Talk:Moore–Penrose inverse

Forced math notation / images

This article is using math image forcing in almost all places. What I mean is that e.g. $AA^{+}A=A\,\!$ is used instead of $AA^{+}A=A$; the former forces the use of png images for math by the \,\! construction.

I do not like this for several reasons:

1. It is ugly (note how png images do a poor job respecting the baseline).
2. It is screen-reader unfriendly.
3. AFAIK the wiki software has some heuristics which allows it to format math the proper way depending on the browser used for viewing, or on user preferences, which is circumvented by using \,\!.

I'll happily delete all occurrences of \,\! in this article, but I thought I'd maybe ask first :) --- Papppfaffe (talk) 20:52, 27 April 2011 (UTC)

I agree. — MFH:Talk 14:56, 12 February 2012 (UTC)

I have removed all uses of \,\!. In my browser, this made no visible difference in the entire article, which is good. I have also removed extraneous uses of \,, which in three cases made no difference and in two cases it looks better to me.  Done --RainerBlome (talk) 10:39, 19 December 2019 (UTC)

Reference to article with NPOV issues

This is a page on a well-established topic and is generally well-sourced. However, one sentence in one section refers to an article under clear violation of NPOV. This sentence also uses language that reflects a general lack of neutrality. I suggest removal of this sentence. -V madhu (talk) 17:06, 26 September 2018 (UTC)

Formatting the Definition

@RainerBlome: With 3 browsers (IE, FireFox, Opera) the version https://en.wikipedia.org/w/index.php?title=Moore%E2%80%93Penrose_inverse&oldid=931017645 looks best. So far, vertical-align is applied to 3 cells in the first row, because the 4th cell is the widest of all cells. top is more conventional as bottom. Your last version does not look well when the window is narrowed. Best regards, --Nomen4Omen (talk) 17:19, 16 December 2019 (UTC)

@Nomen4Omen:, thanks for testing with other browsers! You are right, with very narrow windows the latest version did not work well. I hope I fixed it now, not just for Firefox. Could you please check with IE and Opera, as I don't have those?

I wrote in the edit comment: "top" was wrong anyway. That was mostly wrong itself. I thought that the intent of the `vertical-align` was to get the baselines aligned, but that was wrong. When text starts to wrap due to a narrow window, it should float at the top of the table cells, that's the purpose, as I see now. `vertical-align:top` does exactly that, so `top` it is now. Sorry for my ignorance. :-)

To align the baseline of the lone A in the first row, I first tried to append `^\phantom{+}, but MediaWiki's LaTeX processor did not understand this. I have also tried to use `^\quad` for that purpose, but it does not affect alignment (no surprise there). I have now added an transparent `^+`, the lone A is now aligned. The advantage of `\phantom` would have been that selecting text would not pick up the operator. As it stands, selecting text in that table does not work as expected anyway, so no (more) harm done.

What do you think? --RainerBlome (talk) 23:51, 16 December 2019 (UTC)

Writing K in Blackboard bold?

I'd like to propose writing the field $K$ in Blackboard bold $\mathbb {K} :=\mathbb {R} \vee \mathbb {C}$ or $\mathbb {k} :=\mathbb {R} \vee \mathbb {C} .$ This would be especially helpful if the field $K$ in section Moore–Penrose inverse#Geometric construction is NOT meant to be $\mathbb {R} \vee \mathbb {C} ,$ a thing which remains unclear with the current notation. --Nomen4Omen (talk) 10:36, 19 December 2019 (UTC)

 Done except sections Moore–Penrose inverse#Geometric construction thru Moore–Penrose inverse#Rank decomposition. --Nomen4Omen (talk) 16:26, 19 December 2019 (UTC)

What kind of object is the $R$ in section Moore–Penrose inverse#Projectors. Because ⁠$A\in R^{n\times n}$⁠ can be Hermitian it is quite probable that this $R==\mathbb {K} .$

Section Moore–Penrose inverse#Geometric construction does not seem to require $K==\mathbb {K} ,$ but it contains the grammatically awkward phrase "These imply that ⁠$A^{+}$⁠ is defined on ⁠$\operatorname {ran} A$⁠ to be the inverse of this isomorphism, and on $\left(\operatorname {ran} A\right)^{\perp }$ to be zero." I changed this phrase as I understand it.

