Theorem
Letrepresent a set and
a family of subsets of
such that
1.
2. The union of any elements ofis a member of
3. The intersection of any elements ofis a member of
Then ifis a family of subsets of
such that
if and only if
then
is a topology on
Proof
Sinceand
Supposethen
Alsoso
This can be extended to any intersection of sets.
Supposeis a family of sets of
is a family of subsets of
and
Henceis a topology on
Elements of
are called open sets.