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AxiomOfChoice
Dec16-09, 08:59 AM
Is it true that a function is Riemann integrable on a bounded interval only if it's equal to a continuous function almost everywhere? I'd imagine this is the case, given the Riemann-Lebesgue lemma, which says that a function is RI iff its set of discontinuities has measure zero. (So the "continuous function" is then just f restricted to the complement of its set of discontinuities.) But I might be wrong. Help?

AxiomOfChoice
Dec16-09, 09:11 AM
I've just discovered this is incorrect. Consider the function


f(x) = \begin{cases}
1 & \text{ if } 0\leq x \leq 1/2\\
0 & \text{ if } 1/2 < x \leq 1
\end{cases}


Then f is continuous almost everywhere, but it cannot be equal to a continuous function almost everywhere by an argument involving inverse images of open sets, etc. Bummer.

Hurkyl
Dec16-09, 11:50 AM
However, for every e>0, there exists a set A of measure less than e and a continuous function g such that, outside of A, f and g are equal.