Given a subbasic family C of the product that does not have a finite subcover, we can partition C = ∪i Ci into subfamilies that consist of exactly those cylinder sets corresponding to a given factor space. If Uis open in any T , then T cannot be contained in T0. For topological spaces $A$ and $B$, let $\textrm{Map}(A,B)$ The crux of the matter is how we define "the topology generated by a basis" versus "the topology generated by a subbasis", as well as the difference in the definition of "basis" and "subbasis". ; then the topology generated by X as a subbasis is the topology farbitrary unions of ï¬nite intersections of sets in Sg with basis fS. Proposition 1: Let $(X, \tau)$ be a topological space. If $$\mathcal{B}$$ is a basis of $$\mathcal{T}$$, then: a subset S of X is open iff S is a union of members of $$\mathcal{B}$$.. Then the product topology is the unique topology on $X$ such that for any topological space $A$, $$\textrm{Map}(A,X) \rightarrow \prod\limits_i \textrm{Map}(A, X_i)$$. and $I$ is an arbitrary indexing set. Sum up: One topology can have many bases, but a topology is unique to its basis. Moreover, { U } ∪ F is a finite cover of X with { U } ∪ F ⊆ . Note that this is just a fancy index-juggling way of saying that all sets of the form $\prod_{\beta \in B} U_\beta$, where all $U_\beta$ are open in $X_\beta$ and the set $\{\beta: U_\beta \neq X_\beta \}$ is finite, form an open base for the topology. By using our site, you acknowledge that you have read and understand our Cookie Policy, Privacy Policy, and our Terms of Service. 1 \¢¢¢\ S. n. jn â 0;S. i. Mathematics Stack Exchange is a question and answer site for people studying math at any level and professionals in related fields. B {\displaystyle {\mathcal {B}}} is a subbasis of Ï {\displaystyle \tau } ) and let B â² := { B 1 â© â¯ â© B n | n â N , B 1 , â¦ , B n â B } {\displaystyle {\mathcal {B}}':=\{B_{1}\cap \cdots \cap B_{n}|n\in \mathbb {N} ,B_{1},\ldots ,B_{n}\in {\mathcal {B}}\}} . De nition 1.8 (Subbasis). * Partial order: The topology Ï on X is finer or stronger than the topology Ï' if â¦ ∩ Sn ⊆ U, we thus have Z ⊆ U, which is equivalent to { U } ∪ F being a cover of X. Asking for help, clarification, or responding to other answers. More generally, Tychonoff's theorem, which states that the product of non-empty compact spaces is compact, has a short proof if the Alexander Subbase Theorem is used. In both cases, the topology generated by contains , but at the same time is contained in every topology that contains , hence, it equals the intersection of such topologies (which is the smallest topology containing ). Conversely, given an arbitrary collection ð of subsets of X, a topology can be formed by first taking the collection â¬ of finite intersections of members of ð and then taking the topology ð¯ generated by â¬ as basis. a subset which is also a topological space. (For instance, a base for the topology on the real line is given by the collection of open intervals (a, b) â â (a,b) \subset \mathbb{R}. Collection of subsets whose closure by finite intersections form the base of a topology, https://en.wikipedia.org/w/index.php?title=Subbase&oldid=991948134, Creative Commons Attribution-ShareAlike License, The collection of open sets consisting of all finite, This page was last edited on 2 December 2020, at 17:46. Thus, any basis is a subbasis. As a follow up question, is there any easier way to formally define the product topology on a product space, other than this? Use MathJax to format equations. A subbasis S for a topology on X is a collection of subsets of X whose union equals X. By assumption, if Ci ≠ ∅ then Ci does not have a finite subcover. denote the set of all continuous functions $A \rightarrow B$. A sub-basis Sfor a topology on X is a collection of subsets of X whose union equals X. The topology generated by the subbasis is generated by the collection of finite intersections of sets in as a basis (it is also the smallest topology containing the subbasis). sgenerated by the subbasis S= S T . Let Bbe the collection of all open intervals: (a;b) := fx 2R ja