Dual Character for Compact Lie Groups
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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
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up vote
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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
add a comment |Â
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!
representation-theory lie-groups compactness duality-theorems characters
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
representation-theory lie-groups compactness duality-theorems characters
asked Sep 8 at 13:20
itinerantleopard
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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 $chi_V^*=barchi_V$.
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?
â itinerantleopard
Sep 8 at 15:12
1
to compute the trace, you just need to know the eigenvalues.
â Tsemo Aristide
Sep 8 at 15:13
oh, absolutely right! Thanks!
â itinerantleopard
Sep 8 at 15:16
add a comment |Â
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 $chi_V^*=barchi_V$.
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?
â itinerantleopard
Sep 8 at 15:12
1
to compute the trace, you just need to know the eigenvalues.
â Tsemo Aristide
Sep 8 at 15:13
oh, absolutely right! Thanks!
â itinerantleopard
Sep 8 at 15:16
add a comment |Â
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 $chi_V^*=barchi_V$.
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?
â itinerantleopard
Sep 8 at 15:12
1
to compute the trace, you just need to know the eigenvalues.
â Tsemo Aristide
Sep 8 at 15:13
oh, absolutely right! Thanks!
â itinerantleopard
Sep 8 at 15:16
add a comment |Â
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 $chi_V^*=barchi_V$.
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 $chi_V^*=barchi_V$.
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?
â itinerantleopard
Sep 8 at 15:12
1
to compute the trace, you just need to know the eigenvalues.
â Tsemo Aristide
Sep 8 at 15:13
oh, absolutely right! Thanks!
â itinerantleopard
Sep 8 at 15:16
add a comment |Â
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?
â itinerantleopard
Sep 8 at 15:12
1
to compute the trace, you just need to know the eigenvalues.
â Tsemo Aristide
Sep 8 at 15:13
oh, absolutely right! Thanks!
â 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?
â 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?
â itinerantleopard
Sep 8 at 15:12
1
1
to compute the trace, you just need to know the eigenvalues.
â Tsemo Aristide
Sep 8 at 15:13
to compute the trace, you just need to know the eigenvalues.
â Tsemo Aristide
Sep 8 at 15:13
oh, absolutely right! Thanks!
â itinerantleopard
Sep 8 at 15:16
oh, absolutely right! Thanks!
â itinerantleopard
Sep 8 at 15:16
add a comment |Â
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