if $\Omega$ consists of a finite number atoms then $L^1(\Omega)$ is a finite dimension. why you can show this?
(An atom in $(X, \Omega, \mu)$ is a measurable set $E$ such that $\mu(E)>0$ and for every measurable subset $F\subset E$ either $\mu(F)=0$ or $\mu(F)=\mu(E)$.)