Funtion continuity and open sets

In summary, if a function f is continuous, it will map open sets to open sets. However, if f maps open sets to open sets, it does not necessarily have to be continuous. A counterexample would be a constant function which is continuous but maps any set to a singleton set which is not open. On the other hand, if f is not continuous, it is possible for it to still map open sets to open sets, as shown by the example of a function from a continuous space to a discrete space.
  • #1
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Homework Statement



Suppose that [tex]f : (X,d_X) \to (Y,d_Y)[/tex]. If [tex]f[/tex] is continuous,
must it map open sets to open sets? If [tex]f[/tex] does map open sets to
open sets must [tex]f[/tex] be continuous?

Homework Equations





The Attempt at a Solution



The answer to the first question is yes. The answer to the second question I guess is "no". Is this correct? How can I prove it?
 
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  • #2
Neither is true. For example, a constant function is trivially continuous but maps any set, including any open set, to a singleton set which is not, generally, open.

Conversely, if B has the discrete topology, so that all sets are open, and A is a [0, 1] with the usual topology, the function f(x)= a for all [itex]0\le x< 1/2[/itex] and f(x)= b for all [itex]1/2\le 0\le 1[/itex], where a and b are distinct points in B, is not continuous but maps open sets to open sets.
 
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Related to Funtion continuity and open sets

1. What is the definition of continuity of a function?

The continuity of a function refers to the property of a function where small changes in the input result in small changes in the output. In other words, a function is continuous if its graph has no breaks or gaps.

2. How is the continuity of a function determined?

The continuity of a function can be determined by evaluating the function at a specific point and checking if the limit of the function at that point exists and is equal to the value of the function at that point. If this condition is met for all points in the domain, then the function is continuous.

3. What is the significance of continuity in mathematics?

Continuity is an important concept in mathematics as it allows for the analysis and understanding of functions. It allows us to determine the behavior of a function at a point and make predictions about its behavior in its entire domain. It also plays a crucial role in calculus and is essential in the study of derivatives and integrals.

4. What is the difference between pointwise and uniform continuity?

Pointwise continuity refers to the continuity of a function at a specific point, while uniform continuity refers to the continuity of a function over an entire interval. Pointwise continuity only requires the limit of the function to exist at a point, while uniform continuity requires the limit to be the same for all points in the interval.

5. What are open sets in relation to continuous functions?

In the context of continuous functions, open sets refer to sets where all the points inside the set are mapped to points within the set. In other words, if a function is continuous, then the inverse image of an open set is also an open set. This concept is important in topology and the study of continuous functions.

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