decomposition

[logistic_guy]

here is the question

Find a singular value decomposition of \(\displaystyle A = \begin{bmatrix}1 & 1 \\0 & 0 \\ 1 & 1 \end{bmatrix} \).


my attemb
after finding the eigenvalues, what should i do?

 

[fresh_42]

here is the question

Find a singular value decomposition of \(\displaystyle A = \begin{bmatrix}1 & 1 \\0 & 0 \\ 1 & 1 \end{bmatrix} \).


my attemb
after finding the eigenvalues, what should i do?

[imath]A[/imath] represents a linear transformation [imath] \mathbb{R}^2\stackrel{A}{\longrightarrow }\mathbb{R}^3. [/imath]
An eigenvalue of [imath] A [/imath] is a number [imath] \lambda [/imath] such that [imath] A\cdot v=\lambda \cdot v [/imath] for some vector [imath] v\neq 0. [/imath]

How can [imath] v [/imath] be two-dimensional and three-dimensional at the same time?

 

[fresh_42]

See https://math.berkeley.edu/~hutching/teach/54-2017/svd-notes.pdf

 

[logistic_guy]

[imath]A[/imath] represents a linear transformation [imath] \mathbb{R}^2\stackrel{A}{\longrightarrow }\mathbb{R}^3. [/imath]
An eigenvalue of [imath] A [/imath] is a number [imath] \lambda [/imath] such that [imath] A\cdot v=\lambda \cdot v [/imath] for some vector [imath] v\neq 0. [/imath]

this i understand

How can [imath] v [/imath] be two-dimensional and three-dimensional at the same time?

this i don't know

i'm not talk about eignvalue of \(\displaystyle A\) the eignvalue for \(\displaystyle A^{T}A\)

See https://math.berkeley.edu/~hutching/teach/54-2017/svd-notes.pdf

the pDF file say

\(\displaystyle A = U\Sigma V^{T}\)

\(\displaystyle U\) is \(\displaystyle m\times m\) matrix
\(\displaystyle V\) is \(\displaystyle n\times n\) matrix
\(\displaystyle \Sigma\) is \(\displaystyle m\times n\) matrix

Example 3.3 in pDF file use \(\displaystyle A = 2\times3\) but my \(\displaystyle A = 3\times 2\) i'm confused to follow it

i do my analize before i write the question in this website and i find the eignvalues of \(\displaystyle A^{T}A\)
\(\displaystyle \lambda_1 = 4\)
\(\displaystyle \lambda_2 = 0\)

what's the next step?

 

[fresh_42]

i'm not talk about eignvalue of \(\displaystyle A\) the eignvalue for \(\displaystyle A^{T}A\)

Yes, I know, but you didn't say so.

the pDF file say

\(\displaystyle A = U\Sigma V^{T}\)

\(\displaystyle U\) is \(\displaystyle m\times m\) matrix
\(\displaystyle V\) is \(\displaystyle n\times n\) matrix
\(\displaystyle \Sigma\) is \(\displaystyle m\times n\) matrix

Example 3.3 in pDF file use \(\displaystyle A = 2\times3\) but my \(\displaystyle A = 3\times 2\) i'm confused to follow it

i do my analize before i write the question in this website and i find the eignvalues of \(\displaystyle A^{T}A\)
\(\displaystyle \lambda_1 = 4\)
\(\displaystyle \lambda_2 = 0\)

what's the next step?

The single values are thus the non-negative square roots of the eigenvalues: [imath] \sigma_1=2\, , \,\sigma_2=0 [/imath] and [imath] r=1 [/imath] is the number of non-zero values.

We proceed by theorem 3.2 (in chapter 3: how to find the SVD). We first need the (two-dimensional) eigenvectors [imath] v_1,v_2 [/imath] to the eigenvalues [imath] \lambda_1=4 [/imath] and [imath] \lambda_2=0 [/imath] of [imath] A^TA. [/imath] Note that they should be orthonormal.

