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This question asks if a statement from a textbook (which looks wrong to the OP) contains a typo or mistake.

It is a mistake, indeed.

I answered “Yes, this is an error. Well spotted.”

… and my answer was deleted. I guess because it was deemed “trivial” (I received no notification, and I cannot write comments in deleted questions).

So the original question and the OP are still waiting for answers.

I'd like to know which would be the right answer to such a question? What more is to be said? Should one add useless characters so that the moderators don't deem the answer trivial?

BTW: it looks strange to me that I get notifications when my answer is upvoted/downvoted, but not when it's deleted (I guess that's because it does not change my rep, but still…)

Edited: What I got by asking in Meta was to get the question itself closed as off-topic. I conclude that, if a math student finds some confusing paragraph in her textbook and wants to ask if it's some misunderstanding on her part or some typo in the book, she should not ask in MSE, because such question will be deemed “off-topic” here. Great!

Edit 2: My original wording was inaccurate. I didn't mean “errata” (as a published list of known errors in an book), but just an error or typo. Fixed.

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    $\begingroup$ I would suggest that the right response is to leave a comment saying "Yes, well spotted," and then flag the question for closure, as it is off-topic. $\endgroup$
    – Xander Henderson Mod
    Jan 27, 2022 at 19:35
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    $\begingroup$ Well, I think it's a valid question (not off topic) and it could be of help to other students that have the same doubt (it's a very popular texbook) $\endgroup$
    – leonbloy
    Jan 27, 2022 at 19:37
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    $\begingroup$ Math SE is not meant to be a storehouse for errata. Such questions are not on-topic. $\endgroup$
    – Xander Henderson Mod
    Jan 27, 2022 at 19:40
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    $\begingroup$ In a comment, feel free to suggest that the OP search by text publisher, author, title, errata. We are not here to Google to confirm. Sometime the best comment is suggesting the OP do what they would be well advised to do in the future: "check it out online". And an Errata is something publishers do, available on-line. You merely said, more or less: "I agree, that doesn't seem correct". That doesn't mean it is yet in the errata, leonboy. $\endgroup$
    – amWhy
    Jan 27, 2022 at 20:32
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    $\begingroup$ @leonbloy Yes, in general, those questions are off-topic. The question "Is this a typo?" is not a mathematical question. The question "Can you explain this bit of mathematics from a text?" is not off-topic, though if it turns out that the confusion is caused by a typographical error, the question becomes off-topic post facto. Generally speaking, if a one word answer suffices ("Yes" or "No"), then the question doesn't belong here. $\endgroup$
    – Xander Henderson Mod
    Jan 27, 2022 at 21:04
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    $\begingroup$ (Tangential point. As I understand it: an erratum is an error in print. If you have another one, then you have errata. However, an errata is a list of errata…) $\endgroup$
    – Calvin Khor
    Jan 28, 2022 at 1:07
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    $\begingroup$ @XanderHenderson "Math SE is not meant to be a storehouse for errata. Such questions are not on-topic." $\;-\;$ Is this your opinion as a user, as a mod, or an official MSE stance? I am asking because that's not a self-evident truth to me. For example, next door on SO, one hot tag is language-lawyer which also functions as an ever growing repository of bugs (errata) in both language standards (reference books) and implementations (applications). I am aware, of course, that SO is not MSE, still it's a "sister" site with similarly stated goals in its realm as MSE has in math. $\endgroup$
