Is Every Continuous Function on a Metric Space Bounded?

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  • Thread starter Euge
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    2016
In summary, a continuous function on a metric space is a function that preserves the topological structure of the space and is bounded if its range is limited. An example of a bounded continuous function is f(x) = 1/x on the metric space (0,1], while not every continuous function on a metric space is bounded, such as f(x) = x on [0,1]. It is important to know if a continuous function is bounded as it helps us understand its behavior and range, making it easier to analyze and apply in various fields.
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Euge
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Here is this week's POTW:

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Show that if a metric space $X$ has the property that every real-valued continuous function on $X$ is bounded, then $X$ is compact.
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Remember to read the http://www.mathhelpboards.com/showthread.php?772-Problem-of-the-Week-%28POTW%29-Procedure-and-Guidelines to find out how to http://www.mathhelpboards.com/forms.php?do=form&fid=2!
 
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Hi MHB community,

I've removed one of the directions of the original problem. The edited problem statement you see above is the "meat" of the original statement. Good luck and thanks to all who participate!
 
  • #3
No one answered this week's problem. You can read my solution below.
Proof by contraposition. If $X$ is not compact, then there is an infinite sequence $\{x_n\}_{n\in \Bbb N}$ which has no convergent subsequence. The set $E = \{x_n : n\in \Bbb N\}$ has no point of accumulation, so it is a closed and discrete subset of $X$. Define a function $f : E\to \Bbb R$ by setting $f(x_n) = n$ if $x_n$ is unique and $0$ otherwise. Since $E$ is discrete, $f$ is continuous. By Tietze's extension theorem, $f$ has a continuous extension to all of $X$, call it $F$. As $E$ has infinitely many distinct points, $F(x_n) = n$ for infinitely many $n$. So $F$ is unbounded.
 

FAQ: Is Every Continuous Function on a Metric Space Bounded?

What is a continuous function on a metric space?

A continuous function on a metric space is a function that preserves the topological structure of the metric space, meaning that small changes in the input result in small changes in the output. In other words, if two points in the metric space are close to each other, their images under the function should also be close.

How is a continuous function on a metric space bounded?

A continuous function on a metric space is bounded if its range is limited, meaning there exists a finite number M such that for all x in the metric space, the absolute value of the function's output is less than or equal to M.

What is an example of a continuous function on a metric space that is bounded?

An example of a continuous function on a metric space that is bounded is the function f(x) = 1/x on the metric space (0,1]. This function is continuous, as small changes in the input result in small changes in the output, and it is bounded because its range is limited to values between 0 and 1.

Is every continuous function on a metric space bounded?

No, not every continuous function on a metric space is bounded. For example, the function f(x) = x on the metric space [0,1] is continuous, but its range is unbounded as it includes all real numbers between 0 and 1.

What is the importance of knowing if a continuous function on a metric space is bounded?

Knowing if a continuous function on a metric space is bounded is important because it helps us determine the behavior of the function and its range. Bounded functions are easier to analyze and work with mathematically, and they have useful properties that can be applied in different areas of mathematics and science.

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