@@ -18,13 +18,13 @@ \subsection{Dihedral Groups}\label{dihedral-groups}}
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Let $ P$ be a regular polygon with $ n$ sides and $ V$ its set of vertices.
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We can assume
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\begin {align* }
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- V = \{ e^{2\pi i k /n} : 0 \leq k < n \}
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+ V = \{ e^{2\pi i k /n} : 0 \leq k < n \}
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\end {align* } (nth roots of unity in $ \mathbb {C}$ ).
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Then the symmetries of $ P$ are the isometries (i.e.~distance preserving maps of $ \mathbb {C}$ that map $ V$ to $ V$ ).
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-
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+
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We will show that:
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for $ n \geq 3 $ the set of symmetries of $ P$ , under composition, form a nonabelian group of order $ 2 n$ . This group is called the \emph {dihedral group } of order $ 2 n$ and denoted $ D_{2n}$ .
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- \end {definition }
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+ \end {definition }
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\emph {Warning } - sometimes $ D_{2n}$ is denoted $ D_n$
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@@ -88,7 +88,7 @@ \subsection{Dihedral Groups}\label{dihedral-groups}}
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Algebraically we write,
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\begin {align* }
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- D_{2n} = \left\langle \underbrace {r,\ t}_\text {generators} | \underbrace {r^n = e,\ t^2 = e,\ trt = r^{-1}}_\text {relations} \right\rangle
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+ D_{2n} = \left\langle \underbrace {r,\ t}_\text {generators} | \underbrace {r^n = e,\ t^2 = e,\ trt = r^{-1}}_\text {relations} \right\rangle
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\end {align* }
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Finally, $ D_2 \cong C_2 $ and $ D_4 $ is \Cref {exm:nine }.
@@ -104,7 +104,7 @@ \subsection{Symmetric Groups}\label{symmetric-groups}}
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f: X \to X
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\end {align* } is called a \emph {permutation } of $ X$ .
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Let $ \operatorname {Sym} X$ denote the set of all permutations of $ X$ .
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- \end {definition }
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+ \end {definition }
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\begin {proposition }
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$ \operatorname {Sym} X$ is a group under composition of functions. It is called the symmetric group on $ X$ .
@@ -138,9 +138,9 @@ \subsection{Symmetric Groups}\label{symmetric-groups}}
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G = \begin {pmatrix }
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1 & 2 & \dots & n \\
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\sigma (1) & \sigma (2) & \dots & \sigma (n)
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- \end {pmatrix } \\
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+ \end {pmatrix }
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\end {align* }
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- \end {notation }
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+ \end {notation }
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%
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\begin {example } ~\vspace *{-1.5\baselineskip }
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\begin {align* }
@@ -153,7 +153,7 @@ \subsection{Symmetric Groups}\label{symmetric-groups}}
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2 & 3 & 1 & 4 & 5
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\end {pmatrix } \in S_5
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\end {align* }
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- \end {example }
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+ \end {example }
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\begin {example }
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Composition:
@@ -179,7 +179,7 @@ \subsection{Symmetric Groups}\label{symmetric-groups}}
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or:
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{\centering \includegraphics {02-symmetric-graphical}}
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- \end {example }
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+ \end {example }
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\hypertarget {small-n}{%
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\subsubsection {Small n }\label {small-n }}
@@ -198,30 +198,30 @@ \subsubsection{Small n}\label{small-n}}
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S_3 &= \left \{ \begin {pmatrix }
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1 & 2 & 3 \\
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1 & 2 & 3
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- \end {pmatrix },
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+ \end {pmatrix },
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\begin {pmatrix }
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1 & 2 & 3 \\
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2 & 3 & 1
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- \end {pmatrix },
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+ \end {pmatrix },
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\begin {pmatrix }
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1 & 2 & 3 \\
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3 & 1 & 2
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- \end {pmatrix },
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+ \end {pmatrix },
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\begin {pmatrix }
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1 & 2 & 3 \\
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1 & 3 & 2