What are they?

If you found them, we proceed by theorem 3.2 and definition 2.1.

 

[logistic_guy]

Yes, I know, but you didn't say so.


The single values are thus the non-negative square roots of the eigenvalues: [imath] \sigma_1=2\, , \,\sigma_2=0 [/imath] and [imath] r=1 [/imath] is the number of non-zero values.

We proceed by theorem 3.2 (in chapter 3: how to find the SVD). We first need the (two-dimensional) eigenvectors [imath] v_1,v_2 [/imath] to the eigenvalues [imath] \lambda_1=4 [/imath] and [imath] \lambda_2=0 [/imath] of [imath] A^TA. [/imath] Note that they should be orthonormal.

What are they?

If you found them, we proceed by theorem 3.2 and definition 2.1.

thank

\(\displaystyle \begin{bmatrix}2 - \lambda_1 & 2 \\2 & 2-\lambda_1 \end{bmatrix} \bold{v_1} = \begin{bmatrix}2 - \lambda_1 & 2 \\2 & 2-\lambda_1 \end{bmatrix} \begin{bmatrix}a \\b \end{bmatrix} = \begin{bmatrix}2 - 4 & 2 \\2 & 2-4 \end{bmatrix} \begin{bmatrix}a \\b \end{bmatrix} = \begin{bmatrix}-2 & 2 \\2 & -2 \end{bmatrix} \begin{bmatrix}a \\b \end{bmatrix} = \begin{bmatrix}0 \\0 \end{bmatrix} \)

if i solve this system i get \(\displaystyle \bold{v_1} = \begin{bmatrix}1 \\1 \end{bmatrix}\)

\(\displaystyle \begin{bmatrix}2 - \lambda_2 & 2 \\2 & 2-\lambda_2 \end{bmatrix} \bold{v_2} = \begin{bmatrix}2 - \lambda_2 & 2 \\2 & 2-\lambda_2 \end{bmatrix} \begin{bmatrix}c \\d \end{bmatrix} = \begin{bmatrix}2 - 0 & 2 \\2 & 2-0 \end{bmatrix} \begin{bmatrix}c \\d \end{bmatrix} = \begin{bmatrix}2 & 2 \\2 & 2 \end{bmatrix} \begin{bmatrix}c \\d \end{bmatrix} = \begin{bmatrix}0 \\0 \end{bmatrix} \)

if i solve this system i get \(\displaystyle \bold{v_2} = \begin{bmatrix}1 \\-1 \end{bmatrix}\)

Note that they should be orthonormal.

you mean i divide each one by its magnitude?

\(\displaystyle |\bold{v_1}| = \sqrt{1^2 + 1^2} = \sqrt{1 + 1} = \sqrt{2}\)
\(\displaystyle |\bold{v_2}| = \sqrt{1^2 + -1^2} = \sqrt{1 + 1} = \sqrt{2}\)

the orthonomral vector

\(\displaystyle \bold{v_1} = \begin{bmatrix}\frac{1}{\sqrt{2}} \\\frac{1}{\sqrt{2}} \end{bmatrix}\)
\(\displaystyle \bold{v_2} = \begin{bmatrix}\frac{1}{\sqrt{2}} \\\frac{-1}{\sqrt{2}} \end{bmatrix}\)

is this correct?

 

[fresh_42]

I am once more confused by the fact that you re-norm the vectors but keep the notation. If I now write [imath] \mathbf{v_1} [/imath] how will you know whether I mean [imath] \mathbf{v_1}=(1,1)^T [/imath] or [imath] \mathbf{v_1}=\left(1/\sqrt{2},1/\sqrt{2}\right)^T \;?[/imath] This is not how math works, or written communication!

Let's see.

