Today, we studied that
a set $A$ is positive set if $\lambda(B)\geq0$ for every measurable subset $B$ of $A$.
a set $A$ is negative set if $\lambda(B)\leq0$ for every measurable subset $B$ of $A$.
a set $A$ is null if $A$ is both positive and negative.
Which made me think is every set $A$ with $\lambda(A)\geq0$ is positive?
Also, is every set $A$ with $\lambda(A)=0$ is null set?