# Is it possible for a Lebesgue integral of an unmeasurable function to be finite?

Let f be defined on $\mathbb{R}^d,$ suppose $\int_{\mathbb{R}^d} |f(x)|\,dx<\infty$. Is f then measurable?

My question boils down to "Do I have to check that a given function is measurable, or can I suppose the function is measurable, compute the integral, and if the integral is finite, observe that the function was indeed measurable?"

• How do you compute the integral of a function that is not measurable? Commented Sep 26, 2015 at 21:48
• I suppose you can't. Which is why I was wondering: If you suppose the function is measurable and integrate as usual, and the integral of |f| is finite, does this mean f was measurable? The idea: Without knowing f is measurable, $\int |f|\,dx$ is not defined. If we suppose it is defined and we get something finite, have I shown that the integral was defined in the first place, or is my result meaningless? Commented Sep 26, 2015 at 21:59
• See this question . Commented Sep 27, 2015 at 5:39
• it can happen that $|f|$ is measurable even if $f$ is not! By slightly modifying this construction, we can even have $|f|$ integrable without $f$ being measurable. Commented Sep 27, 2015 at 8:22
• Let $E\subseteq [0,1]$. Suppose $E$ is NOT Lebesgue measurable. Let $f$ be defined on $\mathbb R$ as $f= \chi_{[0,1]-E} - \chi_E$ (where $\chi_{[0,1]-E}$ and $\chi_E$ are the indicator functions of $[0,1]-E$ and $E$ respectively). Then $|f|=\chi_{[0,1]}$ and $\int_{\mathbb R} |f(x)|\,dx = 1 <\infty$. But $f$ is NOT measurable. Commented Sep 27, 2015 at 18:14