This question of mine was made community wiki by someone other than me. I'm aware of discussion (e.g. here and here) arguing that big-list questions should be CW so that the voting system can be used to rank the answers without affecting the reputation of the posters. I suppose someone might have believed my question was a big-list, but I would argue that it is not: I am not looking for a list of all mistaken uses of notation, but a single example of a notation that turned out to be incorrect and led to mathematical mistakes, and if I get such an example I will accept it. The question is certainly soft, but I haven't seen anyone arguing that all soft questions should be CW (and offhand I would probably disagree with that). Is there another convention that applies to make this question mandatory-CW?

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    Hi Mike. I was the one who made it CW (and actually, it's only site mods who can do that to a question, ever since the move to the SE network). If there is some consensus that this was a mistake, then I am happy to remove the CW status. My own feeling though was that, even if you only wanted a single example, the effect of the question would likely be to generate a bunch of very different answers, none arguably definitive over or more authoritative than the others, and that to my understanding is a situation which has historically garnered CW status. – Todd Trimble Aug 10 at 19:07
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    I think that the question you refer to is of "big-list" type, regardless of details of the formulation. -- The only sensible answers to the question are examples of where incorrect notation has led to mistaken proofs -- a negative answer cannot possibly be given. -- Hence I agree with Todd Trimble that the question should be CW. – Stefan Kohl Aug 10 at 19:18
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    @ToddTrimble Thanks for clarifying! But I still disagree. I think there are lots of questions that generate multiple answers without one being more definitive or authoritative, such as questions asking "why is X the way it is" or "give me some intuition for Y", none of which is CW. Can you point to some documented explanation or discussion of the principle you refer to for assigning CW status? – Mike Shulman Aug 10 at 20:03
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    @StefanKohl I don't see what the impossibility of a negative answer has to do with CW status. Many other non-CW questions cannot be given a negative answer, e.g. "why is it true that X?" – Mike Shulman Aug 10 at 20:03
  • Mike, there have been a lot of discussions about CW and how MathOverflow regards it (idiosyncratically, from the perspective of what SE designers seem to have in mind) -- those discussions go back to the days of 'tea' discussions, before the merger with the SE network. There are a lot of gray areas here and there is no official stated policy. It sure looks (or quacks) like, to me, that the initial response of the community was to treat it like big-list, but maybe the community has misunderstood what you were after (cf. your explanatory edit). I've asked my fellow moderators to have a look... – Todd Trimble Aug 11 at 22:24
  • Thanks. I haven't put much time into following all the backs and forths of the "tea/meta" discussions myself, and I expect the same is true for many users (not to mention newcomers), so a stated policy on issues like this would be extremely helpful, not just in giving guidance to users on what to do and what to expect, but in preventing misunderstandings and bad feelings. – Mike Shulman Aug 12 at 4:01
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    (forgot to notify @ToddTrimble) I must admit I feel kind of uncomfortable even raising this issue, because asking my question to not be CW could easily be interpreted as my being greedy for reputation. For me it's more an issue of principle (I don't really need any more rep) and wanting good answers to be worth reputation (in hopes of inspiring some better answers); but when it feels politically uncomfortable to ask a question that's another situation when a clearly stated policy is a good idea. – Mike Shulman Aug 12 at 4:04
  • Mike, I completely understand, and you're raising a good general point. Thanks for speaking up here (in spite of discomfort, also completely understandable). Will try to get back to you soon. – Todd Trimble Aug 12 at 4:51
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    Since @ToddTrimble explicitly mentioned that here on MO community wiki is used differently (mainly preventing reputation gain) from the purpose for which it was designed (lowering the threshold for editing, making the post "owned" by the community), I will add a link to an older discussion about this: Community Wiki in the hands of moderators. (Mainly quid's answer and comments under this post.) I also left a few comments in chat. – Martin Sleziak Aug 12 at 5:49
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    @MartinSleziak Thanks for the links. There is a link there to the stackexchange blog post whose conclusion seems to be that questions (as opposed to answers) should rarely, if ever, be CW: if a question is a "real question" and on-topic for the site, then it doesn't need to be CW, and otherwise it shouldn't be asked in the first place. – Mike Shulman Aug 12 at 21:46
  • As I understand, part of the reason why big-list and soft questions are usually made CW on MO is that one wants to avoid that asking and answering popular soft questions becomes the by far easiest way to quickly and easily gather a lot of points. – Stefan Kohl Aug 13 at 13:45
  • @StefanKohl Thanks for the explanation - I think I can see why that might be a concern, at least hypothetically (wanting high reputation on MO to reflect actual expertise at real mathematics). But couldn't we soften the policy a little while still solving that problem? Maybe allow users who are already above a certain reputation level to ask non-CW soft questions? Or allow each user a small number of non-CW soft questions, like one per month? – Mike Shulman Aug 13 at 14:31
  • To ease the moderator burden we could require authors of such questions to explicitly request non-CW when they ask the question, or afterwards on a meta thread. If such a policy is clearly documented and adhere to, I don't think it would be a problem. – Mike Shulman Aug 13 at 14:31
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    @MikeShulman something similar has happened with one of my questions; but as I never understood and still don't understand the criteria for the CW flag, I accepted the moderator's decision. I guess that the CW flag can't be based on checkable conditions and is, similar to art (and something else), a "I know it when I see it" case. – Manfred Weis Aug 17 at 6:56
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    @AlecRhea Where there is likely to be a slew of texts that answer the question, there's a good chance it will be made CW. – Todd Trimble Aug 19 at 21:50

After consulting with the other site moderators, the decision for now was to let the CW status stand for the question at Main. But this may be a good time to revisit the discussion of some of the ways MathOverflow has been known to use CW, which differ from how many other StackExchange sites use it, and which are consistent with certain traditions since before we were part of the SE network.

