Dual Character for Compact Lie Groups

The name of the pictureThe name of the pictureThe name of the pictureClash Royale CLAN TAG#URR8PPP











up vote
0
down vote

favorite












I've been reading Brian Conrad's notes on compact Lie groups and came across the assertion $$chi_V^* = overlinechi_V $$
where $V$ is a finite dimensional complex representation of a compact Lie group and $V^*$ is its dual representation (defined in the usual way via $(gcdot phi)(v) = phi(g^-1cdot v)$. He says that the proof is the same as in the finite group case, however when I try to work through the computation in the finite case I get an inverse popping up rather than a conjugate.



This led me to do some exploring, and I found in Dummit and Foote that $chi(g^-1)= overlinechi(g)$ for finite groups (and finite dim reps), however the proof relies on the fact that the action of each group element has finite order (hence each minimal polynomial divides $X^k-1$) and then goes on to argue that the group elements just act by diagonal matrices with roots of unity on the diagonal... so I'm not sure how to generalize this to compact Lie groups (or whether its even true outside the finite group setting!).



Is there something basic that I'm missing here? Any help would be greatly appreciated!










share|cite|improve this question

























    up vote
    0
    down vote

    favorite












    I've been reading Brian Conrad's notes on compact Lie groups and came across the assertion $$chi_V^* = overlinechi_V $$
    where $V$ is a finite dimensional complex representation of a compact Lie group and $V^*$ is its dual representation (defined in the usual way via $(gcdot phi)(v) = phi(g^-1cdot v)$. He says that the proof is the same as in the finite group case, however when I try to work through the computation in the finite case I get an inverse popping up rather than a conjugate.



    This led me to do some exploring, and I found in Dummit and Foote that $chi(g^-1)= overlinechi(g)$ for finite groups (and finite dim reps), however the proof relies on the fact that the action of each group element has finite order (hence each minimal polynomial divides $X^k-1$) and then goes on to argue that the group elements just act by diagonal matrices with roots of unity on the diagonal... so I'm not sure how to generalize this to compact Lie groups (or whether its even true outside the finite group setting!).



    Is there something basic that I'm missing here? Any help would be greatly appreciated!










    share|cite|improve this question























      up vote
      0
      down vote

      favorite









      up vote
      0
      down vote

      favorite











      I've been reading Brian Conrad's notes on compact Lie groups and came across the assertion $$chi_V^* = overlinechi_V $$
      where $V$ is a finite dimensional complex representation of a compact Lie group and $V^*$ is its dual representation (defined in the usual way via $(gcdot phi)(v) = phi(g^-1cdot v)$. He says that the proof is the same as in the finite group case, however when I try to work through the computation in the finite case I get an inverse popping up rather than a conjugate.



      This led me to do some exploring, and I found in Dummit and Foote that $chi(g^-1)= overlinechi(g)$ for finite groups (and finite dim reps), however the proof relies on the fact that the action of each group element has finite order (hence each minimal polynomial divides $X^k-1$) and then goes on to argue that the group elements just act by diagonal matrices with roots of unity on the diagonal... so I'm not sure how to generalize this to compact Lie groups (or whether its even true outside the finite group setting!).



      Is there something basic that I'm missing here? Any help would be greatly appreciated!










      share|cite|improve this question













      I've been reading Brian Conrad's notes on compact Lie groups and came across the assertion $$chi_V^* = overlinechi_V $$
      where $V$ is a finite dimensional complex representation of a compact Lie group and $V^*$ is its dual representation (defined in the usual way via $(gcdot phi)(v) = phi(g^-1cdot v)$. He says that the proof is the same as in the finite group case, however when I try to work through the computation in the finite case I get an inverse popping up rather than a conjugate.



      This led me to do some exploring, and I found in Dummit and Foote that $chi(g^-1)= overlinechi(g)$ for finite groups (and finite dim reps), however the proof relies on the fact that the action of each group element has finite order (hence each minimal polynomial divides $X^k-1$) and then goes on to argue that the group elements just act by diagonal matrices with roots of unity on the diagonal... so I'm not sure how to generalize this to compact Lie groups (or whether its even true outside the finite group setting!).



      Is there something basic that I'm missing here? Any help would be greatly appreciated!







      representation-theory lie-groups compactness duality-theorems characters






      share|cite|improve this question













      share|cite|improve this question











      share|cite|improve this question




      share|cite|improve this question










      asked Sep 8 at 13:20









      itinerantleopard

      1327




      1327




















          1 Answer
          1






          active

          oldest

          votes

















          up vote
          1
          down vote



          accepted










          The analogy comes from the following fact, if the group is compact, you have an invariant scalar product $b$. Let $rho$ be the representation, and $lambda$ an eigenvalue of $rho(g)$ whose eigenvector is $v$, with $b$, you define the isomorphism $Vrightarrow V^*$ by $vrightarrow b(.,v)=v^*$. $rho(g)(v^*)(u)=v^*(g^-1(u))=b(g^-1(u),v)=b(gg^-1(u),gv)=b(u,g(v))=b(u,lambda v)=barlambda b(u,v)=barlambda v^*(u)$. This implies that $Trace(g_V^*)=barTrace(g_V)$, and $chi_V^*=barchi_V$.






