$T \subseteq X$ is a set and $σ(Τ)$ is the intersection of all σ-Algebras containing $T$ (or simpler the smallest σ-Algebra containing T).
What I've tried so far is find a set $A$ so that $A \subseteq σ(Τ)\subseteq A$. But I have no intuitional ideas about this $A$ set, let alone a solution to the problem.