Are all functions from a discrete topological space to itself continuous?

In summary, a discrete top (X,T) with T open sets implies that any function f:X->X is continuous, since every open set in the power set of X is open. In the concrete topology, where the only open sets are X and the empty set, the only continuous functions are constant functions. This is because for any function f, its inverse images of the only open sets are open.
  • #1
Nusc
760
2
I need to show all fxn f: X -> X are cts in the discrete top. and that the only cts fxns in the concrete top are the csnt fxns.


Let (X,T) be a discrete top with T open sets.

Let f: X->X. WTS that f:X->X is cts if for every open set G in the image of X, f^-1(G) = V is an open in X when V is a subset of X.


Since (X,T) is a discrete top, V in the power set of X must be open. Since T is open and T = PX this implies that f is cts.

Is there anything wrong with that?

As for the xecond half, I'm not sure. When they say cnst fxns do they mean f(c) = c?
 
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  • #2
Nothing wrong with that. In the discrete topology, every set is open. If f is any function, A any open set (any set) then f-1(A) is a set and therefore open.

No, f(c)= c is the identity function: f(x)= x. A constant function is of the form f(x)= c for all x in the set.

I'm not sure what you mean by the "concrete" topology. Is that the topology in which the only open sets are X and the empty set? (I would call it the "indiscrete" topology.)
 
  • #3
Yes that's the concrete top
 
  • #4
Are you sure it's true that the only continuous functions are the constant functions? If f(x)= x, the identity function, then f-1(empty set)= empty set and f-1(X)= X. That is, the inverse images of the only open sets are open.
 

FAQ: Are all functions from a discrete topological space to itself continuous?

What is continuity?

Continuity is a mathematical concept that describes a function's behavior at a given point. A function is considered continuous if its graph is a continuous, unbroken curve with no abrupt changes or gaps. This means that the function's output values change smoothly as its input values change.

Why is continuity important in mathematics?

Continuity is important in mathematics because it allows us to make predictions and draw conclusions about a function's behavior without having to evaluate every single point on its graph. It also helps us to identify and understand the properties of functions, such as differentiability and integrability.

How do you determine if a function is continuous?

A function is continuous if it meets three criteria: 1) the function is defined at the given point, 2) the limit of the function as it approaches the given point exists, and 3) the value of the function at the given point is equal to the limit. If any of these criteria are not met, the function is not continuous at that point.

What is the difference between point discontinuity and removable discontinuity?

Point discontinuity, also known as a jump discontinuity, occurs when the limit of a function at a given point does not exist. This means that there is a sudden jump or gap in the graph of the function at that point. Removable discontinuity, on the other hand, occurs when a function is undefined at a given point, but its limit exists and can be filled in or removed by redefining the function at that point.

How can continuity be applied in real-world situations?

Continuity has many real-world applications, such as in physics, engineering, and economics. In physics, continuity is used to describe the smooth flow of energy and matter. In engineering, it is used to ensure the stability and safety of structures and systems. In economics, it is used to analyze and predict the behavior of markets and economies.

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