Effect of orthonormal projection on rank

In summary, the conversation discusses the relationship between rank(R) and rank(A) when given a QR factorization A = QR. It is determined that the rank of A is equal to the rank of R, as well as the dimension of Q(S) when Q is full rank. The reason for this is because Q is orthnormal and one-to-one, causing no overlap in the image and reducing the rank. Therefore, the rank of QR and R are equal.
  • #1
kalleC
11
0

Homework Statement


Given rank(R) and a QR factorization A = QR, what is the rank(A)


Homework Equations





The Attempt at a Solution


I want to know if multiplication by a full rank orthonormal matrix Q and an upper trapezoidal matrix R yields rank(R)=rank(Q*R)=rank(A)

This is mostly guesswork by me but I'd like to use it for a question I need to answer.
 
Physics news on Phys.org
  • #2
Well, the rank of a matrix is the dimension of the image, right? If the image of R is a subspace S of dimension rank(R), then what's the dimension of Q(S) if Q is full rank?
 
  • #3
They are equal? As the only way Q(S) would be dissimilar would be if rank(Q)<rank(R).

But does not the reason for this have anything to do with Q being orthnormal? Otherwise couldn't Q act on R and cause some of the image to overlap effectively reducing the rank?
 
  • #4
Q is full rank, so it's one to one. So yes, rank(QR)=dim(Q(S))=dim(S)=rank(R). So rank(QR)=rank(R).
 
  • #5
Thank you very much =)
 

1. What is an orthonormal projection?

An orthonormal projection is a mathematical operation that involves projecting a vector onto a subspace in a way that preserves the length and angle of the vector. In simpler terms, it is a way to find the closest vector in a subspace that approximates a given vector.

2. How does orthonormal projection affect the rank of a matrix?

The orthonormal projection of a matrix does not change its rank. This is because the projection does not alter the linearly independent vectors in the original matrix, which are the basis for the rank.

3. Can orthonormal projection increase the rank of a matrix?

No, orthonormal projection cannot increase the rank of a matrix. The rank of a matrix is determined by the number of linearly independent columns or rows, which remains unchanged after projection.

4. What is the relationship between orthonormal projection and matrix multiplication?

Orthogonal projection can be represented as a matrix multiplication of the original matrix and the projection matrix. The projection matrix is constructed using the orthogonal basis vectors of the subspace onto which the original matrix is being projected.

5. Are there any practical applications of orthonormal projection in science?

Yes, orthonormal projection has various applications in science, such as data compression, signal processing, and image reconstruction. It is also commonly used in machine learning and data analysis to reduce the dimensionality of data while preserving important features.

Similar threads

  • Calculus and Beyond Homework Help
Replies
5
Views
1K
  • Calculus and Beyond Homework Help
Replies
2
Views
2K
  • Calculus and Beyond Homework Help
Replies
4
Views
2K
  • Calculus and Beyond Homework Help
Replies
7
Views
1K
  • Calculus and Beyond Homework Help
Replies
4
Views
1K
  • Calculus and Beyond Homework Help
Replies
3
Views
2K
  • Calculus and Beyond Homework Help
Replies
3
Views
1K
  • Calculus and Beyond Homework Help
Replies
4
Views
3K
  • Calculus and Beyond Homework Help
Replies
7
Views
3K
  • Calculus and Beyond Homework Help
Replies
3
Views
1K
Back
Top