Is the Group of Symmetries of a Pentagram Isomorphic to the Dihedral Group?

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Homework Help Overview

The discussion revolves around the group of symmetries of a regular pentagram and its relationship to the dihedral group of order 10. Participants are exploring the nature of these symmetries, particularly focusing on the distinction between rotational and reflectional symmetries.

Discussion Character

  • Conceptual clarification, Assumption checking, Mathematical reasoning

Approaches and Questions Raised

  • Participants are attempting to understand why the symmetry group of the pentagram, initially thought to be isomorphic to the cyclic group of order 5, is actually isomorphic to the dihedral group. Questions about the nature of reflectional symmetries and their implications for the group's order are raised.

Discussion Status

The discussion is active, with some participants providing insights into the role of reflectional symmetries and how they contribute to the overall order of the symmetry group. There is an acknowledgment of the need to consider both types of symmetries to fully understand the group's structure.

Contextual Notes

Participants are operating under the assumption that the group of symmetries must account for both rotations and reflections, and there is a specific focus on the implications of these symmetries for the group's order. The original poster's initial assumption about the order of the group is being critically examined.

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Homework Statement


The group of symmetries of a regular pentagram is isomorphic to the dihedral group of order 10.

Show that this is true.

The Attempt at a Solution


It seems to me that the group shown by the "star" has order 5, since, by following the lines from one point, it takes 5 total paths to get back to the original point.

So I thought it would isomorphic to the cyclic group of order 5...how is it isomorphic to the dihedral group?
 
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The rotational symmetries are isomorphic to the cyclic group of order 5, but there are also reflectional symmetries that need to be considered.
 
By reflectional symmetries do you mean the inverse? As in, if I choose one path from point A to point B to point C, then the reflectional symmetry is from point C to point B to point A?
 
If you center your pentagram about the origin then the reflection across the x-axis would be a reflectional symmetry. You should be able to generate your group from that reflection and the rotation of 72 degrees about the origin.
 
Oh okay...so when I said this---

fk378 said:

The Attempt at a Solution


It seems to me that the group shown by the "star" has order 5, since, by following the lines from one point, it takes 5 total paths to get back to the original point.

So I thought it would isomorphic to the cyclic group of order 5...how is it isomorphic to the dihedral group?

---was I wrong?
 
Yes the group will have order 10. If you label a vertex A and label a vertex adjacent to A B. Then there are 5 positions that A can be moved to and each of those allows exactly 2 positions for B.
 

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