Theorem 3.37. A basis for the product topology on [Icej Xa is the collection of all sets of the form IIce Ua, where Ug is open in Xa for each a and Ug = Xa for all but finitely тапy a.

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Could you show me how to do 3.37 in detail?

**Definition.** Let \(\{X_{\alpha}\}_{\alpha \in \lambda}\) be a collection of topological spaces. The product \(\prod_{\alpha \in \lambda} X_{\alpha}\), or Cartesian product, is the set of functions

\[
\{f : \lambda \rightarrow \bigcup_{\alpha \in \lambda} X_{\alpha} \mid \text{for all } \alpha \in \lambda, f(\alpha) \in X_{\alpha}\}.
\]

Here \(f(\alpha)\) is called the \(\alpha\)th coordinate of \(f\). The spaces \(X_{\alpha}\) are sometimes called factors of the infinite product. Thus a point in the product may be thought of as a function that associates to each \(\alpha\) an element \(f(\alpha)\) of the factor \(X_{\alpha}\).

**Definition.** For each \(\beta\) in \(\lambda\), define the projection function \(\pi_{\beta} : \prod_{\alpha \in \lambda} X_{\alpha} \rightarrow X_{\beta}\) by \(\pi_{\beta}(f) = f(\beta)\). We define the product topology on \(\prod_{\alpha \in \lambda} X_{\alpha}\) to be the one generated by the subbasis of sets of the form \(\pi_{\beta}^{-1}(U_{\beta})\), where \(U_{\beta}\) is open in \(X_{\beta}\).

**Theorem 3.37.** A basis for the product topology on \(\prod_{\alpha \in \lambda} X_{\alpha}\) is the collection of all sets of the form \(\prod_{\alpha \in \lambda} U_{\alpha}\), where \(U_{\alpha}\) is open in \(X_{\alpha}\) for each \(\alpha\) and \(U_{\alpha} = X_{\alpha}\) for all but finitely many \(\alpha\).
Transcribed Image Text:**Definition.** Let \(\{X_{\alpha}\}_{\alpha \in \lambda}\) be a collection of topological spaces. The product \(\prod_{\alpha \in \lambda} X_{\alpha}\), or Cartesian product, is the set of functions \[ \{f : \lambda \rightarrow \bigcup_{\alpha \in \lambda} X_{\alpha} \mid \text{for all } \alpha \in \lambda, f(\alpha) \in X_{\alpha}\}. \] Here \(f(\alpha)\) is called the \(\alpha\)th coordinate of \(f\). The spaces \(X_{\alpha}\) are sometimes called factors of the infinite product. Thus a point in the product may be thought of as a function that associates to each \(\alpha\) an element \(f(\alpha)\) of the factor \(X_{\alpha}\). **Definition.** For each \(\beta\) in \(\lambda\), define the projection function \(\pi_{\beta} : \prod_{\alpha \in \lambda} X_{\alpha} \rightarrow X_{\beta}\) by \(\pi_{\beta}(f) = f(\beta)\). We define the product topology on \(\prod_{\alpha \in \lambda} X_{\alpha}\) to be the one generated by the subbasis of sets of the form \(\pi_{\beta}^{-1}(U_{\beta})\), where \(U_{\beta}\) is open in \(X_{\beta}\). **Theorem 3.37.** A basis for the product topology on \(\prod_{\alpha \in \lambda} X_{\alpha}\) is the collection of all sets of the form \(\prod_{\alpha \in \lambda} U_{\alpha}\), where \(U_{\alpha}\) is open in \(X_{\alpha}\) for each \(\alpha\) and \(U_{\alpha} = X_{\alpha}\) for all but finitely many \(\alpha\).
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