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- \end {pmatrix },
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+ \end {pmatrix },
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\begin {pmatrix }
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1 & 2 & 3 \\
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3 & 2 & 1
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- \end {pmatrix },
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+ \end {pmatrix },
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\begin {pmatrix }
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1 & 2 & 3 \\
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2 & 1 & 3
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\end {pmatrix } \right \} \\
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& \cong D_6
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\end {align* }
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- \end {example }
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+ \end {example }
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\begin {remark } ~
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\begin {enumerate }
@@ -235,7 +235,7 @@ \subsubsection{Small n}\label{small-n}}
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\begin {pmatrix }
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1 & 2 & 3 & 4 & \dots & n \\
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2 & 3 & 1 & 4 & \dots & n
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- \end {pmatrix },
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+ \end {pmatrix },
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\begin {pmatrix }
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1 & 2 & 3 & 4 & \dots & n \\
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1 & 3 & 2 & 4 & \dots & n
@@ -249,7 +249,7 @@ \subsubsection{Small n}\label{small-n}}
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r = \begin {pmatrix }
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1 & 2 & 3 & 4 \\
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2 & 3 & 4 & 1
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- \end {pmatrix },\ t =
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+ \end {pmatrix },\ t =
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\begin {pmatrix }
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1 & 2 & 3 & 4 \\
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4 & 3 & 2 & 1
@@ -294,7 +294,7 @@ \subsubsection{Small n}\label{small-n}}
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$ o(\sigma ) = k$ , $ \sigma $ is like the rotations of k points.
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\begin {figure }
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\centering \includegraphics [width=0.5\linewidth ]{02-sigma-graphical}
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- \end {figure }
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+ \end {figure }
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\item
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a 2-cycle is called a \emph {transposition }.
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\end {enumerate }
@@ -350,7 +350,7 @@ \subsubsection{Small n}\label{small-n}}
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\begin {align* }
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\begin {pmatrix }1 & 2 & 3\end {pmatrix } \begin {pmatrix }2 & 3\end {pmatrix } &= \begin {pmatrix }1 & 2\end {pmatrix } \begin {pmatrix } 3 \end {pmatrix } \\
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&= \begin {pmatrix }1 & 2\end {pmatrix } \text { suppress 1-cycles.} \\
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- \begin {pmatrix }2 & 3\end {pmatrix } \begin {pmatrix }1 & 2 & 3\end {pmatrix } &= \begin {pmatrix }1 & 3\end {pmatrix }
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+ \begin {pmatrix }2 & 3\end {pmatrix } \begin {pmatrix }1 & 2 & 3\end {pmatrix } &= \begin {pmatrix }1 & 3\end {pmatrix }
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\end {align* }
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\end {example }
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@@ -498,7 +498,7 @@ \subsubsection{Small n}\label{small-n}}
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\node (C31) [left of=C32] {$ \langle r \rangle $ };
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\node (C33) [below right of=A4] {$ \langle rt \rangle $ };
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\node (C34) [right of=C33] {$ \langle r^2 t \rangle $ };
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-
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+
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\node (1) [below=3cm,at=(A4.south)] {$ \left\{ e\right \} $ };
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\draw (A4) -- (C31);
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\draw (A4) -- (C32);
@@ -508,7 +508,7 @@ \subsubsection{Small n}\label{small-n}}
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\draw (C32) -- (1);
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\draw (C33) -- (1);
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\draw (C34) -- (1);
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- \end {tikzpicture }
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+ \end {tikzpicture }
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\end {center }
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We put the largest subgroups at the top and work our way down.
@@ -564,7 +564,7 @@ \subsubsection{Small n}\label{small-n}}
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\mathitem
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\begin {align* }
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- \operatorname {sgn}(\sigma ) &= 1 = \operatorname {sgn}(\rho ) \\
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+ \operatorname {sgn}(\sigma ) &= 1 = \operatorname {sgn}(\rho ) \\
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\implies \operatorname {sgn}(\sigma \rho ) &= \operatorname {sgn}(\sigma ) \operatorname {sgn}(\rho ) \\
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&= 1
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\end {align* } by Theorem \ref {thm:two }
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