          share|cite|improve this answer




















          • Oh, okay, the argument with the invariant (hermitian?) scalar product $b$ makes sense to me, but to arrive at the conclusion $Tr(g_V^*) = overlineTr(g_V)$ don't you need a basis of eigenvectors of the transformation $vmapsto gcdot v$? I know that finite group representations always act via diagonalizable matrices, but is the same true for general Lie groups?
            – itinerantleopard
            Sep 8 at 15:12






          • 1




            to compute the trace, you just need to know the eigenvalues.
            – Tsemo Aristide
            Sep 8 at 15:13










          • oh, absolutely right! Thanks!
            – itinerantleopard
            Sep 8 at 15:16










          Your Answer




          StackExchange.ifUsing("editor", function ()
          return StackExchange.using("mathjaxEditing", function ()
          StackExchange.MarkdownEditor.creationCallbacks.add(function (editor, postfix)
          StackExchange.mathjaxEditing.prepareWmdForMathJax(editor, postfix, [["$", "$"], ["\\(","\\)"]]);
          );
          );
          , "mathjax-editing");

          StackExchange.ready(function()
          var channelOptions =
          tags: "".split(" "),
          id: "69"
          ;
          initTagRenderer("".split(" "), "".split(" "), channelOptions);

          StackExchange.using("externalEditor", function()
          // Have to fire editor after snippets, if snippets enabled
          if (StackExchange.settings.snippets.snippetsEnabled)
          StackExchange.using("snippets", function()
          createEditor();
          );

          else
          createEditor();

          );

          function createEditor()
          StackExchange.prepareEditor(
          heartbeatType: 'answer',
          convertImagesToLinks: true,
          noModals: false,
          showLowRepImageUploadWarning: true,
          reputationToPostImages: 10,
          bindNavPrevention: true,
          postfix: "",
          noCode: true, onDemand: true,
          discardSelector: ".discard-answer"
          ,immediatelyShowMarkdownHelp:true
          );



          );













           

          draft saved


          draft discarded


















          StackExchange.ready(
          function ()
          StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmath.stackexchange.com%2fquestions%2f2909614%2fdual-character-for-compact-lie-groups%23new-answer', 'question_page');

          );

          Post as a guest






























          1 Answer
          1






          active

          oldest

          votes








          1 Answer
          1






          active

          oldest

          votes









          active

          oldest

          votes






          active

          oldest

          votes








          up vote
          1
          down vote



          accepted










          The analogy comes from the following fact, if the group is compact, you have an invariant scalar product $b$. Let $rho$ be the representation, and $lambda$ an eigenvalue of $rho(g)$ whose eigenvector is $v$, with $b$, you define the isomorphism $Vrightarrow V^*$ by $vrightarrow b(.,v)=v^*$. $rho(g)(v^*)(u)=v^*(g^-1(u))=b(g^-1(u),v)=b(gg^-1(u),gv)=b(u,g(v))=b(u,lambda v)=barlambda b(u,v)=barlambda v^*(u)$. This implies that $Trace(g_V^*)=barTrace(g_V)$, and $chi_V^*=barchi_V$.






          share|cite|improve this answer




















          • Oh, okay, the argument with the invariant (hermitian?) scalar product $b$ makes sense to me, but to arrive at the conclusion $Tr(g_V^*) = overlineTr(g_V)$ don't you need a basis of eigenvectors of the transformation $vmapsto gcdot v$? I know that finite group representations always act via diagonalizable matrices, but is the same true for general Lie groups?
            – itinerantleopard
            Sep 8 at 15:12






          • 1




            to compute the trace, you just need to know the eigenvalues.
            – Tsemo Aristide
            Sep 8 at 15:13










          • oh, absolutely right! Thanks!
            – itinerantleopard
            Sep 8 at 15:16














          up vote
          1
          down vote



          accepted










          The analogy comes from the following fact, if the group is compact, you have an invariant scalar product $b$. Let $rho$ be the representation, and $lambda$ an eigenvalue of $rho(g)$ whose eigenvector is $v$, with $b$, you define the isomorphism $Vrightarrow V^*$ by $vrightarrow b(.,v)=v^*$. $rho(g)(v^*)(u)=v^*(g^-1(u))=b(g^-1(u),v)=b(gg^-1(u),gv)=b(u,g(v))=b(u,lambda v)=barlambda b(u,v)=barlambda v^*(u)$. This implies that $Trace(g_V^*)=barTrace(g_V)$, and $chi_V^*=barchi_V$.






          share|cite|improve this answer




















          • Oh, okay, the argument with the invariant (hermitian?) scalar product $b$ makes sense to me, but to arrive at the conclusion $Tr(g_V^*) = overlineTr(g_V)$ don't you need a basis of eigenvectors of the transformation $vmapsto gcdot v$? I know that finite group representations always act via diagonalizable matrices, but is the same true for general Lie groups?
            – itinerantleopard
            Sep 8 at 15:12






