Is the Adjoint of a Unitary Matrix Still Unitary?

In summary, the conversation discusses the proof that the adjoint of the product of two matrices is equal to the product of their adjoints. The proof works for both real and imaginary elements, as taking the complex conjugate does not change the indices. The definition of the adjoint is also mentioned as a simpler way to understand the proof.
  • #1
Kolahal Bhattacharya
135
1

Homework Statement



I am to prove that adjoint of(AB)= adjoint of B times adjoint of A

Homework Equations





The Attempt at a Solution



I satrted this as U=adjoint of AB
u_ik=sum(j)[(a_ij)*(b_jk)]
I know then,I may take tarnspose of both sides so that we have:[(u_ki)~]=sum(j){[(b_kj)~][(a_ji)~]}
then [U~]=[B~][A~]
Then,we are done.But,this proves for real elements.I am not sure that this proves also for imaginary elements...
Please help.
 
Physics news on Phys.org
  • #2
Kolahal Bhattacharya said:

Homework Statement



I am to prove that adjoint of(AB)= adjoint of B times adjoint of A

Homework Equations





The Attempt at a Solution



I satrted this as U=adjoint of AB
u_ik=sum(j)[(a_ij)*(b_jk)]
I know then,I may take tarnspose of both sides so that we have:[(u_ki)~]=sum(j){[(b_kj)~][(a_ji)~]}
then [U~]=[B~][A~]
Then,we are done.But,this proves for real elements.I am not sure that this proves also for imaginary elements...
Please help.

The proof works exactlythe same way, you just have to include a complex conjugate of the elements when you take the adjoint. Taking the complex conjugate does not change anything to the indices, so the proof still works...you just have complex conjugates everywhere.
 
  • #3
OK,I thought of this possibility as I am not using any extra property of complex matrices.
Do I need to write (a_ij)* in those cases?
 
  • #4
I think you guys are making this too complicated. By definition:

<x,Ay>=<adjoint(A)x,y> for all x,y.

So:

<x,ABy>=<x,A(By)>=<adjoint(A)x,By>=<adjoint(B)*adjoint(A)x,y> for all x,y.
 
Last edited:

1. What is a unitary matrix?

A unitary matrix is a square matrix with complex entries whose conjugate transpose is equal to its inverse. In other words, multiplying a unitary matrix by its conjugate transpose results in the identity matrix.

2. What is the significance of unitary matrices in science?

Unitary matrices are important in many areas of science, particularly in quantum mechanics. They are used to represent unitary transformations, which preserve the inner product of vectors. This property is essential in studying quantum systems and understanding the behavior of particles.

3. How is the unitary matrix problem solved?

The unitary matrix problem can be solved using a variety of methods, such as the Gram-Schmidt process, QR decomposition, and singular value decomposition. These methods involve finding an orthonormal basis for the matrix and transforming it into a unitary matrix.

4. Can all matrices be transformed into unitary matrices?

No, not all matrices can be transformed into unitary matrices. Only square matrices with complex entries can be transformed into unitary matrices, and even then, not all of them can be transformed. The transformation depends on the properties and structure of the original matrix.

5. What applications do unitary matrices have in data analysis?

Unitary matrices are commonly used in data analysis for tasks such as data compression, signal processing, and image processing. They can also be used for dimensionality reduction, where a high-dimensional dataset is transformed into a lower-dimensional space while preserving important information.

Similar threads

  • Advanced Physics Homework Help
Replies
15
Views
1K
Replies
27
Views
935
  • Linear and Abstract Algebra
Replies
7
Views
1K
  • Linear and Abstract Algebra
Replies
4
Views
1K
Replies
1
Views
962
  • Linear and Abstract Algebra
Replies
3
Views
1K
  • Differential Geometry
Replies
3
Views
2K
  • Introductory Physics Homework Help
Replies
8
Views
1K
  • Linear and Abstract Algebra
Replies
2
Views
1K
  • Calculus and Beyond Homework Help
Replies
12
Views
2K
Back
Top