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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!

Peter C
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    Welcome to MSE! What is your definition of continuity? – Arpit Kansal Jan 20 '16 at 17:28
  • Thank you! Sorry if this sounds a bit rushed as I have a programming class in 20 minutes.

    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
  • Other than the range, are there any restrictions on $f$? For example, do you know that $f$ is continuous at $a$? Without such a restriction, your statement is false. – Rory Daulton Jan 20 '16 at 18:52
  • Yes yes. Your question made me realise that I made a mistake when I wrote the question. Edited the first part of the question "Let $f\colon A \to \mathbb R$ be continuous at $a\in A$" – Peter C Jan 20 '16 at 20:09
  • Shouldn't there be some clarification as to what $A$ is? – hardmath Jan 20 '16 at 22:46
  • Believe it or not the top paragraph is the entire question. I'm thinking that I have to use the inequality involving epsilon and somehow isolate $\mid f(x) \mid$ (which is what I have done on the 3rd paragraph. I'm trying to progress by finding an equivalent that's equal or smaller than $ \mid f(x) + f(a) \mid $ and also connected to one of the inequalities. But this is a 2 mark question so I might be over complicating it. – Peter C Jan 20 '16 at 22:54

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