Topology/Product Spaces

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Let \{(X_i, \tau_i)\}_{i\in I} be an indexed collection of topological spaces and let X=\prod_{i\in I} X_i. For each i\in I let \pi_i:X\to X_i be the ith coordinate projection. The product topology is the topology τ on X generated by sets of the form \pi_i^{-1}[U_i] where i\in I and U_i\in\tau_i.