[math](A^TA)\mathbf{v_1}= \begin{pmatrix}2&2\\2&2\end{pmatrix} \begin{pmatrix}1/\sqrt{2}\\1/\sqrt{2}\end{pmatrix}=\begin{pmatrix}2/\sqrt{2}+2/\sqrt{2}\\2/\sqrt{2}+2/\sqrt{2}\end{pmatrix}=\begin{pmatrix}4/\sqrt{2}\\4/\sqrt{2}\end{pmatrix}=4\cdot \begin{pmatrix}1/\sqrt{2}\\1/\sqrt{2}\end{pmatrix}[/math]and the vector norm is [imath] 1. [/imath]
The equation [imath] (A^TA)\mathbf{v_2}=\begin{pmatrix}0\\0\end{pmatrix}=0\cdot \mathbf{v_2} [/imath] is also correct, [imath] \mathbf{v_2} [/imath] has vector norm [imath] 1, [/imath] too. Both vectors are also orthogonal since
[math] \begin{pmatrix}1/\sqrt{2}\, , \,1/\sqrt{2}\end{pmatrix}\cdot \begin{pmatrix}1/\sqrt{2}\\-1/\sqrt{2}\end{pmatrix}=0. [/math]
Yes, you are correct. I first thought you made a scaling mistake - that's why I checked it here - but your scaling is ok. Now you can use [imath] \sigma_1=2 [/imath] and definition 2.1 to build [imath] \Sigma, [/imath] and theorem 3.2 to build [imath] V [/imath] and [imath] U. [/imath] Note that [imath] r=1 [/imath] and you have some arbitrariness by determining the second column in [imath] U. [/imath] However, [imath] U [/imath] must be an orthogonal matrix, i.e. we need [imath] U^T\cdot U=I. [/imath]

 

[logistic_guy]

I am once more confused by the fact that you re-norm the vectors but keep the notation. If I now write [imath] \mathbf{v_1} [/imath] how will you know whether I mean [imath] \mathbf{v_1}=(1,1)^T [/imath] or [imath] \mathbf{v_1}=\left(1/\sqrt{2},1/\sqrt{2}\right)^T \;?[/imath] This is not how math works, or written communication!

you're right it's better to use different vector like \(\displaystyle \bold{s_1} = \begin{bmatrix}1 \\1 \end{bmatrix}\) and \(\displaystyle \bold{s_2} = \begin{bmatrix}1 \\-1 \end{bmatrix}\)

Yes, you are correct. I first thought you made a scaling mistake - that's why I checked it here - but your scaling is ok. Now you can use [imath] \sigma_1=2 [/imath] and definition 2.1 to build [imath] \Sigma, [/imath] and theorem 3.2 to build [imath] V [/imath] and [imath] U. [/imath] Note that [imath] r=1 [/imath] and you have some arbitrariness by determining the second column in [imath] U. [/imath] However, [imath] U [/imath] must be an orthogonal matrix, i.e. we need [imath] U^T\cdot U=I. [/imath]

the pDF file you send say \(\displaystyle V = n\times n\) matrix but it don't say how many component it hold
is it correct to assume only two?
if \(\displaystyle \bold{v_1}\) is the first component and \(\displaystyle \bold{v_2}\) is the second component
it mean we already find \(\displaystyle V\)

\(\displaystyle V = \begin{bmatrix}\frac{1}{\sqrt{2}} & \frac{1}{\sqrt{2}} \\\frac{1}{\sqrt{2}} & -\frac{1}{\sqrt{2}} \end{bmatrix}\)

i'm wrong?

 

[fresh_42]

you're right it's better to use different vector like \(\displaystyle \bold{s_1} = \begin{bmatrix}1 \\1 \end{bmatrix}\) and \(\displaystyle \bold{s_2} = \begin{bmatrix}1 \\-1 \end{bmatrix}\)


the pDF file you send say \(\displaystyle V = n\times n\) matrix but it don't say how many component it hold
is it correct to assume only two?
if \(\displaystyle \bold{v_1}\) is the first component and \(\displaystyle \bold{v_2}\) is the second component
it mean we already find \(\displaystyle V\)

