Bokuan Li
May 27th at 05:24

You probably don’t need $* \in \{\cap, \cup\}$ if you only use it for $\cap$. The union for (O2) shouldn’t be over the product $I_{j} \times J$, since $I_{j}$ depends on $J$. Instead, it should be something like: Let $\{U_{j}\}_{j \in J}\subset \tau(\mathcal{B})$. For each $j \in J$, let $I_{j}$ such that $U_{j} = \bigcup_{i \in I_j}B_{i, j}$, then

\[\bigcup_{j \in J}U_{j} = \bigcup_{j \in J}\bigcup_{i \in I_j}B_{i, j}= \bigcup_{(k, f) \in J \times \prod_{j \in J}I_j}B_{k, f\paren{k}}\]