ (or accordingly for the infinite case) we have $|f(x)-0|<\epsilon$. But clearly $$ |f(x)-0|<\epsilon\iff |f(x)|<\epsilon\iff \bigl||f(x)|-0\bigr|<\epsilon.$$ Hence the condition for $f$ and $|f|$ is in fact the same.

Note that for any nonzero limit $a$ we'd only have the other direction $$ |f(x)-a|<\epsilon\implies\bigl||f(x)|-|a|\bigr|<\epsilon.$$

Solution 3:

Yes

Try

$|f(x)|= ||f(x)|-0| < \epsilon $