Skip to main content
Mathematics LibreTexts

3.2: Alternating Groups

  • Page ID
    131874
  • \( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \)

    \( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash {#1}}} \)

    \( \newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\)

    ( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\)

    \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\)

    \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\)

    \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\)

    \( \newcommand{\Span}{\mathrm{span}}\)

    \( \newcommand{\id}{\mathrm{id}}\)

    \( \newcommand{\Span}{\mathrm{span}}\)

    \( \newcommand{\kernel}{\mathrm{null}\,}\)

    \( \newcommand{\range}{\mathrm{range}\,}\)

    \( \newcommand{\RealPart}{\mathrm{Re}}\)

    \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\)

    \( \newcommand{\Argument}{\mathrm{Arg}}\)

    \( \newcommand{\norm}[1]{\| #1 \|}\)

    \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\)

    \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\AA}{\unicode[.8,0]{x212B}}\)

    \( \newcommand{\vectorA}[1]{\vec{#1}}      % arrow\)

    \( \newcommand{\vectorAt}[1]{\vec{\text{#1}}}      % arrow\)

    \( \newcommand{\vectorB}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \)

    \( \newcommand{\vectorC}[1]{\textbf{#1}} \)

    \( \newcommand{\vectorD}[1]{\overrightarrow{#1}} \)

    \( \newcommand{\vectorDt}[1]{\overrightarrow{\text{#1}}} \)

    \( \newcommand{\vectE}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash{\mathbf {#1}}}} \)

    \( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \)

    \( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash {#1}}} \)

    \(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)
    Definition: Alternating groups

    Alternating groups \(A_n\) is the set of all even permutations associated with composition.  \(|A_n|=\frac{n!}{2}\). \(A_n\) is a subgroup of the symmetric group \(S_n.\)

    Example \(\PageIndex{1}\)

    \( A_3=\{1, (1, 2, 3), (3, 2, 1)\}. A_3\) is a cyclic group of order \(3.\)

    Theorem \(\PageIndex{1}\)

    \(A_n\)  is non abelian for \(n \ge 4\).

    Proof:

    Let \(a,b \in A_4\).

    We will show that \(ab \ne ba, \; \forall a,b \in A_n\), \(n \ge 4\).

    Note \(A_4=\{e, (1,2)(3,4), (1,3)(2,4), (1,4)(2,3), (1,3)(1,2), (1,4)(1,2),\)

    \((1,4)(1,3),(2,4)(2,3), (3,1)(3,2), (4,2)(4,1), (4,1)(4,3),(4,2)(4,3) \}\). 

    Let \(a=(1,2)(3,4)\) and \(b=(4,1)(4,2)\).

    Consider \(ab=(1,2)(3,4)(4,1)(4,2)=(1,3,4,2)\).

    Further consider, \(ba=(4,1)(4,2)(1,2)(3,4)=(1,4,3,2)\).

    Since \((1,3,4,2) \ne (1,4,3,2), \; \forall a,b \in A_4\), thus \(A_4\) is non-abelian.

    Since \(A_4 \le A_n, \; \forall n \ge 4\), \(A_n\) is non-abelian for \(N\ge 4\).◻

    Example \(\PageIndex{1}\)

    Find a cyclic subgroup of \(A_8\) that has order 4.  

    Consider the subgroup \(\{e, (1,2,3,4),(1,3)(2,4),(4,3,2,1)\}\) which is a subgroup of \(A_8\) of order 4.

    Then consider \((1,2,3,4)(1,2,3,4)=(1,3)(2,4) \in A_8\).

    Further \((1,2,3,4)(1,3)(2,4)=(1,4,3,2) \in A_8\).

    Further \((1,2,3,4)(1,4,3,2)=e\).

    Thus \(\{e, (1,2,3,4),(1,3)(2,4),(4,3,2,1)\}\), a subgroup of \(A_8\) that is order 4 and cyclic.

     

    Example \(\PageIndex{2}\)

     Find a non-cyclic subgroup of \(A_8\) that has order 4.

    Consider \(\{e,(1,2),(2,3),(3,4)\}\) which is a subgroup of \(A_8\) of order 4.

    Then consider \((1,2)^n\).

    There is no \(n\) for which \((1,2)^n=(3,4) \text{ or } (2,3)\).

    Similarly, there is no \(n\) for which \((2,3)^n=(1,2)\text{ or } (3,4)\).

    Similarly, there is no \(n\) for which \((3,4)^n=(1,2)\text{ or } (2,3)\).

    Thus \(\{e,(1,2),(2,3),(3,4)\}\) is not cyclic.

    Version 2:

    Let \(G=\{e,i,j,k\}\).

    Let \(i=(1,2)(3,4)\), \(j=(5,6)(7,8)\) and \(k=(1,2)(3,4)(5,6)(7,8)\).

    Consider \(ij=k\), \(ik=j\), and \(jk=i\).

    Thus there is closure in \(G\).

    Further, consider \(i^{-1}=i\), \(j^{-1}=j\), and \(k^{-1}=k\).

    Thus inverses exist for all elements of \(G\).

    Note there is no \(n\) for which \(i^n=j\) or \(k\).

    Similarly, there is no \(n\) for which \(j^n=i\) or \(k\).

    Similarly, there is no \(n\) for which \(k^n=j\) or \(k\).

    Since there is no \(<g> \in G\), \(G\) is not cyclic.

    .....


    This page titled 3.2: Alternating Groups is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Pamini Thangarajah.

    • Was this article helpful?