Orthogonality Relations for Characters

Oct 13, 2025 by wbrum.org 98 views

Introduction

Recently, I have been learning linear representation of finite groups using notes written by my professor, which supplement Serre's classic book. Below is a corollary in the notes on the class function part that has left me confused. I would like to discuss this corollary with the community and seek clarification.

Background

In the study of linear representations of finite groups, class functions play a crucial role. A class function is a function on the conjugacy classes of a group that is constant on each conjugacy class. In other words, it is a function that is invariant under conjugation. Class functions are important because they can be used to construct representations of a group.

The Corollary

The corollary in question is as follows:

Corollary: Let G be a finite group and let χ be a class function on G. Then, for any irreducible representation V of G, we have

χ(g) = ∑_{i=1}^k χ(g)Tr(V_i(g))

where g ∈ G, V_i is an irreducible representation of G, and Tr(V_i(g)) is the trace of the matrix V_i(g).

My Confusion

I am having trouble understanding the proof of this corollary. The proof in my notes involves using the fact that the character of an irreducible representation is a class function, and then using the orthogonality relations for characters to derive the desired result. However, I am not convinced that this proof is correct.

Orthogonality Relations for Characters

The orthogonality relations for characters state that for any two irreducible representations V and W of a group G, we have

∑_{g∈G} χ_V(g)χ_W(g^(-1)) = |G| δ_V,W

where δ_V,W is the Kronecker delta, which is 1 if V = W and 0 otherwise.

Proof of the Corollary

The proof of the corollary in my notes involves using the orthogonality relations for characters to derive the desired result. Specifically, we have

χ(g) = ∑_{i=1}^k χ(g)Tr(V_i(g))

where g ∈ G, V_i is an irreducible representation of G, and Tr(V_i(g)) is the trace of the matrix V_i(g).

To see this, we note that

χ(g) = ∑_{i=1}^k χ(g)Tr(V_i(g))

= ∑{i=1}^k ∑{j=1}^k χ(g)Tr(V_i(g))χ(V_j(g^(-1)))

= ∑{i=1}^k ∑{j=1}^k χ(g)Tr(V_i(g))δ_{i,j}

= ∑_{i=1}^k χ(g)Tr(V_i(g))

where we have used the orthogonality relations for characters to derive the desired result.

My Concerns

I am not convinced that this proof is correct. Specifically, I am concerned that the use of the orthogonality relations for characters is not justified. In particular, I am not sure that the sum over g in the orthogonality relations is well-defined.

Discussion

I would like to discuss this corollary with the community and seek clarification. Specifically, I would like to know:

Is the proof of the corollary in my notes correct?
Is the use of the orthogonality relations for characters justified?
Are there any other ways to prove the corollary?

Conclusion

In conclusion, I am having trouble understanding the proof of the corollary in question. I would like to discuss this corollary with the community and seek clarification. Specifically, I would like to know if the proof is correct, if the use of the orthogonality relations for characters is justified, and if there are any other ways to prove the corollary.

References

Serre, J.-P. (1977). Linear Representations of Finite Groups. Springer-Verlag.
Fulton, W., & Harris, J. (1991). Representation Theory: A First Course. Springer-Verlag.

Additional Information

Introduction

In our previous article, we discussed a corollary in the notes on the class function part of linear representations of finite groups. We expressed our confusion about the proof of this corollary and sought clarification from the community. In this article, we will provide a Q&A section to address some of the questions and concerns raised by our readers.

Q: Is the proof of the corollary in my notes correct?

A: Unfortunately, the proof of the corollary in your notes is not correct. The use of the orthogonality relations for characters is not justified, and the sum over g in the orthogonality relations is not well-defined.

Q: What is the correct proof of the corollary?

A: The correct proof of the corollary involves using the fact that the character of an irreducible representation is a class function, and then using the properties of class functions to derive the desired result. Specifically, we have

χ(g) = ∑_{i=1}^k χ(g)Tr(V_i(g))

where g ∈ G, V_i is an irreducible representation of G, and Tr(V_i(g)) is the trace of the matrix V_i(g).

Q: How do we justify the use of the orthogonality relations for characters?

A: To justify the use of the orthogonality relations for characters, we need to show that the sum over g in the orthogonality relations is well-defined. This involves using the fact that the character of an irreducible representation is a class function, and then using the properties of class functions to derive the desired result.

Q: What are some other ways to prove the corollary?

A: There are several other ways to prove the corollary. One approach involves using the fact that the character of an irreducible representation is a class function, and then using the properties of class functions to derive the desired result. Another approach involves using the fact that the character of an irreducible representation is a linear combination of the characters of the irreducible representations of the group, and then using the properties of linear combinations to derive the desired result.

Q: Can you provide more details about the properties of class functions?

A: Class functions are functions on the conjugacy classes of a group that are constant on each conjugacy class. In other words, they are functions that are invariant under conjugation. The properties of class functions include:

They are constant on each conjugacy class.
They are invariant under conjugation.
They can be used to construct representations of a group.

Q: Can you provide more details about the properties of irreducible representations?

A: Irreducible representations are representations of a group that cannot be decomposed into smaller representations. The properties of irreducible representations include:

They are representations of a group that cannot be decomposed into smaller representations.
They are linear combinations of the characters of the irreducible representations of the group.
They can be used to construct representations of a group.

Q: Can you provide more details about the properties of linear combinations?

A: Linear combinations are sums of the characters of the irreducible representations of a group. The properties of linear combinations include:

They are sums of the characters of the irreducible representations of a group.
They can be used to construct representations of a group.
They are linear combinations of the characters of the irreducible representations of the group.

Conclusion

In conclusion, we have provided a Q&A section to address some of the questions and concerns raised by our readers. We hope that this section has been helpful in clarifying the proof of the corollary in question. If you have any further questions or concerns, please do not hesitate to contact us.

References

Serre, J.-P. (1977). Linear Representations of Finite Groups. Springer-Verlag.
Fulton, W., & Harris, J. (1991). Representation Theory: A First Course. Springer-Verlag.

Additional Information

We would like to thank our readers for their questions and concerns. We hope that this Q&A section has been helpful in clarifying the proof of the corollary in question. If you have any further questions or concerns, please do not hesitate to contact us.