Take two topological spaces $(X,T_{X})$ and $(Y,T_{Y})$. What are the necessary conditions on $(X,T_{X})$ and $(Y,T_{Y})$ such that $$T_{1} := \{A \times B\mid A \in T_{X},B \in T_{Y}\}$$ ia a topology on $X\times Y$ and $$T_{2} := \{U \subseteq X \mid X \backslash U \text{ is infinite}\} \cup \{\emptyset,X\}$$ is a topology on $X$.
2026-03-28 15:21:30.1774711290
Conditions for the product of two topologies to be a topology
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As discussed, if X is finite, $T_2$ is indiscrete.
If X is infinite, then $T_2$ is discrete because for
x in X, {x} is open since X - {x} is infinite.