# Improper integral does not exists -> not Lebesgue-measurable?

If I have the measure space $(\mathbb{R},\mathcal{B},\lambda)$ with the Lebesgue-measure $\lambda$ and the improper Riemann-integral of $\lvert f\rvert$ does not exists. Is then $f$ not Lebesgue-integrable?

For example I found here:

$\int_{\mathbb{R}}\frac{1}{1+\lvert x\rvert}\, d\lambda$.

It is $\lim_{R\to\infty}\int_0^R \frac{1}{1+\lvert x\rvert}\, dx=\infty$.

Does this mean that the function $\frac{1}{1+\lvert x\rvert}$ is NOT Lebesgue-integrable, i.e. that $\int_{\mathbb{R}}\left\lvert\frac{1}{1+\lvert x\rvert}\right\rvert\, d\mu=\int_{\mathbb{R}}\frac{1}{1+\lvert x\rvert}\, d\mu\geq\infty$?

I only know the fact, that, if the improper Riemann-integral of $\lvert f\rvert$ exists, that then $f$ is Lebesgue-integrable...

• I fail to see the connection between your question and your example. But yes: if $|f|$ is a function that is continuous almost everywhere whose improper Riemann integral doesn't exist, then $f$ is not Lebesgue-integrable. Your example has a value neither under Riemann integration nor Lebesgue integration. – Omnomnomnom Dec 12 '13 at 18:11
• It is possible that the function may be Lebesgue integrable but that the Riemann integral does not exist, for example, $f=1_\mathbb{Q}$. – copper.hat Dec 12 '13 at 18:14