Can a subbasis help determine which topology is finer?

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Homework Help Overview

The discussion revolves around the criteria for determining which topology is finer when two topologies are defined by a subbasis. The original poster is exploring whether similar criteria apply as with bases, specifically in the context of subbases and their relation to subspace topologies.

Discussion Character

  • Exploratory, Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • The original poster questions if there exists a criterion for comparing topologies defined by subbases, drawing a parallel to the criteria for bases. Some participants seek clarification on the definition of subbasis and its implications for topology.

Discussion Status

The discussion is active, with participants clarifying definitions and exploring the implications of using subbases. There is an openness to learning and understanding the nuances of topology, particularly regarding the generation of topologies from subbases.

Contextual Notes

The original poster is motivated by a specific goal related to proving that a certain collection forms a subbasis for the subspace topology, indicating a potential gap in information or assumptions that may need to be addressed.

Bashyboy
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Homework Statement


If two different topologies on a given space ##X## are given by a basis, then we have the following criteria for determining which topology is finer:

Let ##\mathcal{B}## and ##\mathcal{B}'## be bases for the for topologies ##\tau## and ##\tau'##, respectively, on ##X##. Then the following are equivalent:

(1) ##\tau \subseteq \tau'##

(2) For each ##x \in X## and each basis element ##B \in \mathcal{B}##, there is a basis element ##B' \in \mathcal{B}'## such that ##x \in B' \subseteq B##.

Homework Equations

The Attempt at a Solution



I am wondering, do we have a similar criteria if the two topologies are given by a subbasis ##\mathcal{S}## and ##\mathcal{S}'##? My motivation for asking this question is that I am trying to show that ##\{Y \cap S ~|~ S \in S \}## forms a subbasis for the subspace topology on ##Y \subseteq X##, and I thought that it might help me in proving this.
 
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Do you mean by subbasis ##\mathcal{S}## a basis of the from ##(X,\mathcal{B})## induced topology of ##Y##?
 
Here is the definition of subbasis with which I am working:

A subbasis ##\mathcal{S}## for a topology on ##X## is a collection of subsets of ##X## whose union equals ##X##. The topology generated by the subbasis ##\mathcal{S}## is defined to be the collection ##\tau## of all unions of finite intersections of elements in ##\mathcal{S}##.
 
I can't see any problems. The topology generated by the sets of ##\mathcal{S}## is the least finest topology which contains these sets. Refinements thus should be possible to define analogously if everything is done with rigor. (But as topology is notoriously strange, I would be delighted to learn something new.)
 

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