\(\displaystyle V = \begin{bmatrix}\frac{1}{\sqrt{2}} & \frac{1}{\sqrt{2}} \\\frac{1}{\sqrt{2}} & -\frac{1}{\sqrt{2}} \end{bmatrix}\)

Looks ok. Let me think out loud while checking how the matrices are built. We start with [imath] A [/imath] which is a matrix with 3 rows and 2 columns, so [imath] m=3 [/imath] and [imath] n=2 [/imath] according to the paper that says that [imath] A [/imath] is a [imath] m\times n [/imath] matrix. This is the notation I am used to: first the rows and then the columns. Let's see how far we get by this convention.

[imath]\Sigma [/imath] is the easiest one to start with. It is of the same shape as [imath] A, [/imath] has [imath] \sigma_1=2 [/imath] on its first diagonal entry and zeros elsewhere, i.e.
[math] \Sigma=\begin{pmatrix}2&0\\0&0\\0&0\end{pmatrix} .[/math]Then theorem 3.2 says that [imath] V [/imath] consists of the two orthonormal eigenvectors of [imath] A^TA [/imath] which is the one you have, too. Let me write it as
[math] V= \dfrac{1}{\sqrt{2}}\begin{pmatrix}1&1\\1&-1\end{pmatrix}[/math]instead. The theorem says further that the first column of [imath] U [/imath] is
[math] \mathbf{u_1}:=\sigma_1^{-1}A \mathbf{v_1}=\dfrac{1}{2\sqrt{2}}\begin{pmatrix}1&1\\0&0\\1&1\end{pmatrix}\cdot \begin{pmatrix}1\\1\end{pmatrix}=\dfrac{1}{2\sqrt{2}}\begin{pmatrix}2\\0\\2\end{pmatrix}=\dfrac{1}{\sqrt{2}}\begin{pmatrix}1\\0\\1\end{pmatrix}=\begin{pmatrix}1/\sqrt{2}\\0\\1/\sqrt{2}\end{pmatrix}.[/math]Now we need to find vectors [imath] \mathbf{u_2}\, , \,\mathbf{u_3} [/imath] with [imath] 3 [/imath] components that are orthonormal two [imath] \mathbf{u_1}[/imath] and each other.

i'm wrong?

No. You are right. Now find the rest of [imath] U [/imath] and finally check [imath] U\Sigma V^T\stackrel{?}{=}A. [/imath]

 

[logistic_guy]

Looks ok. Let me think out loud while checking how the matrices are built. We start with [imath] A [/imath] which is a matrix with 3 rows and 2 columns, so [imath] m=3 [/imath] and [imath] n=2 [/imath] according to the paper that says that [imath] A [/imath] is a [imath] m\times n [/imath] matrix. This is the notation I am used to: first the rows and then the columns. Let's see how far we get by this convention.

i use this notation too

No. You are right. Now find the rest of [imath] U [/imath] and finally check [imath] U\Sigma V^T\stackrel{?}{=}A. [/imath]

thank

Now we need to find vectors [imath] \mathbf{u_2}\, , \,\mathbf{u_3} [/imath] with [imath] 3 [/imath] components that are orthonormal two [imath] \mathbf{u_1}[/imath] and each other.

i think that's the hard partthe idea simple but process take a lot of calculation
my think the dot product betwwen two vector is orthonomral if it's zero

\(\displaystyle \bold{u_1}\cdot \bold{u_2} = 0\)

\(\displaystyle \frac{1}{\sqrt{2}}\begin{bmatrix}1 \\0 \\1 \end{bmatrix}\cdot \begin{bmatrix}a \\b \\c \end{bmatrix} = 0\)

\(\displaystyle \frac{1}{\sqrt{2}}\times a + 0\times b + \frac{1}{\sqrt{2}} \times c = 0\)

\(\displaystyle \frac{1}{\sqrt{2}}(a + c) = 0\)

\(\displaystyle a + c = 0\)

\(\displaystyle a = -c\)

