Sunday, 26 June 2016

calculus - Absolute function continuous implies function piecewise continuous?



I have a simple true/false question that I am not sure on how to prove it.





If $|f(x)|$ is continuous in $]a,b[$ then $f(x)$ is piecewise continuous in $]a,b[$




Anyone that can point me in the right direction or give a counterexample, even though I think it's true. Thanks in advance!


Answer



Here is a counter example to the statement:



Define $f(x)$ to be $1$ if $x$ is rational and $-1$ if $x$ is irrational. Now $f$ is not continuous anywhere, but $|f|$ is identically $1$ and thus continuous.


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