Continuous functions in topology

In summary, a continuous function in topology is one where the inverse image of an open set in the codomain is open in the domain. This does not necessarily mean that the function is surjective, as there may be open sets in the codomain that have a null inverse image in the domain. The definition of topology allows for such cases to exist.
  • #1
ehrenfest
2,020
1

Homework Statement


In topology, a f: X -> Y is continuous when

U is open in Y implies that f^{-1}(U) is open in X

Doesn't that mean that a continuous function must be surjective i.e. it must span all of Y since every point y in Y is in an open set and that open set must have a pre-image open in X, so there must be some x in X s.t. f(x) = y?

What is wrong with my logic?


Homework Equations





The Attempt at a Solution

 
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  • #2
If you think it isn't true, the try and construct a counterexample. :smile:

If it's true, this might help you see why. If it's false, analysis of the counterexample might help you spot the flaw.
 
  • #3
So you're asking about the case when there is no x in X st... That would seem to be the empty set. Now what was the definition of topology?
 
  • #4
I think matt grime answered it, but my counter example is:

f:R->R
f(x) = abs(x)

the set (-4, -2) is open but when when you pull it back you get the null set, which is open in the usual topology on R. I think I see now!
 

Related to Continuous functions in topology

1. What is a continuous function in topology?

A continuous function in topology is a function between two topological spaces that preserves the topological structure. This means that the preimage of an open set in the target space is an open set in the domain space. In simpler terms, it is a function that does not cause any abrupt changes or breaks in the topological structure of the spaces.

2. How is continuity defined in topology?

In topology, continuity is defined using the notion of open sets. A function f between two topological spaces X and Y is continuous if the preimage of every open set in Y is an open set in X. This means that the function does not cause any sudden changes in the topological structure of the spaces.

3. What is the importance of continuous functions in topology?

Continuous functions are important in topology because they allow us to study and understand the topological properties of spaces and how they are related. They also help us to identify continuous deformations between spaces, which is a fundamental concept in topology.

4. Can a function be continuous in one direction but not the other?

Yes, it is possible for a function to be continuous in one direction but not the other. This is known as one-sided continuity. For example, a function can be continuous from the right but not from the left at a particular point.

5. What is the difference between pointwise continuity and global continuity?

Pointwise continuity refers to the continuity of a function at a specific point, whereas global continuity refers to the continuity of a function across its entire domain. A function can be pointwise continuous at every point but not globally continuous, and vice versa.

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