\(\displaystyle \bold{u_2} = \begin{bmatrix}1 \\b \\-1 \end{bmatrix}\)

the system don't give the value of \(\displaystyle b\)

 

[fresh_42]

You can use the same trick as before. You had [imath] \mathbf{v_1}\perp \mathbf{v_2} [/imath] with (up to a factor) [imath] \mathbf{v_1}=(1,1) [/imath] and [imath] \mathbf{v_2}=(1,-1). [/imath] This situation remains if we add a zero in the middle:
[math]\dfrac{1}{\sqrt{2}} \begin{pmatrix}1\\0\\1 \end{pmatrix}=\mathbf{u_1} \perp \mathbf{u}_2=\dfrac{1}{\sqrt{2}}\begin{pmatrix}1\\0\\-1\end{pmatrix}.[/math]
But, yes, you can choose any value for [imath] b [/imath]. But that would make things unnecessarily complicated. [imath] b=0 [/imath] is fine. Especially, as it allows us to write down [imath] \mathbf{u_3} [/imath] immediately. With [imath] b=0, [/imath] the two vectors [imath] \mathbf{u_1},\mathbf{u_2} [/imath] span a two-dimensional plane in [imath] \mathbb{R}^3 [/imath] with no disturbances from the coordinate in the middle, since both are zero. This means that the plane they span is the [imath] (x,z) [/imath]-plane! So the [imath] y [/imath]-coordinate spans the missing dimension that is automatically perpendicular to the [imath] (x,z) [/imath]-plane, i.e. we get [imath] \mathbf{u_3}=\begin{pmatrix}0\\1\\0\end{pmatrix} [/imath] as the third vector for free.

 

[logistic_guy]

thank fresh_42 very much

you explained it nicely

But, yes, you can choose any value for [imath] b [/imath]. But that would make things unnecessarily complicated. [imath] b=0 [/imath] is fine.

this for me first time i know. it'll help me in next questions

This means that the plane they span is the [imath] (x,z) [/imath]-plane! So the [imath] y [/imath]-coordinate spans the missing dimension that is automatically perpendicular to the [imath] (x,z) [/imath]-plane, i.e. we get [imath] \mathbf{u_3}=\begin{pmatrix}0\\1\\0\end{pmatrix} [/imath] as the third vector for free.

i like this trick
if i don't see trick i just take the cross product of \(\displaystyle \bold{u_1} \times \bold{u_2}\), right?
i do cross product before it take a lot of calculation and i don't want it this time

so the solution

\(\displaystyle V = U\Sigma V^{T} = \begin{bmatrix}\frac{1}{\sqrt{2}} & 0 & \frac{1}{\sqrt{2}} \\0 & 1 & 0 \\ \frac{1}{\sqrt{2}} & 0 & -\frac{1}{\sqrt{2}} \end{bmatrix}\begin{bmatrix}2 & 0 \\0 & 0 \\ 0 & 0\end{bmatrix}\begin{bmatrix}\frac{1}{\sqrt{2}} & \frac{1}{\sqrt{2}} \\ \frac{1}{\sqrt{2}} & -\frac{1}{\sqrt{2}} \end{bmatrix}\)

if \(\displaystyle \sigma_1 = 2\) and \(\displaystyle \sigma_2 = 1\), \(\displaystyle \Sigma = \begin{bmatrix}2 & 0 \\0 & 1 \\ 0 & 0\end{bmatrix}\), right?

i see the sigma matrix have only \(\displaystyle \sigma_1\) and \(\displaystyle \sigma_2\) why it's necessary to add the last row? i mean do we must make the \(\displaystyle \Sigma\) matrix \(\displaystyle 3 \times 2\) to make it match matrix \(\displaystyle A\) or there's another reason?

i also see the matrix \(\displaystyle V = V^{T}\) i know it's symmetry. do it mean anything else in this case?

