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If I have a very nice function as $f\in \mathcal{C}_0^\infty$, (infinitely many times differentiable with all derivatives bounded and continuous and with compact support).

Can I apply integration by parts in:

$\int |f'(x)|g(x) dx$?

by writing $|f'(x)| = (f'(x))_+ + (f'(x))_{-} \ $ ?

The point is.. is it true that $(f'(x))_{+} = (f_+(x))'$. If $f$ is so nice (make a plot) it seems to me that it shouldn't be a problem.

Thank you very much for any help! :)

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    The subscript$+$means: $f_+(x) = \max\{f(x), 0\}$ and $f_{-}(x)=-\min \{f(x),0\}$.2012-11-19

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