Show Hermitian Identity: (AB)^+ = A^+ B^+

In summary, to show that (AB)^+ = A^+ B^+ using index notation and the fact that + is the Hermitian transpose, we can use the distributive property of complex conjugates and the definition that (xy)^* = (x^*)(y^*). However, it is important to note that (AB)_ij is not equal to A_ij B_ij, as AB is a matrix product.
  • #1
evlyn
15
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Homework Statement



Show that (AB)^+ = A^+ B^+ using index notation


Homework Equations



+ is the Hermitian transpose


The Attempt at a Solution



I know that AB = Ʃa_ik b_kj summed over k

so (AB)^+ = (Ʃa_ik b_kj)^+ = Ʃ (a_ik b_kj)^+ = Ʃ (a_ik)^+(b_kj)^+ = A^+ B^+

I am not really sure if this makes sense, I don't know if it is acceptable to distribute the transpose within the sum.
 
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  • #2
(AB)^+ is equal to (B^+)(A^+), not (A^+)(B^+). That may be a sign something is going wrong. (A^+)_ij=(A_ji)*, where * is complex conjugate. Start from there.
 
  • #3
That helped.

So now using the definition (usually a good thing) I have:

(AB)^+_ij = [AB_ji]^* = A_ji ^* B_ji^* = B_ij^+ A_ij^+ = B^+ A^+

I know that the complex conjugate is distributive can I just assume that for the proof?
 
  • #4
evlyn said:
That helped.

So now using the definition (usually a good thing) I have:

(AB)^+_ij = [AB_ji]^* = A_ji ^* B_ji^* = B_ij^+ A_ij^+ = B^+ A^+

I know that the complex conjugate is distributive can I just assume that for the proof?

Yes, you can use (xy)^*=(x^*)(y^*). That's fine. But now you've lost the matrix product part. AB is a product. (AB)_ij isn't equal to A_ij B_ij.
 

FAQ: Show Hermitian Identity: (AB)^+ = A^+ B^+

What is the Hermitian identity for matrix multiplication?

The Hermitian identity for matrix multiplication states that the conjugate transpose of the product of two matrices is equal to the product of their individual conjugate transposes in reverse order.

How is the Hermitian identity used in matrix algebra?

The Hermitian identity is used to simplify complex matrix calculations, particularly in the context of linear algebra. It allows for the manipulation of matrix equations by taking advantage of properties such as commutativity and associativity.

What is the difference between a Hermitian matrix and a non-Hermitian matrix?

A Hermitian matrix is a square matrix that is equal to its own conjugate transpose. This means that the matrix is symmetric across its diagonal, with real values on the diagonal and complex conjugate values on the off-diagonal. A non-Hermitian matrix does not have this property and can have any arrangement of real and complex values.

Can the Hermitian identity be extended to more than two matrices?

Yes, the Hermitian identity can be extended to any number of matrices. In this case, the conjugate transpose of the product of the matrices will be equal to the product of their individual conjugate transposes in reverse order.

How is the Hermitian identity related to the concept of adjoint matrices?

The Hermitian identity is closely related to the concept of adjoint matrices. In fact, the Hermitian identity can be seen as a special case of the more general adjoint identity, which states that the adjoint of the product of two matrices is equal to the product of their individual adjoints in reverse order.

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