Pinwheel Symmetry Formulas Exposed

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Discussion Overview

The discussion revolves around the combinatorial problem of determining the number of distinct colorings of a pinwheel with 8 identically shaped pins, where each pin can be colored in one of 4 colors. Participants explore the implications of rotational and reflectional symmetries in this context, as well as the relevance of Burnside's lemma in calculating distinct configurations.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant suggests that the total number of configurations is given by \(4^8\) but questions whether reflectional symmetries should be considered, proposing \(4^5\) as a potential adjustment.
  • Another participant cites a formula from Wikipedia involving Burnside's lemma, indicating that the number of distinct colorings can be calculated using \({1\over 8}\sum_{d\mid 8}\varphi(d)4^{8/d}\).
  • Concerns are raised about the relevance of combinatorics in an abstract algebra class, with a participant expressing confusion over the inclusion of such topics.
  • A participant explains that distinct colorings account for configurations that can be rotated into one another, emphasizing the role of group theory in this analysis.
  • One participant expresses frustration with the textbook's wording, suggesting it may be ambiguous and unhelpful for learners.
  • Another participant mentions the difficulty in finding proofs for the formulas discussed and highlights the importance of Burnside's lemma in understanding the problem.

Areas of Agreement / Disagreement

Participants express differing views on the correct approach to calculating the number of distinct colorings, with some supporting the use of Burnside's lemma while others question the necessity of considering reflectional symmetries. The discussion remains unresolved regarding the best method to apply.

Contextual Notes

Participants note limitations in the textbook's clarity and the potential ambiguity in the problem statement, which may affect understanding and application of the concepts discussed.

mathjam0990
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Question:

How many different pinwheels with 8 identically shaped pins are there if each pin can be colored one of 4 different colors?

My answer:

4^8 because each pin has 4 choices Is this correct? I think this only includes rotational symmetries. Do I need to consider reflectional symmetries? If so, would reflectional symmetries = 4^5?

Thank you for your help!
 
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I'm not a specialist in combinatorics, but Wikipedia says the number is ${1\over 8}\sum_{d\mid 8}\varphi(d)4^{8/d}$.

By the way, how can a pinwheels spin if it has several pins? Do you mean 8 curls?
 
This is actually for an abstract algebra class. Not sure why they are teaching us combinatorics :( Also, I stated the question quote from our textbook so I am not sure. Thanks for the info though, I appreciate it.
 
mathjam0990 said:
This is actually for an abstract algebra class. Not sure why they are teaching us combinatorics :( Also, I stated the question quote from our textbook so I am not sure. Thanks for the info though, I appreciate it.

Because Burnside's lemma involves the use of group (action) theory.

Usually, we talk about "distinct colorings"; for example, if we have for the "pins":

1 = red
2 = yellow
3 = green
4 = blue
5 = red
6 = yellow
7 = green
8 = blue

this is regarded as the same coloring as:

1 = yellow
2 = green
3 = blue
4 = red
5 = yellow
6 = green
7 = blue
8 = red

since the $8$-cycle $(1\ 2\ 3\ 4\ 5\ 6\ 7\ 8)$ takes the first to the second (that particular $8$-cycle generates the rotation group).

$4^8$ is the size of the total "configuration space" $X$ of all possible colorings where we keep track of which "pin" got painted what color. The number of "distinct colorings" is much smaller, since we might rotate one configuration into another, with no way to tell them apart (this assumes all eight "pins" are identical).

I daresay a pinwheel is being used here to avoid the harder question of including reflection symmetries (like you would have using a necklace, with beads).
 
Thank you Deveno for your help. The question in the book definitely stated quote pinwheel. I think it's a poorly written book and a bit ambiguous at times which doesn't help someone like me who is already not too great at this stuff. Anyway, thanks for the info!
 

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