If $f$ is a Lebesgue measurable function on $R^n$, we define $K(t)=\lambda\{x\in R^n:|f(x)|>t\}.$ I want to prove that
$\int_0^{\infty}K(t)dt=\int_{R^n} |f(x)|dx$
If $ f\in L^1 (R^n)$, then $\lim_{s\rightarrow t^-} K(s)=K(t).$
I have no idea about these two problems.