          • 1




            to compute the trace, you just need to know the eigenvalues.
            – Tsemo Aristide
            Sep 8 at 15:13










          • oh, absolutely right! Thanks!
            – itinerantleopard
            Sep 8 at 15:16












          up vote
          1
          down vote



          accepted







          up vote
          1
          down vote



          accepted






          The analogy comes from the following fact, if the group is compact, you have an invariant scalar product $b$. Let $rho$ be the representation, and $lambda$ an eigenvalue of $rho(g)$ whose eigenvector is $v$, with $b$, you define the isomorphism $Vrightarrow V^*$ by $vrightarrow b(.,v)=v^*$. $rho(g)(v^*)(u)=v^*(g^-1(u))=b(g^-1(u),v)=b(gg^-1(u),gv)=b(u,g(v))=b(u,lambda v)=barlambda b(u,v)=barlambda v^*(u)$. This implies that $Trace(g_V^*)=barTrace(g_V)$, and $chi_V^*=barchi_V$.






          share|cite|improve this answer












          The analogy comes from the following fact, if the group is compact, you have an invariant scalar product $b$. Let $rho$ be the representation, and $lambda$ an eigenvalue of $rho(g)$ whose eigenvector is $v$, with $b$, you define the isomorphism $Vrightarrow V^*$ by $vrightarrow b(.,v)=v^*$. $rho(g)(v^*)(u)=v^*(g^-1(u))=b(g^-1(u),v)=b(gg^-1(u),gv)=b(u,g(v))=b(u,lambda v)=barlambda b(u,v)=barlambda v^*(u)$. This implies that $Trace(g_V^*)=barTrace(g_V)$, and $chi_V^*=barchi_V$.







          share|cite|improve this answer












          share|cite|improve this answer



          share|cite|improve this answer










          answered Sep 8 at 14:15









          Tsemo Aristide

          52.5k11244




          52.5k11244











          • Oh, okay, the argument with the invariant (hermitian?) scalar product $b$ makes sense to me, but to arrive at the conclusion $Tr(g_V^*) = overlineTr(g_V)$ don't you need a basis of eigenvectors of the transformation $vmapsto gcdot v$? I know that finite group representations always act via diagonalizable matrices, but is the same true for general Lie groups?
            – itinerantleopard
            Sep 8 at 15:12






          • 1




            to compute the trace, you just need to know the eigenvalues.
            – Tsemo Aristide
            Sep 8 at 15:13










          • oh, absolutely right! Thanks!
            – itinerantleopard
            Sep 8 at 15:16
















          • Oh, okay, the argument with the invariant (hermitian?) scalar product $b$ makes sense to me, but to arrive at the conclusion $Tr(g_V^*) = overlineTr(g_V)$ don't you need a basis of eigenvectors of the transformation $vmapsto gcdot v$? I know that finite group representations always act via diagonalizable matrices, but is the same true for general Lie groups?
            – itinerantleopard
            Sep 8 at 15:12






          • 1




            to compute the trace, you just need to know the eigenvalues.
            – Tsemo Aristide
            Sep 8 at 15:13










          • oh, absolutely right! Thanks!
            – itinerantleopard
            Sep 8 at 15:16















          Oh, okay, the argument with the invariant (hermitian?) scalar product $b$ makes sense to me, but to arrive at the conclusion $Tr(g_V^*) = overlineTr(g_V)$ don't you need a basis of eigenvectors of the transformation $vmapsto gcdot v$? I know that finite group representations always act via diagonalizable matrices, but is the same true for general Lie groups?
          – itinerantleopard
          Sep 8 at 15:12




          Oh, okay, the argument with the invariant (hermitian?) scalar product $b$ makes sense to me, but to arrive at the conclusion $Tr(g_V^*) = overlineTr(g_V)$ don't you need a basis of eigenvectors of the transformation $vmapsto gcdot v$? I know that finite group representations always act via diagonalizable matrices, but is the same true for general Lie groups?
          – itinerantleopard
          Sep 8 at 15:12




          1




          1




          to compute the trace, you just need to know the eigenvalues.
          – Tsemo Aristide
          Sep 8 at 15:13




          to compute the trace, you just need to know the eigenvalues.
          – Tsemo Aristide
          Sep 8 at 15:13












          oh, absolutely right! Thanks!
          – itinerantleopard
          Sep 8 at 15:16




          oh, absolutely right! Thanks!
          – itinerantleopard
          Sep 8 at 15:16

















           

          draft saved


          draft discarded















































           


          draft saved


          draft discarded














          StackExchange.ready(
          function ()
          StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmath.stackexchange.com%2fquestions%2f2909614%2fdual-character-for-compact-lie-groups%23new-answer', 'question_page');

          );

          Post as a guest













































































          這個網誌中的熱門文章

          How to combine Bézier curves to a surface?

          Carbon dioxide

          Why am i infinitely getting the same tweet with the Twitter Search API?