Can a subbasis help determine which topology is finer?

  • Thread starter Bashyboy
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In summary, the criteria for determining which topology is finer when given by two different bases on a given space is equivalent to the following: (1) one topology is a subset of the other, and (2) for every element in the first basis, there exists an element in the second basis that contains and is contained in the first element. This criteria may also apply when the topologies are given by subbases, as long as the subbasis is defined as a collection of subsets whose union is the space and the generated topology is defined as the collection of all unions of finite intersections of elements in the subbasis.
  • #1
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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  • #2
Do you mean by subbasis ##\mathcal{S}## a basis of the from ##(X,\mathcal{B})## induced topology of ##Y##?
 
  • #3
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}##.
 
  • #4
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.)
 

1. What is a subbasis topology?

A subbasis topology is a type of topology in which a set of subsets, called the subbasis, is used to generate the open sets of the topology. The subbasis sets are not required to be open sets themselves, but they must cover the entire space and any intersection of subbasis sets must be a union of subbasis sets.

2. How is a subbasis topology different from a basis topology?

A subbasis topology is a more general type of topology compared to a basis topology. While a basis topology is generated by a collection of open sets that can be used to form any open set in the topology, a subbasis topology is generated by a set of subsets that may not be open sets themselves. This allows for a wider variety of topologies to be created, but also makes it more difficult to prove certain properties.

3. Can a topology have more than one subbasis?

Yes, a topology can have multiple subbasis sets. In fact, a topology can have infinitely many subbasis sets. However, the subbasis sets must still cover the entire space and any intersection of subbasis sets must be a union of subbasis sets in order to generate a valid topology.

4. What does it mean for a topology to be finer?

A topology is said to be finer than another topology if it has more open sets. In other words, if every open set in the first topology is also open in the second topology, then the second topology is finer than the first. This means that the second topology is able to distinguish between more points in the space compared to the first topology.

5. How do I determine if a topology is finer than another topology?

To determine if a topology is finer than another topology, you can compare their respective bases or subbasis sets. If the second topology has a larger collection of bases or subbasis sets, then it is finer than the first topology. Alternatively, you can also check if every open set in the first topology is also open in the second topology, as this is another way to show that the second topology is finer.

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