Is (topology, union, empty set) with basis as generator a monoid?

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  • Set - $\tau $
  • Generator - basis of $\tau $
  • Operator - $\cup $
  • Unit - empty set

Is it a monoid?

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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.