    – dxiv
    Jan 28, 2022 at 7:04
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    $\begingroup$ @leonbloy I believe there is nothing wrong with asking questions on MSE about (potential) math errors in published works. I also believe it is of public service to answer such questions, towards both the OP and future readers. That said, your answer could/should have added a justification, however terse, of why "yes". Even if that meant simply (re)stating the obvious - because that's how it was defined in the paragraph right above. $\endgroup$
    – dxiv
    Jan 28, 2022 at 7:04
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    $\begingroup$ Frankly, the kind of gate-keeping claiming this sort of questions to be off topic is precisely what many potential contributors to the site find most off-putting about it. This is what drives people away. I can only say I strongly disagree with it. $\endgroup$
    – YiFan
    Jan 28, 2022 at 8:19
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    $\begingroup$ Typos in mathematical text can often hinder understanding and it is great if someone wants to get the confusion sorted out by asking here. However in this case the asker has guessed the typo already and wants a confirmation. The best approach would have been a confirmation in comment. Note that the reasons for deletion of your answer are different from that of closure of the question. Once the typo is clarified / confirmed I don't know what other users can add via answer. So I don't see a problem with closure of the question. $\endgroup$
    – Paramanand Singh Mod
    Jan 28, 2022 at 8:52
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    $\begingroup$ Related? – Are questions caused by typographical errors in a textbook on-topic? $\endgroup$
    – Martin R
    Jan 28, 2022 at 8:53
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    $\begingroup$ @ParamanandSingh Except this is not your run-of-the-mill "typo". It's not a "$K$" instead of an "$M$", it's a "$2^M$". Which, in the context, could raise legitimate questions in one's mind, like - thought this was about the message set of count $M$, but maybe it is about the pass/fail combinations thereof of count $2^M$, and I am missing the point entirely... hmm, let me ask on MSE. I don't think the question warranted closure, and I do think some future reader facing the same "typo" could conceivably benefit from a confirmation. $\endgroup$
    – dxiv
    Jan 28, 2022 at 9:04
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    $\begingroup$ @XanderHenderson: I suggest that you, as a moderator, should be more careful making statements such as "Math SE is not meant to be a storehouse for errata. Such questions are not on-topic." As a moderator, your words carry much more significance than those of a general user. As Martin R noted, this is not the MSE community consensus that that questions are off-topic, as indicated by the highly upvoted answers to the earlier Meta question here. $\endgroup$
    – Moishe Kohan
    Jan 28, 2022 at 18:42
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    $\begingroup$ This said, I do find the posted answer to the linked question on main suboptimal and it would definitely benefit from more work. Personally, I would have left this as a comment, but this is my personal preference and in general SE discourages answering questions in comments. $\endgroup$
    – Moishe Kohan
    Jan 28, 2022 at 18:46
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    $\begingroup$ ... and after asking this question in meta I've suddenly start receiving downvotes on old questions of mine in MSE Interesting :-/ math.stackexchange.com/questions/4269319/… math.stackexchange.com/questions/3185389/… math.stackexchange.com/questions/3351005/… math.stackexchange.com/questions/3152094/… $\endgroup$
    – leonbloy
    Jan 29, 2022 at 22:12

