Suppose we know the conclusion of problem 8,
Problem 8 Let $ X $ be a finite measure space. If $ f $ is measurable, let
$ E_n = \{x \in X : n-1 \leq |f(x)| < n \} $. Then
$ f \in L^1 $ if and only if $ \sum_{n=1}^{\infty}nm(E_n) < \infty. $
First, if $ m(X)=/infty $, it's done. Hence let's suppose that $ m(X)<\infty $