Sections Moore–Penrose inverse#Limit relations and Moore–Penrose inverse#Derivative seem to require $K==\mathbb {K} ,$ because of the limits. --Nomen4Omen (talk) 15:07, 20 December 2019 (UTC)

You wrote "Because ⁠$A\in R^{n\times n}$⁠ can be Hermitian it is quite probable that this $R==\mathbb {K} .$" Sheesh, I found that one hard to parse. :-) Took me minutes to understand, I'm not joking. I'm not a native speaker, but I'd add a comma.

For reference, that paragraph was introduced by IP edit https://en.wikipedia.org/w/index.php?title=Moore%E2%80%93Penrose_inverse&diff=761062582&oldid=753802446 , the edit comment does not help here.

If I understand correctly, you mean because "is ⁠$A$⁠ hermitian?" can be asked, ⁠$R$⁠ must be ⁠$\mathbb {K}$⁠. Sounds good to me. I guess that at the time they wrote this, the author meant ⁠$K$⁠.

Regarding the original question, in the geometric construction section, in how far would it help if ⁠$K$⁠ is not meant to be ⁠$\mathbb {R} \vee \mathbb {C}$⁠? To keep the section as general as it is?

I suggest to use the {{done}} marker only when all issues discussed in the entire discussion section are done. For me, the "done" marker means "no need to look at this section any more, can be archived".

--RainerBlome (talk) 19:34, 22 December 2019 (UTC)

Because we do not really know of Moore–Penrose inverses over ⁠$K\neq \mathbb {R}$⁠ or ⁠$K\neq \mathbb {C}$⁠, and we know that ⁠$K=\mathbb {R} \vee \mathbb {C}$⁠ always works, I propose to write ⁠$\mathbb {K}$⁠ throughout the article and let the discernment up to the knowledgeable reader. --Nomen4Omen (talk) 10:24, 23 December 2019 (UTC)

I agree that the same notation should be used throughout the article. As long as we define it, we can use either variant of K. Personally, I would have left it at plain K until LaTeX understands unicode 𝕂. (:-)

Regarding the geometric construction section, I still do not understand why you did not change the notation there along with the rest of the article. As far as I can see, K is not really restricted to R or C anywhere in the article, except by the notation definition.

In the "Limit relations" section, ð is an element of R, not of K.

In the "Derivative" section, K is not mentioned, and d/dx is not clearly defined. I assume that the total derivative or Fréchet derivative is meant. I have no access to the reference given there, can anyone provide a free one? In the implied "lim h -> 0", h is an element of K, but K can be anything as long as you manage to drive h to zero. For example, a normed vector space would work. I can imagine that the derivative as given in that section does not always exist. At least d/dx A(x) and d/dx A^T(x) need to exist for the given expression to work.

Regarding K other than R or C, I disagree. We do know other kinds of suitable spaces K, see Moore–Penrose inverse#Generalizations. Everything follows from the axioms at the top. Whenever these all work, the inverse discussed here exists and is unique. As an example, if we call the field of biquaternions ⁠$\mathbb {B}$⁠ (the "ordinary" variant with complex coefficients), it works with all B^n×m matrices, as Theorem 3.5 in this paper shows. This allows us to generalize the definition of ⁠$\mathbb {K}$⁠ from ⁠$\mathbb {R} \vee \mathbb {C}$⁠ to ⁠$\mathbb {R} \vee \mathbb {C} \vee \mathbb {B}$⁠. I have added that to the article in the Generalizations section.

What do you mean by discernment? --RainerBlome (talk) 00:25, 24 December 2019 (UTC)

Added generalization to all fields along with example

Added generalization to all fields along with example. Could this be expressed more clearly? --Svennik (talk) 10:47, 27 November 2020 (UTC)

Without having looked at this in detail, the restriction "doesn't always have a pseudoinverse" leads me to believe that that paragraph would be better suited in the article on generalized inverses. One of the salient points of the Moore–Penrose inverse is that it always exists (for R, C and B Matrices). --RainerBlome (talk) 23:52, 17 January 2021 (UTC)

But when it exists, it's unique, unlike generalized inverses. --Svennik (talk) 13:26, 18 January 2021 (UTC)

Remark to changes of 20 Dec 2020

1. As user:84.246.203.253 prefers (MOS:VAR !!) ⁠$\mathbf {K}$⁠, others such as me prefer ⁠$\mathbb {k}$⁠.
   Reason: it is typographically closer to ⁠$\mathbb {R}$⁠ and ⁠$\mathbb {C}$⁠.
2. Changed also $K$ to ⁠$\mathbb {k}$⁠ in §§ Moore–Penrose inverse#Geometric construction and Moore–Penrose inverse#Rank decomposition​, because of the occurrence of $A^{+}$ in both §§ — which would be undefined if ⁠$K\not \in \{\mathbb {R} ,\mathbb {C} \}$⁠.
3. Changed AB in formulas to A B, so that it is easier to change one of them by means of an editor which supports searching for whole words.
4. A few typos.