3 Answers 3

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Some aspects of this are similar to the issues with solution verification posts: sometimes those posts are just unanswerable.

An answer of the form

"Yes, this is an error. Well spotted."

contains no useful information (besides inviting others to just believe in the answer-er, for whatever reasons). I would argue that even if the answer-er is the author themselves, posting a "yes" answer is not useful - we should be critical while reading any text and never take anyone's word for granted.

Put it another way, whenever one feels that a "yes" answer would suffices, it raises the question: why would the OP asks the question in the first place? Did they explain well their doubts? E.g. In the linked question, is there any mathematical reason to believe that the set is $\{1, \cdots, 2^M\}$ instead of $\{1, \cdots, M\}$? If no, then why the question? If yes, they should explain that and from there, other users might be able to give meaningful mathematical answers.

When the OP is not able to explain their doubt, for example in this question, it makes sense to close the post as missing contexts (or Needs details or clarity). Personally I think questions of this form are on topic, just that most of the time those posts are missing contexts.

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    $\begingroup$ Often, the OP is feeling confused and just needs reassurance about their reasoning from someone else perceived to be more authoritative. I think it is a perfectly good answer that would have been very much helpful to the OP. $\endgroup$
    – YiFan
    Jan 28, 2022 at 6:03
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    $\begingroup$ @YiFan unfortunately this site isn't about providing reassurance to posters -- there's a close-reason of 'seeking personal advice' just for that. The stated aim of the site is to provide a repository of questions and answers in mathematics. $\endgroup$
    – postmortes
    Jan 28, 2022 at 7:55
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    $\begingroup$ No, the site is not about providing reassurance to posters per se. Yes, it is to provide a repository of high quality Q&As. But it is also about helping people about mathematics and allowing them to learn. This is a major reason why many people participate, and to deny this is a major reason driving many potential contributors away. $\endgroup$
    – YiFan
    Jan 28, 2022 at 8:17
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    $\begingroup$ @YiFan: yes learning and helping others to learn math is the reason I participate here. But still I think that such confirmation type of answers are suitable for comments. If more explanation is needed it can be added as an answer. $\endgroup$
    – Paramanand Singh Mod
    Jan 28, 2022 at 10:13
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    $\begingroup$ @Yifan, like postmortes and Paramanand have suggested, reassurance is accomplished in comments. leonboy's answer should be transferred to a comment. $\endgroup$
    – amWhy
    Jan 28, 2022 at 15:22
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    $\begingroup$ FWIW, I do agree with both of you that adding this as a comment would be more ideal and it's what I would have done. However, I don't think there's anything wrong with putting it as an answer either, and I don't think it should be deleted. Certainly I disagree with the idea that this answer contains "no useful information", as claimed in this post. (I even more strongly disagree with the idea that the original question was off-topic, but that's tangential to the point here.) $\endgroup$
    – YiFan
    Jan 29, 2022 at 8:12
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    $\begingroup$ @amWhy > leonboy's answer should be transferred to a comment No, because it's not a comment. It's an answer. It answers the question. A comment leaves the question waiting for an answer. That makes no sense to me. $\endgroup$
    – leonbloy
    Jan 29, 2022 at 22:16
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    $\begingroup$ @leonbloy: there is an extremely strong tradition, on MSE, of answering questions in the comments. It makes no sense to me, but it is there. Many users do it, some moderators do it, and no moderator has ever done anything about it (as opposed to many sister SE sites, where moderators routinely admonish users again answering in the comments). We have thousands of "unanswered" questions in the unanswered queue that have an answer in the comments. $\endgroup$
    – Martin Argerami
    Jan 30, 2022 at 4:29
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    $\begingroup$ @MartinArgerami To me an Answer is something that requires thought, work and time. Any banality can be helpful to someone. Posting, as an answer, something copy/pasted from a textbook (either one on a bookshelf or in one's internal working memory), or explaining a definition someone did not understand is, in my opinion, extremely unappetizing. It's ok if a newbie does that for they have scant opportunity to earn a minimum required number of points, but I'm quite willing to downvote veterans who indulge in such practices. $\endgroup$
    – Jyrki Lahtonen
    Jan 31, 2022 at 6:53
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    $\begingroup$ @JyrkiLahtonen Perhaps I'm misunderstanting, but I find your comment rather offensive. Some people here answer questions to earn reputation points, or to help students who had valid doubts, or just becaase one loves math. It's not your business to judge motives. "Trivial" (to type) answers might not deserve upvotes, but if they answer the question, they are useful and should be accepted. $\endgroup$
    – leonbloy
    Jan 31, 2022 at 22:23
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    $\begingroup$ @leonbloy To an extent I see your point, and I realize that I am in a minority here. But the feeling won't go away either. In my opinion gluttony on low hanging fruits has made the site a lot worse than what it could be. It was not obvious ten years ago, but that's how it turned out. A contradiction built into the design of the site more than anything else. $\endgroup$
    – Jyrki Lahtonen
    Feb 1, 2022 at 3:56
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    $\begingroup$ @JyrkiLahtonen: When someone wants help, and it’s easy for you to give that help, wouldn’t you give it? I agree there’s no thrill in shooting turkeys for sport — but this is more like shooting turkeys to feed someone hungry. $\endgroup$
    – Peter LeFanu Lumsdaine
    Feb 1, 2022 at 15:28
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    $\begingroup$ @PeterLeFanuLumsdaine This is a false dichotomy. The choice is not between "giving aid" and "not giving aid". It is entirely possible to help the person who needs help without polluting the database here with questions that are too local, or are not a good fit for this format. For example, you can use the comments to give a quick "Yes, this is a typo", while also voting to close the question. $\endgroup$
    – Xander Henderson Mod
    Feb 1, 2022 at 15:31
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    $\begingroup$ In relation to solution-verification or proof-verification, there is the following discussion (and more) math.meta.stackexchange.com/questions/29119/… $\endgroup$
    – Calvin Khor
    Feb 2, 2022 at 7:43
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    $\begingroup$ @leonbloy I guess I had better learn to just shrug my shoulders, shake my head, and move on (in some order). My emotional investment to the site is so high. And that makes it difficult to give up on this, but... I'm just one user. The points in Xander's last comment do have merit. This is a very delicate area. $\endgroup$
    – Jyrki Lahtonen
    Feb 3, 2022 at 19:31
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This Answer is in addition to Arctic Char's (which I agree with). Regarding the similarities to , there are the discussions

