A base for a topological space is not necessarily closed under union. For instance, the set of open intervals is a base for the standard topology on $\mathbb{R}$, but the set of open intervals is not closed under union.
However, if the base is closed under union, then it should be a monoid, since set union is an associative operation.
No.
A base for a topological space is not necessarily closed under union. For instance, the set of open intervals is a base for the standard topology on $\mathbb{R}$, but the set of open intervals is not closed under union.
However, if the base is closed under union, then it should be a monoid, since set union is an associative operation.