–Nomen4Omen (talk) 17:48, 20 December 2020 (UTC)

Please introduce notation

Special:Diff/1090367874 introduced properties that can be expressed in a simple sentence, but instead using notation that is not universally understood. Syntax seems to be "A matrix is a set of tuples of coordinates (which are themselves tuples of two index values), and values in ⁠$\mathbb {K}$⁠". Please prefer to use simpler notation, and introduce the notation in the notation section. I also think that the notation is questionable, but I'm not even 100% sure I understand the author's intent.

• Special:Diff/1168148932 should have changed ⁠$\mathbb {k}$⁠ to ⁠$\mathbb {K}$⁠, shouldn't it? If yes, the error could have been avoided if the local convention of using braces (\mathbb{k}) had been used in the first place. I fixed this.
• The ⁠$\times$⁠ operator seems overloaded, meaning two different things in the same expression. Neither of these exactly matches its use ("m x n") in the notation section.
• Dimensions m and n are permitted to be zero, but this is unlikely to make sense. I fixed this.

RainerBlome (talk) 00:31, 16 September 2023 (UTC)

Biquaternion nonsense

I think some of the hypercomplex stuff on Wikipedia is kind of trivial, and is given false prominence. For instance, the "biquaternion" nonsense is just the 2x2 complex matrices. So it's literally talking about block matrices in an obscure and confusing way. Wikipedia might need a cleanup of this stuff. --Svennik (talk) 15:07, 16 September 2023 (UTC)

Can we please avoid judgment like "nonsense" and "trivial"? Like any encyclopedia, Wikipedia contains a lot of info that many deem trivial. But what seems trivial to you or me may not be so obvious to other readers. The whole point of an encyclopedia is to provide knowledge that the reader did not have before, or to confirm it. But back on topic:

Yes, biquaternions (Let's denote them with $\mathbb {Q}$.) behave like 2 × 2 complex matrices. If the question was whether the concept of the pseudoinverse of a biquaternion is valid, there would indeed not be much value in mentioning this, for most readers. But the paragraph you removed talks about the pseudoinverse of $m\times n$ matrixes over $\mathbb {K} =\mathbb {Q}$, whose elements are biquaternions.

Maybe such a matrix of 2x2 matrixes can be "flattened", and, (blindly) following your argument, is equivalent to some kind of block matrix with complex elements, but if so, this is far from obvious to me. Do you agree?

RainerBlome (talk) 18:15, 19 September 2023 (UTC)

Unless the biquaternions are given an unusual involution, an $m\times n$ matrix over the biquaternions is essentially a $2m\times 2n$ matrix over the complex numbers, with the conjugate-transpose operation being the usual one. Formally, this is an isomorphism of two dagger categories: One consisting of matrices over the complex numbers with an even number of rows and columns, and the other consisting of matrices over the "biquaternions". Under this isomorphism, the result you're referring to is not very hard. I think a remark about the use of block matrices to extend the notion of pseudoinverse from *-fields to (some) *-algebras would be appropriate here. I don't know how clear this is. I'll take on board what you said about being more careful with my language. Thank you. --Svennik (talk) 19:13, 19 September 2023 (UTC)

Define Support

There seem to be many overlapping terms in this topic which can be confusing. For example "column space" is the same thing as "image". But the term "support" is used but not defined. Please can someone knowledgeable define it? Bodger Boffin (talk) 14:33, 9 October 2023 (UTC)

 Done—Anita5192 (talk) 15:16, 9 October 2023 (UTC)

Projection vs Orthogonal Projection

In the Definition section, it is stated that "Note that ... are orthogonal projection operators, as follows from ..." (Sorry, copy and paste deleted the math expressions.) But I believe the conditions cited, P^2 = P, are those for "projection," not "orthogonal projection." The Properties / Projectors section properly cites both conditions, namely P^2 = P for projection, and, in addition, P^H = P, for orthogonal projection.

I would edit the article, but I'm not an expert in this area. QRManual (talk) 14:47, 18 April 2024 (UTC)