Currently it's only the site moderators who can make questions CW, or who can undo that action. If a question is CW, then so automatically are the answers. On the other hand, if a question is not CW, a user who has answered can make his/her post CW independently, without affecting other posts within the thread.

Speaking as an active moderator, most times I make a question CW are after it was flagged as a possible candidate for such, although sometimes I'll do it without prior prompting if it seems to me well within the wheelhouse of questions we've traditionally made CW.

Questions that are typically or at least often made CW include

  • Career advice questions,

  • Questions regarding style (e.g., in writing papers),

  • Big-list questions, especially if they quantify over all mathematical fields.

A justification is that, especially in such cases, there may be strong differences of opinion, each with weight and argumentation behind it, regarding which answers are better than others. [For example, whether to punctuate at the end of a display line, or whether in a paper one should refer to an earlier Proposition using a capital P.] Here it seems sensible to put it to the community and sort it out with large numbers of votes, and the community generally feels more invited to vote up or down when reputation points are not at stake.

(By the way, when I asked, site moderators felt that Mike's question came under the umbrella of the third bulleted point.)

An answer is often made CW in order to avoid an unseemly appearance of existing for the sake of gaining reputation. For example, a question may have answered in comments, and someone culls the comments together to form an answer, but makes the answer CW in order not to take credit (in the form of reputation points) for answers by others. As a community service. This can also be done in cases where different people answered different parts of a multi-part series of questions within a single post, whereupon arises the nettlesome difficulty of which answer to officially accept. The difficulty can be neatly finessed by assembling all the partial answers into a single master answer and then marking that one CW, courteously assigning all credit where credit is due. Marking that as the official answer then seems a satisfactory solution.

Another example: if someone asks a question but already knows an answer, and posts the answer concurrently with the question, then she might make the answer CW, again to signal that it's not meant to step in front of other answers (after all, often the early bird gets the worm where points are concerned), but more along the lines of selflessly adding to the public commons.

None of this is actually written down as MathOverflow official policy. That in itself might make some people uncomfortable; my hope is that this answer might help lessen the discomfort a little. My understanding as expressed above is based mostly on informal traditions and things that have frequently been said over the years, much of it in comments at Main, or in flags. So some of it is a bit hard for me to quantify. One early discussion of such matters may be found at 'tea':, and here's another: More recently there was this: Why was this question made community wiki?

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    I forgot to mention that often questions about history have been put up as CW candidates, as it can be very hard ascertaining an answer that seems definitive, but that seems much touchier ground to me, so I didn't mention it. – Todd Trimble Aug 18 at 21:51
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    Also: this is more a comment, but it's just a fact that the issue of reputation can be a slight embarrassment, as has been said very often in varying degrees of strength. Some notable users find them childish and refuse to associate themselves with points, answering only in comments, such is their distaste. Some would love to give their badges away. Yet others forthrightly declare they enjoy the little ego strokes of rep points and that this is a motivating factor. Still others seem to take the points way too seriously (to my taste, anyway!). The various sides may war within the same soul. – Todd Trimble Aug 18 at 21:59
  • You've confused me, Todd – did you forget to mention questions of history etc., etc., or did you decide not to mention it because it seems touchier ground? It can't be both. – Gerry Myerson Aug 18 at 22:11
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    @GerryMyerson You're right: that was confusing. I had considered in the course of composing this answer mentioning it as one class of historical examples, but then that slipped my mind. After the fact and upon reflection, I decided that it was just as well -- for one thing, I don't think that type of question has been much wiki-hammered as of late, and for another it seems inherently more delicate ground with respect to wiki-hammering. – Todd Trimble Aug 18 at 23:19
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    Thanks for the explanation! I still respectfully disagree that my question is big-list. I have a clear criterion for a correct answer; I am asking for a single example of an X, not a big list of all X's (and so far no real X's have been given). In general we leave the criterion of correctness up to the question author through the discretion to accept, allowing the community a lesser say through votes; and I don't see why this question should be different. Other questions asking for examples are not CW, despite that many answers are possible; we let the author choose the answer they prefer. – Mike Shulman Aug 19 at 3:39
  • I would be interested to hear from the other moderators (by private email if preferred) their reasoning for why they think my question is a big-list. – Mike Shulman Aug 19 at 3:39
  • One problem with different handling of CW on MO than in SE network might be that users familiar with the way CW works on other sites might think that they can edit those posts more freely than ordinary post - while it is possible that some posts on MO were made CW mainly as reputation-avoidance tool - as described in this answer - and without any intent to make the post "owned" by community. – Martin Sleziak Aug 19 at 23:31
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    To add some context to the previous comment, here is a quote from FAQ on community wiki: "Community Wiki posts work by partly transferring ownership of the post from the original author to the community. They make the post easier to edit and maintain by a wider group of users, but they do not contribute to any user's reputation." Community-wiki posts can be edited by users with 100 reputation points (without going through any review queue). – Martin Sleziak Aug 19 at 23:31
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    @MartinSleziak Thank you. To the best of my knowledge, this has not been a problem in any practical sense -- not yet, anyway. – Todd Trimble Aug 20 at 2:43
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    For what interest it may have, here is another proposed way to use CW, from the very early days of MO:… It was applied here:… – Todd Trimble Aug 20 at 23:43

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