What does $||XY||_1$ mean?
Solution 1:
Under independence $||XY||_1=||X||_1||Y||_1$ holds even if $E|X|$ or $E|Y|$ is infinity (by Tonelli's Theorem). But $E(XY)=(EX)(EY)$ holds if the expectations are finite.
Under independence $||XY||_1=||X||_1||Y||_1$ holds even if $E|X|$ or $E|Y|$ is infinity (by Tonelli's Theorem). But $E(XY)=(EX)(EY)$ holds if the expectations are finite.