  1. The problem with proof verification ,

  2. Why do users give answers using alternative method when I'm asking what went wrong with my solution? (I like the answer here), and

  3. Why do users give answers using alternative method when I'm asking what went wrong with my solution?

(and so on.) A number of times the same point is raised that a mere yes seems like an appeal to some sort of authority. A good Answer in my opinion would try to mitigate this (and give the correction, of course).

Specifically for this question, here are some points that could be addressed:

  1. Where is the typo, exactly? We have the book name, but its still not clear. This is because the book in question has (at least) two editions: Second Ed (Zbl 1140.94001) and First Ed (Zbl 0762.94001). A page number would also be nice (this is in the picture included in the question, but it is better IMO to have it explicitly in text).
  2. Has the author addressed the typo? I could not find an errata. But if the error is from the first edition, perhaps the error is fixed in the second edition. For more complicated typos and still-alive authors, it might be worth bringing the error to their attention (and then sharing the personal correspondence here.)
  3. Why is it obviously a typo? Probably because $2^M$ exceeds $M$ for $M$ large.
  4. Why is the typo likely human error? This one seems to have been covered by this comment,

Agreed, this is very probably a typo. $2^M$ does not make any sense in this context. Probably the authors already had the rate in mind, which is defined next. – Andreas Lenz Jan 28 at 7:55

but a fuller quotation from the text (more fitting for an Answer) would be nice.

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    $\begingroup$ Nice points raised here. +1 $\endgroup$
    – Paramanand Singh Mod
    Feb 2, 2022 at 9:00
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A correct answer in my opinion should be:

  1. Whether it is or isn't a typo

  2. If it is a typo, why it is obvious its a mistake/typo [if OP already has supplied such an argument then an alternative argument is very nice but simply confirming that the OP's logic is valid is quite helpful for the OP as it helps them develop more confidence in their reasoning ability]

  3. What it should have been? with an explanation of why. Now depending on the typo this might be impossible to reconstruct/guess at. But sometimes it is possible, and whenever it is possible I think a good answer has the onus of having to do so.

  4. This might be controversial but I think its fair game to then attempt to solve the reconstructed problem, while at the same stroke. Depending on the OP this may or may not be necessary. If the OP seems to be pretty advanced to the point that they probably know how to solve it but want to fact check if their typo-recognition abilities are fine then I might choose not solve the problem and I dont know how to concretely explain how to notice such posters but its obvious when you run into one. Otherwise if it's a more beginner seeming user, why not just throw them a bone while at it is my thought process.

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