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I am troubled by this. The authors of the book explicitly ask others not to post solutions to the exercises in their book on the web (Arora and Barak, "Computational Complexity", page vi):

We ask that you do not publish solutions for the book’s exercises on the web though, so other people can use them as homework and exam questions as well.

Should we have a different policy regarding answering this kind of situations (where the authors of the questions explicitly ask others not to post answers to their questions)?

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    $\begingroup$ Source: Final draft of the introduction, see page vi, the paragraph after the itemized list. $\endgroup$ – Tsuyoshi Ito May 27 '12 at 20:05
  • $\begingroup$ @Tsuyoshi, thanks. $\endgroup$ – Kaveh May 27 '12 at 20:10
  • $\begingroup$ Such policies are established to help lazy teachers. Do we want to chime in on this? This is closely tied to our homework debate; I don't see how we help anybody by refusing to put knowledge out there and we can not prevent anyone from cheating, anyway, nor are we responsible for cheaters of for catching them. $\endgroup$ – Raphael May 27 '12 at 22:08
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    $\begingroup$ @Raphael, I would suggest that you be more careful about how you characterize other people, you interpret actions to find people's motives and that can be dangerous. $\endgroup$ – Kaveh May 27 '12 at 23:12
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    $\begingroup$ ps: this is not about homework or cheating, it is about respecting the will of our colleagues regarding questions they have designed for their book, e.g. this discussion doesn't apply if someone rephrases a question from such a textbook in their own words. $\endgroup$ – Kaveh May 27 '12 at 23:15
  • $\begingroup$ @Kaveh: I might have exaggerated a tad, but I stand by the statement at its core: nobody is hurt by published solutions but teachers who want to reuse others' problems for grading. If you can offer another (and more plausible) interpretation of the quoted passage, please share it. (I know that protecting solutions by heavy licenses is common practice in textbook publishing, but that does not change my opinion.) $\endgroup$ – Raphael May 27 '12 at 23:23
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    $\begingroup$ @Raphael, the point of (good) assignments is to help students learn. Unfortunately it is a reality that many students don't understand this, so it is not really the benefit of the teachers but the students, the teacher doesn't really loose anything. Anyway, coming up with good suitable-difficulty and useful questions is much more difficult than you might think if you haven't thought a course. Hopefully you will see what I mean when you do. :) $\endgroup$ – Kaveh May 27 '12 at 23:57
  • $\begingroup$ @Kaveh: You are right on both counts: students who do not solve their exercises on their own are doing it wrong, and it is their own fault. Creating good exercises is hard (although rephrasing helps making Googling solutions harder). Neither explains why we should keep solutions a secret, though. $\endgroup$ – Raphael May 28 '12 at 0:02
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    $\begingroup$ It doesn't matter whether posting solutions is intrinsically good or bad. Our colleagues have asked the community, as a courtesy, not to publicly post solutions to the problems in their textbook. Ignoring their request violates Wheaton's Rule. $\endgroup$ – JeffE May 29 '12 at 14:18
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I don't care what the authors say, I will not refrain answering a question because someone somewhere requested people not to answer this question.

The text of the question belongs to the author of the book, and they may request that the text not be posted; in fact, that is automatically the case due to copyright law. The semantic content of the question does not belong to anyone, and anyone may ask the question, whether they first read it in that book or they made it up on their own. The authors of the book do not have a monopoly on the question.

Acknowledging the source of a question is required by academic traditions. Refusing to answer questions is very much against academic traditions.

Even if you're favorable to the authors' request (which I'm not), consider this from a practical angle: let's say someone wants to ask a question that is an exercise in this book, and they're aware of the policy. They can easily work around the policy by pretending to have come up with the question on their own. It's unenforceable.

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  • $\begingroup$ I am not sure if copyright would limit posting the question, it might be considered a fair use. It seems to me that in these cases it be would be better if the text was not copied, the question was rephrased in the OP's own words, and no reference was given (since that would make it more difficult to use Google to find an answer to the question). $\endgroup$ – Kaveh May 27 '12 at 20:29
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    $\begingroup$ @Kaveh Copyright applies as a general matter. Here, if I read the question correctly, the only passage that is copied verbatim is “Prove the existence of a universal TM for space bounded computation (analogously to the deterministic universal TM of Theorem 1.9)”. There is no creativity to the phrasing of this sentence, so I do not think copyright can apply to this sentence alone. Even if it does, asking for an explanation of the sentence is likely to be ruled fair use. $\endgroup$ – Gilles May 27 '12 at 20:33
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    $\begingroup$ I do not like removing the attribution because crediting ideas is required by academic guidelines. Furthermore, the SE question is not the exercise, but a request for a clarification related to the way the exercise is presented in the book. The book reference could be removed from the title, maybe. $\endgroup$ – Gilles May 27 '12 at 20:34
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    $\begingroup$ There is a big difference between answering one person's question and posting the answer for public consumption by Google. The authors have absolutely no objection to the former; their request is about the latter. Do I think the request is pointless? Yes. But I feel obliged to respect it anyway. $\endgroup$ – JeffE May 29 '12 at 14:19
  • $\begingroup$ @JeffE, I agree with your comments and Tsuyoshi's answer, but it seems it is going to be a personal decision for each user. $\endgroup$ – Kaveh May 29 '12 at 16:05
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    $\begingroup$ @JeffE Since you quoted Wheaton's rule: I consider requesting a monopoly on certain mathematical ideas and demanding their censorship to be very “dicky”, and I will not honor this request. If you don't want people to discuss your ideas, don't sell a book, sign an NDA. $\endgroup$ – Gilles May 29 '12 at 17:59
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As blufox said, the specific question does not ask for a solution. But I think that you raised a good point apart from the specific question: should we follow a request by an author not to post solutions to exercises in a book on the web?

Well, I do not have a solid opinion on the matter in general. Given that there are different reasonable opinions, I do not think that it should be an enforced policy. However, in the case of Arora and Barak, I think I would follow personally. I value this textbook highly, and I find it reasonable for me to follow the authors’ small request in return to the efforts which they put into writing it. Just my two cents.

Of course, as Gilles pointed out, it is not always possible to realize that the question is from Arora and Barak, especially if an asker intentionally hides or misrepresents the source. But that does not change the fact that I intend to follow the authors’ request.

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I would say that if anyone notices such a case (i.e a question which is an exercise from a book and the authors have specifically written that they're unhappy with solutions being posted online), then they should put this in a comment or edit the question to this effect. Then users who might want to answer can make an informed decision whether to go against the authors' policy or to comply.

But clarification questions like the one you linked should definitely be okay. Arora-Barak is a wonderful book, but it does have some typos and may not be perfectly clear in some places, especially to a novice in the field. That is of course true for almost any technical book, especially introductory textbooks that contain a lot of new material and are in their first edition.

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I am the author of the question, In this case, I did not ask for the solution. My problem was with the interpretation of the question which I felt was not clear. This is clear (I hope) from the line

"However, this seems a dubious interchange of space and time. If on the other hand, t actually meant that the second machine stops within t space, too, then the second part does not make sense any more because it says SU uses Ct cells, which is not t. So my question is how do I interpret this? Is the first interpretation really possible?"

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  • $\begingroup$ Apologies, updated the post so it doesn't imply that there is something wrong with the question. $\endgroup$ – Kaveh May 27 '12 at 20:25

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