Proving eigenvalues = 1 or -1 when A = A transpose = A inverse A is circulant

In summary, when A is a circulant matrix satisfying the conditions A=A^T=A^-1, all of its eigenvalues must equal either 1 or -1. This can be proven by considering the relation detA*detB=detAB and the fact that detA is the product of the eigenvalues.
  • #1
stihl29
25
0

Homework Statement


Prove all eigenvalues = 1 or -1 when A is circulant and satisfying
A=A^T=A^-1
I can think of an example, the identity matrix, but i can't think of a general case or how to set up a general case.

Homework Equations





The Attempt at a Solution


I can only show by example for identity matrix
 
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  • #2
remember that detA=detA^T=[detA^-1]^-1?
 
  • #3
I don't remember ever learning that, sorry for being clueless, but i don't see the relation.

OH, maybe since det A is the product of eigenvalues, and because 1 or -1 is the only number ^-1 that stays the same ?
is that right?
 
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  • #4
You can justify that by eigenvalues, but the more straightforward method is, to consider the relation detA*detB=detAB, so detA*det(A^-1)=detI, I think now you can see where it's going.
 
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Related to Proving eigenvalues = 1 or -1 when A = A transpose = A inverse A is circulant

1. How do you prove that eigenvalues of a matrix are equal to 1 or -1?

To prove that eigenvalues of a matrix are equal to 1 or -1, we need to first show that the matrix is symmetric, meaning that it is equal to its transpose. Then, we need to show that the matrix is also equal to its inverse. Finally, we need to use the property of circulant matrices, which state that the eigenvalues of a circulant matrix are the same as its first row. By setting the first row of the matrix to be all ones, we can easily see that the eigenvalues must be either 1 or -1.

2. What is a circulant matrix?

A circulant matrix is a square matrix in which each row is a cyclic permutation of the row above it. This means that each row is shifted one position to the right compared to the row above it. Circulant matrices have many interesting properties, including easy calculation of eigenvalues and eigenvectors.

3. Why is it important to prove that A = A transpose = A inverse A is circulant in order to show that eigenvalues are 1 or -1?

Proving that a matrix is circulant allows us to easily calculate its eigenvalues. This is because the eigenvalues of a circulant matrix are equal to its first row, and by setting the first row to be all ones, we can immediately see that the eigenvalues must be either 1 or -1. This makes it much simpler to prove that the eigenvalues are equal to 1 or -1 when A = A transpose = A inverse A.

4. Can a matrix have eigenvalues other than 1 or -1 when A = A transpose = A inverse A is circulant?

No, a matrix that satisfies the condition A = A transpose = A inverse A is circulant can only have eigenvalues of 1 or -1. This is because the property of circulant matrices states that the eigenvalues are equal to the first row of the matrix, which we can easily see must be all ones in this case. Therefore, there are no other possible eigenvalues.

5. Are there any real world applications of proving eigenvalues = 1 or -1 when A = A transpose = A inverse A is circulant?

Yes, there are many real world applications of circulant matrices and their properties. One example is in signal processing, where circulant matrices are used to efficiently perform convolution operations. Another example is in quantum mechanics, where circulant matrices are used to represent cyclic permutations in quantum systems. Additionally, circulant matrices have applications in image processing, numerical analysis, and more.

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