Can a Finite Group Acting on a Set Have No Fixed Points?

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In summary, a finite group can act on a set without fixed points, known as a free action. This has significant implications in mathematics, such as in topology and geometry. To prove that a finite group has no fixed points, the Orbit-Stabilizer Theorem can be used. Real-life applications of this concept include coding theory and cryptography. However, a group must have a well-defined action on a set in order to have fixed points.
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Euge
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MHB
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Here is this week's POTW:

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Let $X$ be a finite set with more than one element, and $G$ be a finite group acting transitively on $X$. Show that some element of $G$ is free of fixed points.

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Remember to read the https://mathhelpboards.com/showthread.php?772-Problem-of-the-Week-%28POTW%29-Procedure-and-Guidelines to find out how to https://mathhelpboards.com/forms.php?do=form&fid=2!
 
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  • #2
This week's problem was solved correctly by castor28 and Olinguito. You can read castor28's solution below.
Let $|X|=n>1$. As the action is transitive, there is only one orbit. By Burnside's lemma, this is equal to the average number of points fixed by an element of $G$.
The identity of $G$ fixes all the $n$ points. If every element of $G$ fixed at least one point, the average would be grater than $1$.
 

1. Can a finite group acting on a set have no fixed points?

Yes, it is possible for a finite group to act on a set and have no fixed points. This means that every element in the set is moved by at least one element in the group.

2. What is a fixed point in group theory?

In group theory, a fixed point is an element in a set that remains unchanged when acted upon by elements in a group. In other words, the element is mapped to itself by the group action.

3. How can a finite group have no fixed points?

A finite group can have no fixed points if the group action is not transitive. This means that there are elements in the set that cannot be reached by any element in the group.

4. Are there any real-life applications of groups with no fixed points?

Yes, group actions with no fixed points are commonly used in coding theory, specifically in error-correcting codes. They are also used in combinatorics and graph theory.

5. Is it possible for a finite group to have no fixed points if the set is infinite?

No, if the set is infinite, then a finite group acting on it will always have at least one fixed point. This is because an infinite set has infinitely many elements, and the group action must eventually repeat, resulting in at least one fixed point.

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