Induced topology

In topology and related areas of mathematics, an induced topology on a topological space is a topology that makes a given (inducing) function or collection of functions continuous from this topological space.[1][2]

A coinduced topology or final topology makes the given (coinducing) collection of functions continuous to this topological space.[3]

Definition

The case of just one function

Let be sets, .

If is a topology on , then the topology coinduced on by is .

If is a topology on , then the topology induced on by is .

The easy way to remember the definitions above is to notice that finding an inverse image is used in both. This is because inverse image preserves union and intersection. Finding a direct image does not preserve intersection in general. Here is an example where this becomes a hurdle. Consider a set with a topology , a set and a function such that . A set of subsets is not a topology, because but .

There are equivalent definitions below.

The topology coinduced on by is the finest topology such that is continuous . This is a particular case of the final topology on .

The topology induced on by is the coarsest topology such that is continuous . This is a particular case of the initial topology on .

General case

Given a set X and an indexed family (Yi)iI of topological spaces with functions

the topology on induced by these functions is the coarsest topology on X such that each

is continuous.[1][2]

Explicitly, the induced topology is the collection of open sets generated by all sets of the form , where is an open set in for some i I, under finite intersections and arbitrary unions. The sets are often called cylinder sets. If I contains exactly one element, all the open sets of are cylinder sets.

Examples

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gollark: We have, er, 14ish.
gollark: It's 25 votes for the next ~5 days.
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References

  1. Rudin, Walter (January 1, 1991). Functional Analysis. International Series in Pure and Applied Mathematics. 8 (Second ed.). New York, NY: McGraw-Hill Science/Engineering/Math. ISBN 978-0-07-054236-5. OCLC 21163277.CS1 maint: ref=harv (link) CS1 maint: date and year (link)
  2. Adamson, Iain T. (1996). "Induced and Coinduced Topologies". A General Topology Workbook. Birkhäuser, Boston, MA. p. 23. doi:10.1007/978-0-8176-8126-5_3. Retrieved July 21, 2020. ... the topology induced on E by the family of mappings ...
  3. Singh, Tej Bahadur (May 5, 2013). "Elements of Topology". Books.Google.com. CRC Press. Retrieved July 21, 2020.

Sources

  • Hu, Sze-Tsen (1969). Elements of general topology. Holden-Day.

See also

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