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The question "The probability for a streak when tossing a coin" is on hold. [Edit: It has been reopened.] I disagree with closing the question and voted to reopen.

This problem may sound like it is easy. As far as I know, it is not, even though the comment on the problem and quick closure make it sound like it might be just an exercise. You can set up a recurrence which isn't so easy to analyze, and there is a complicated approximation in Feller's classic book. If there is a simple way to solve this problem that Feller and others have overlooked, I'd like to know about it. I think it would be appropriate to put the recursion and approximation in an answer on MO (others would be better than I would be at writing such an answer), and not push the question to MSE.

I think there is a tendency for people to misjudge the level of combinatorics problems just because they may sound simple. Please be careful when considering whether to vote to close a problem if it is outside your main area of expertise, even if it looks like there are votes to close already.

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  • $\begingroup$ @Q.W.: Are you András Bátkai? If so, then I think your comment "it is good if you indicate where you got struck" is highly misleading. It suggests that a student should be able to work it out, and that we can't guess what the difficult part is. If the question were to count the streaks of length $M$, that would fit. It doesn't fit the question of what the probability is. This is not the first time you have made such a comment leading to a combinatorics question being closed. tea.mathoverflow.net/discussion/1591/… $\endgroup$
    – Douglas Zare
    Aug 22, 2013 at 14:04
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    $\begingroup$ Sorry, I was at a public computer and did not want to use my password. Here my comment repeated: I made my comment not because this is easy, but a rather old and well-investigated question. If the question lists the classical literature and ask for recent developments, then it is perfectly legitimate. But the OP did not do that. $\endgroup$
    – András Bátkai
    Aug 22, 2013 at 15:58
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    $\begingroup$ I made a comment because I thought the question was not suitable (not because I thought it was easy). I hope if other people think differently, they also comment on the question and also on my comment. I do not believe that those who voted are irresponsible and act just because I bade a small comment. I am that an important person... $\endgroup$
    – András Bátkai
    Aug 22, 2013 at 16:03
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    $\begingroup$ I still think that the question, as it is, not suitable because of its wording and because it seems the OP is not aware of the classical literature. But you can make it an interesting question. Feel free to edit and make it a good question! $\endgroup$
    – András Bátkai
    Aug 22, 2013 at 16:04
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    $\begingroup$ I clicked on the names of the people who voted to close. I believe not one of them had "combinatorics" as one of their top 10 tags. The fifth voter had "probability" as tag number 10, but I think the others didn't have probability in their top 10 tags. Does it tell you anything that the many people who do seem to specialize in the area of the problem who can vote to close were not the ones voting to close? I think people who did not fully understand the problem dismissed it as an exercise, just like in the previous example. Please avoid doing that. $\endgroup$
    – Douglas Zare
    Aug 23, 2013 at 0:04
  • $\begingroup$ you should discuss this with those who vote. And you should edit questions to become better if you think the question is important and if the asker is not on the level. You are sure he or she understands the comments and answers? I really hope I am mistaken. $\endgroup$
    – András Bátkai
    Aug 23, 2013 at 7:29
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    $\begingroup$ You keep saying I should talk to the people who vote to close instead of to you. (You said the same thing last time, too.) I hope they read this discussion, too, but I would like you to stop declaring combinatorics problems to be exercises which combinatoricists say are not exercises. Telling people their reasonable questions should be closed is unfriendly and misleading to them and to the people who trust you and vote to close. It wastes the time of other people like me who take the trouble to reopen a question which should not have been closed. Please stop. $\endgroup$
    – Douglas Zare
    Aug 23, 2013 at 8:17
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    $\begingroup$ And you keep saying I declare these problems to be exercises. I do not. $\endgroup$
    – András Bátkai
    Aug 23, 2013 at 8:44
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    $\begingroup$ You said, "As it stands, it is a (maybe) hard exercise." That is a direct quote from you. tea.mathoverflow.net/discussion/1591/… You also said, "indicate what you already know and where your problem lies so that people can help you" and "indicate where you got struck, what exactly the problem is. It is easier to explain then, and the answerers can address your problems." These say the combinatorics problems are exercises. Please stop doing this unless you are sure the problems really are exercises. $\endgroup$
    – Douglas Zare
    Aug 23, 2013 at 11:04
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    $\begingroup$ I do not really want to get involved in this discussion (a second time). But I think that the word excercise is used in various different ways in this conversation. And what as far as I understand @AndrásBátkai meant with the sentence quoted by Douglas Zare, and in that context this was pretty clear, at least to me, is that that other question was presented like an excercise (without any context or motivation), which is something other than saying it is an excercise. $\endgroup$
    – user9072
    Aug 23, 2013 at 19:28

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