Is Quotient Map Closed? Proof and Explanation | Math Homework

In summary, the conversation discusses whether the quotient map f is closed in addition to being open. The individual proposing that it is closed provides a proof using the definition of an open quotient map, but the other individual points out that there is not a one-to-one correspondence between open sets in X and Y. They also give a counterexample to show that the map is not always closed.
  • #1
pivoxa15
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Homework Statement


THe quotient map f is open but is it also closed?

The Attempt at a Solution


I think it is. Consider f: X->Y

FOr every open set V in Y there exists by definition an open set f^-1(V) in X. There is a one to one correspondence between open sets in X and open sets in Y by definition.

So for every closed set V complement in Y there exists a closed set f^-1(V) complement in X. So f is both closed and open.
 
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  • #2
Are you asking whether or not an open quotient map is closed? If so, then what you did does not prove that. There isn't a 1-1 correspondence between open sets in X and open sets in Y. Look carefully at the definition. If f:X->Y is a quotient map, then U is open in Y iff f^-1(U) is open in X. This does not exhaust all the open sets in X. So you haven't proved that f takes closed sets to closed sets.

Anyway, this is false. For a counterexample, let [itex]\pi_1 : \mathbb{R}^2 \to \mathbb{R}[/itex] be the projection map onto the first coordinate. Then [itex]\pi_1[/itex] is an open surjection, so it's a quotient map. However it's not closed. (I'll let you find a closed set that doesn't get mapped to a closed set.)
 
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1. Is quotient map closed?

The answer is yes, a quotient map is closed. This means that the image of a closed set under a quotient map is also closed.

2. How do you prove that a quotient map is closed?

To prove that a quotient map is closed, we need to show that the image of a closed set under the quotient map is also closed. This can be done by using the definition of a quotient map and showing that the preimage of the complement of the image is open.

3. What is a quotient map in mathematics?

A quotient map is a type of function in mathematics that maps one set onto another, while preserving the structure of the original set. It is often used in topology to define the quotient space of a topological space by identifying certain points or sets together.

4. Can you explain the concept of quotient map with an example?

Yes, consider the function f:ℝ → ℝ/ℤ, where ℝ is the set of real numbers and ℝ/ℤ is the set of real numbers modulo the integers. This function maps each real number to its equivalence class in ℝ/ℤ. This is an example of a quotient map as it preserves the structure of the real numbers while mapping them onto the quotient space ℝ/ℤ.

5. How is quotient map related to topological spaces?

Quotient maps are often used in topology to define the quotient space of a topological space. This is done by identifying certain points or sets together and then using the quotient map to map the original space onto the quotient space. Quotient maps are also important in understanding topological properties such as continuity and compactness.

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