 

[fresh_42]

There is no [imath] \sigma_2 [/imath] since the sigmas that constitute [imath] \Sigma [/imath] are only the [imath] \sigma_i [/imath] with [imath] i\leq r [/imath] and [imath] r, [/imath] which counts the non-zero eigenvalues is [imath] r=1 [/imath] because
[imath] \lambda_2=0 [/imath] and only [imath] \lambda_1\neq 0. [/imath] This means that [imath] \Sigma=\begin{pmatrix}2&0\\0&0\\0&0\end{pmatrix} .[/imath]


In case your question was: "Given another example with [imath] \sigma_1=2 [/imath] and [imath] \sigma_2=1 [/imath] what would we get for [imath] \Sigma \,?[/imath]" then you are right. The paper orders the sigmas from large to small, so [imath] \sigma_1=2>\sigma_2=1>0 [/imath] and [imath] \Sigma= \begin{pmatrix}2&0\\0&1\\0&0\end{pmatrix}.[/imath] Yes. If that was behind that little word "if", then you are right.


I think it doesn't matter since theorem 3.2 only requires that
[math] \mathbf{u_1}=\sigma_1^{-1}A\mathbf{v_1}=\begin{pmatrix} 1/\sqrt{2}\\0\\1/\sqrt{2}\end{pmatrix} [/math] and the rest of the matrix [imath] U [/imath] is just any orthonormal complement. I have written [imath] U [/imath] with switched columns, (extended) [imath] \mathbf{v_2} [/imath] second and [imath] \mathbf{u_3}=(0,1,0)^T [/imath] last.


Hint: calculations will be a little easier if you write [imath] U=\dfrac{1}{\sqrt{2}}\begin{pmatrix}1&0&1\\0&\sqrt{2}&0\\1&0&-1\end{pmatrix} [/imath] and [imath] V=\dfrac{1}{\sqrt{2}}\begin{pmatrix}1&1\\1&-1\end{pmatrix}. [/imath]

By the way: Thanks for the question. I learned SVD now and the paper is nice to have since it is written as a recipe.

 

[Harry_the_cat]

attempt not attemb

 

[logistic_guy]

By the way: Thanks for the question. I learned SVD now and the paper is nice to have since it is written as a recipe.

you mean you don't solve this quesiton beforethen you're very clever teacher

The paper orders the sigmas from large to small, so [imath] \sigma_1=2>\sigma_2=1>0 [/imath] and [imath] \Sigma= \begin{pmatrix}2&0\\0&1\\0&0\end{pmatrix}.[/imath] Yes. If that was behind that little word "if", then you are right.

you mean the order of sigma is important, large value come first
if \(\displaystyle \sigma_1 = 2\) and \(\displaystyle \sigma_2 = 5\)

\(\displaystyle \Sigma = \begin{bmatrix}5 & 0\\0 & 2\\0 & 0\end{bmatrix}\), right?

if both \(\displaystyle \sigma_1 \neq 0\) and \(\displaystyle \sigma_2 \neq 0\) which one to chose to work with formula?
\(\displaystyle \sigma_i^{-1}A\mathbf{v_1}\)

Hint: calculations will be a little easier if you write [imath] U=\dfrac{1}{\sqrt{2}}\begin{pmatrix}1&0&1\\0&\sqrt{2}&0\\1&0&-1\end{pmatrix} [/imath] and [imath] V=\dfrac{1}{\sqrt{2}}\begin{pmatrix}1&1\\1&-1\end{pmatrix}. [/imath]

i think i like this notation. i'll adabt in future questions

attempt not attemb

thank
english isn't my language

 

[Harry_the_cat]

thank
english isn't my language

Yes I thought so, just trying to help

 

[fresh_42]

you mean you don't solve this quesiton beforethen you're very clever teacher

I may have seen it before but I didn't remember. The paper deserves merit. It was my second hit on Google. The first one was from MIT but was less convincing.

