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Given a square and a permutation of 90 degrees counter-clockwise, what is the order of this finite symmetry group? Here's an attempt: $$\rho =\begin{bmatrix} 1 &2 &3 &4 \\ \rho (1) &\rho(2) &\rho(3) &\rho(4) \end{bmatrix}=\begin{bmatrix} 1 &2 &3 &4 \\ 2&3 &4 &1 \end{bmatrix}$$

I want to find the order of this symmetry group.

This is a group of $S_{4}$. I'm tempted to say the order of $S_{n}$ is $4$ but this doesn't look right given the constraint of a 90 degrees counter-clockwise permutation. That means, if we have the square with sides labelled 1,2,3 and 4 as reflected in the matrix above, only the below are permitted $$1\rightarrow 2 \\ 2\rightarrow 3\\ 3\rightarrow 4\\ 4\rightarrow 1.$$

Have I gone wrong anywhere? Also, the square is a $D_{4}$, how do I determine $D_{4}\leq S_{n}$?

Suppose, the reflection is $\phi$:

$$\phi =\begin{bmatrix} 1 &2 &3 &4 \\ 2&1 &4 &3 \end{bmatrix}$$ (notation implies subgroup)

Hrodelbert
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  • Welcome to Math.SE! I am not following your question $100%$: am I right that your goal is to derive the order of the symmetry group of the square with corners $1,2,3,4$ starting from the assumption that it has to be a subgroup of $S_4$? Note that $S_4$ has order $4!$. – Hrodelbert Jun 03 '15 at 09:13
  • There was a typing error. 4! was what I had in mind but I'm couldn't convinced myself geometrically. It is a subgroup of 4. That is a given. ignoring the reflection for the moment, with permutation of 90 degrees CCW alone, there is no way 1 could be mapped to 4 in a single 'move'. – opensource.11 Jun 03 '15 at 09:15
  • When you write "this finite symmetry group" (emphasis added), do you mean the group generated by the rotation $\rho$, the group of all symmetries of the square, or something else? (Note that the set ${\rho}$ is not closed under composition, nor is the set ${e, rho}$ consisting of $\rho$ and the identity map.) – Andrew D. Hwang Jun 03 '15 at 09:33
  • I do not know. It was a question copied from the blackboard during lesson :( – opensource.11 Jun 03 '15 at 09:34
  • @user86418 why is it not closed under composition? – opensource.11 Jun 03 '15 at 09:35
  • @opensource.11: If you rotate a square through a quarter turn and then through a quarter turn again, the overall result is not a rotation through a quarter turn. Briefly, "${\rho}$ is not closed under composition". :) (My guess is "this group" refers to "the group generated by the rotation $\rho$", a.k.a., the smallest group of symmetries of the square that contains a quarter-turn, but it's best to clarify with your instructor.) Incidentally, it's also a good idea to make a square from paper and to number its vertices. Then you can experiment by physically manipulating a piece of paper. – Andrew D. Hwang Jun 03 '15 at 09:48

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