# Singular values by QR decomposition

I have a hypothesis on some relationship between matrices, but no proof for it. I have a feeling this should exist already, could anyone point me in the right direction?

If I have a square matrix $A$ (let's say full rank, but otherwise arbitrary) and calculate its QR decomposition as $A = QR$ so that the diagonal of R is positive (so R is unique), my hypothesis is that the diagonal of $R$, which gives its eigenvalues, also gives the singular values of $A$. In other words, $diag(R) = eig(R) = \sqrt{eig(R^T R)} = sing(A)$. This seems to imply that $eig(R)^2 = eig(R^2) = eig(R^T R)$.

This seems to be true in practice, but from what properties does this follow? I tested this by generating lots of matrices w. random values between -1 and 1 and calculating these numbers using this program:

import numpy as np
from utils import qr_pos

def is_diagonal(A):
return np.count_nonzero(A - np.diag(np.diagonal(A))) == 0

def test_eig(n=30, tol=0.00001):
M = np.random.rand(N, N) # Random matrix
M = 2*M - 1              # Entries from -1 to 1
Q, R = qr_pos(M)         # QR dec. so that diag(R) is positive
if is_diagonal(R):
print 'R is diagonal!'
RTR = np.transpose(R) * R
sinvals = np.sqrt(np.linalg.eigvals(RTR))
diff = np.linalg.norm(sinvals - np.diag(R))

# print 'Singular values:', sinvals
# print 'Diagonal of R:', np.diag(R)
# print 'Difference:', diff
return diff < tol, M

success = True
N = 300
for i in range(N):
equal, M = test_eig()
print 'i',
if not equal:
print
print 'Case found where the hypothesis does not hold:', M
success = False
break
if success:
print
print 'Hypothesis holds for', N, 'random test cases.'


Which outputs:

i i i i ... i i i
Hypothesis holds for 300 random test cases.

• The QR decomposition does not by itself give either the singular values or the eigenvalues of a matrix. QR decomposition may well be used in an iterative procedure to find singular values or eigenvalues. – hardmath Jan 23 '18 at 16:58
• The eigenvalues of $R$ are not usually the singular values of $A$ (in fact they can't be unless $R$ is diagonal). However, both $R$ and $A$ have the same singular values. – Omnomnomnom Jan 23 '18 at 17:10
• @Omnomnomnom I added a program I used to test my hypothesis. It seems to be the case that the eigenvalues of $R$ are the singular values of $A$ in many cases, and in none of the cases is $R$ diagonal. How can this be the case? – nardi Jan 23 '18 at 22:05
• Note that unless $R$ is diagonal, we cannot say $R^2 = R^T R$. Would it suffice to give an example where nonsingular $R^2$ and $R^T R$ have different eigenvalues? – hardmath Jan 23 '18 at 23:32
• @nardi not sure what's wrong with your code, but I've put a specific example down into an answer – Omnomnomnom Jan 23 '18 at 23:56

## 2 Answers

Here's a quick example:

$$M = \pmatrix{1&2\\1&1} \implies\\ Q = \frac 1{\sqrt{2}}\pmatrix{1 & 1\\1 & -1}, \quad R = \frac 1{\sqrt{2}} \pmatrix{2 & 3\\ 0 & 1}$$ The singular values of $M$ (and of $R$) are $\sqrt{\frac{7 \pm 3\sqrt{5}}{2}}$, which doesn't match the diagonal entries of $R$.

• You're right, this is not true. I made an error in that I used * instead of np.dot, so I was not getting the proper matrix product. – nardi Jan 24 '18 at 16:26

Accepted answer is nice, this is just another way of seeing this.

Hypothesis : Assume the diagonal of $$R$$ contains the singular values.

If you use the Gram-Schmidt process to compute the $$A = QR$$, you have $$r_{1,1} = \|a_1\|$$ with $$a_1$$ the first column of $$A$$, meaning that $$\|a_1\|$$ is a singular value of $$A$$, which is not always true.

Moreover using every permutation of $$A$$ and of its transpose gives that the norm of each columns and rows of $$A$$ are singular values of $$A$$, which exceed the number of possible singular values.

So the hypothesis is wrong.

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