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The elements of a topology \(\class{blue}{\mathcal{T}}\) of a topological space \( (\mathbb{X}, \class{blue}{\mathcal{T}}) \) must satisfy three properties:

- The empty set \( \emptyset \) and the whole set \(X\) are both inside \(\mathcal{T}\).
- The union of
*any*(even infinitely many) elements \(\mathbb{T}_i\) of \(\class{blue}{\mathcal{T}}\) is also an element of \(\class{blue}{\mathcal{T}}\). - The intersection of
*finitely*many elements \(\mathbb{T}_i\) of \(\class{blue}{\mathcal{T}}\) is also an element of \(\class{blue}{\mathcal{T}}\).