I remember a dialogue with my professor at university when I saw the announcement of his courses for the upcoming semester. I commented: "I didn't know you are familiar with <some topic I can't remember which>." He replied: "I am not. I want to learn it. That's why I'm holding the lecture!" Being forced to explain something means being prepared for some unusual questions means having to learn it deeper than just "reading it".

you mean the order of sigma is important, large value come first
if \(\displaystyle \sigma_1 = 2\) and \(\displaystyle \sigma_2 = 5\)

\(\displaystyle \Sigma = \begin{bmatrix}5 & 0\\0 & 2\\0 & 0\end{bmatrix}\), right?

Yes, that's correct. I think the order is only used to avoid discrepancies with the ordering of the columns of the three matrices [imath] U,\Sigma,V. [/imath] Their columns have to match the corresponding values [imath] \sigma_k [/imath] so that the equations will be correct. Ordering the [imath] \sigma_k [/imath] by size makes it clearer and avoids confusion.

if both \(\displaystyle \sigma_1 \neq 0\) and \(\displaystyle \sigma_2 \neq 0\) which one to chose to work with formula?
\(\displaystyle \sigma_i^{-1}A\mathbf{v_1}\)

Your matrix in the example is correct. The paper says [imath] \sigma_1\ge \sigma_2\ge \ldots\ge \sigma_n\ge 0 [/imath] and establishes thus an order for column 1 to column n this way. I would bet that the reverse order would work as well as long as we use the reverse order of column vectors in the matrices, too. But "filling up the rest" is much clearer than starting with the arbitrariness in this rest and administrating the size of this rest only to begin with it. Remember that we had [imath] \mathbf{u_1},\ldots,\mathbf{u_r} [/imath] and any orthonormal complement for [imath] \mathbf{u_{r+1}},\ldots,\mathbf{u_n}. [/imath] Now imagine writing: Start with an arbitrary orthonormal complement [imath] \mathbf{u_1},\ldots,\mathbf{u_{n-r}} [/imath] of [imath] \mathbf{u_{n-r+1}},\ldots,\mathbf{u_n} [/imath], etc. That would probably work, too, but would be much more inconvenient.

 

[logistic_guy]

Yes I thought so, just trying to help

i do appreciate your help

I may have seen it before but I didn't remember. The paper deserves merit. It was my second hit on Google. The first one was from MIT but was less convincing.

I remember a dialogue with my professor at university when I saw the announcement of his courses for the upcoming semester. I commented: "I didn't know you are familiar with <some topic I can't remember which>." He replied: "I am not. I want to learn it. That's why I'm holding the lecture!" Being forced to explain something means being prepared for some unusual questions means having to learn it deeper than just "reading it".

nice story i'll write it down

Yes, that's correct. I think the order is only used to avoid discrepancies with the ordering of the columns of the three matrices [imath] U,\Sigma,V. [/imath] Their columns have to match the corresponding values [imath] \sigma_k [/imath] so that the equations will be correct. Ordering the [imath] \sigma_k [/imath] by size makes it clearer and avoids confusion.

thank

Your matrix in the example is correct. The paper says [imath] \sigma_1\ge \sigma_2\ge \ldots\ge \sigma_n\ge 0 [/imath] and establishes thus an order for column 1 to column n this way. I would bet that the reverse order would work as well as long as we use the reverse order of column vectors in the matrices, too. But "filling up the rest" is much clearer than starting with the arbitrariness in this rest and administrating the size of this rest only to begin with it. Remember that we had [imath] \mathbf{u_1},\ldots,\mathbf{u_r} [/imath] and any orthonormal complement for [imath] \mathbf{u_{r+1}},\ldots,\mathbf{u_n}. [/imath] Now imagine writing: Start with an arbitrary orthonormal complement [imath] \mathbf{u_1},\ldots,\mathbf{u_{n-r}} [/imath] of [imath] \mathbf{u_{n-r+1}},\ldots,\mathbf{u_n} [/imath], etc. That would probably work, too, but would be much more inconvenient.

thank fresh_42 very much

i appreciate your nice explanation very much