Let $f\colon A \to \mathbb R$ be continuous at $a\in A$. Show that $| f |\colon A \to \mathbb R$ defined by $| f |(x) = | f(x)|$ is continuous at $a$.
Given $\epsilon > 0$. And for all $\epsilon > 0$ there exists $\gamma > 0$. I know that since $f\colon A \to \mathbb R$ is continuous at $a\in A$ then it follows that $\mid x - a\mid < \gamma$ and $\mid f(x) - f(a) \mid < \epsilon$
$\mid f(x) - f(a) \mid = \mid 2f(x) - f(a) - f(x) \mid = \mid 2f(x)\mid - \mid f(a) + f(x)\mid$
Ok guys, after bashing my head I realised that I over complicated the whole thing and noticed what to do. You can ignore my reasoning above.
Since $f$ is continuous at $a$ all $\epsilon > 0$ and there exists $\gamma > 0$ such that $ x \in A$
This is what I really missed:
$\mid \mid f(x) \mid - \mid f(a) \mid \mid \le \mid f(x) - f(a) \mid $
so
$\mid \mid f(x) \mid - \mid f(a) \mid \mid \le \mid f(x) - f(a) \mid \lt \epsilon $
hence
$\mid \mid f(x) \mid - \mid f(a) \mid \mid \lt \epsilon $
therefore $\mid f \mid$ is continuous at $a$ also.
Took forever but the questions did help me. Always surprising to see how simple it really is. Thanks!
For all $\epsilon > 0 $ there exists $\omega > 0$ s.t. $\mid x-a \mid < \omega \implies \mid f(x) - f(a) \mid < \epsilon$
– Peter C Jan 20 '16 at 17:42