# Spivak Chapter 2, problems 27 (and 28)

To be honest, I have no idea how to even start this problem. I'm sorry I don't have any work to show, but I'm just at a blank. Help?

Chapter 2: Problem 27:

"University B, once boasted 17 tenured professors of mathematics. Tradition prescribed that at their weekly luncheon meeting, faithfully attended by all 17, any members who had discovered an error in their published work should make an announcement of this fact, and propmptly resign. Such an announcement had never actually been made, because no professor was aware of any errors in her or his work. This is not to say that no errors existed, however. In fact, over the years, in the work of every member of the department at least one error had been found, by some other member of the department. This error had been mentioned to all other members of the department, but the actual author of the error had been kept ignorant of the fact, to forestall any resignations. One fateful year, the department was augmented by a visitor from another university, one Prof.X, who had come with hopes of being offered a permanent position at the end of the academic year. Naturally, he was apprised, by various members of the department, of the published errors which had been discovered. When the hoped-for appointment failed to materialize, Prof. X obtained his revenge at the last luncheon of the year. "I have enjoyed my visit here very much", he said, "but I feel that there is one thing that I have to tell you. At least one of you has published an incorrect result, which has been discovered by others in the department." What happened the next year?"

Chapter 2: Problem 28:

After figuring out, or looking up, the answer to Problem 27, consider the following: Each member of the department already knew that Prof.X asserted, so how could his saying it change anything?

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Could you write the problem or post a link to it? –  user10444 Jan 8 '13 at 16:39
@Lob, not all of us have the book, and even if they did it would be nice of you to spare us the effort of getting off the couch. –  nbubis Jan 8 '13 at 16:41
@Lob: Welcome to MSE! There are people who do not have easy access to the reference you cite and it is great that you cite the reference, however, can you write out the problem. Also, MSE typically only want one question per posting in case users are searching. Is this homework? If so, please tag it as such. Regards –  Amzoti Jan 8 '13 at 16:41
@Freddy It presuposes a lot of things about human nature that don't belong to mathematics... If we apply very specific and restrictive rules, it could be reduced to a much simpler problem. From my point of view, it's just a way to make something simple very complex. –  MyUserIsThis Jan 8 '13 at 18:39
@Ferfer93 It is a mathematics puzzle. It is not meant to suggest a professor would actually resign if they learn about an error in their paper. For the sake of the problem, if the professor learns about the error they resign. –  Danikar Jan 8 '13 at 18:44

## 1 Answer

First simplify the problem to only 2 professors, call them Prof. A and Prof. B (instead of 17).

On the next meeting after Professor X's statement. Prof. A will expect Prof. B to resign since Prof. A knows about Prof. B's error. When Prof. B does not resign, Prof. A will know it is because Prof. B is aware of an error of Prof. A's. Therefore, Prof. A knows about his error and must resign on the next meeting. Similarly Prof. B will have found out his error and will also resign.

Now think about how this works for 3 professors. Then you can generalize it to n professors and use it for your 17 professor problem.

If a professor knows about 0 errors, he must resign as soon as Professor X makes his statement because he would know the error was his.

If a professor knows exactly $k$ errors, and no one has resigned before the $k$th meeting after Professor X's statement, then he knows all members of the department know about at least $k$ errors which are not their own. So the professor now knows there must be at least $k + 1$ errors that everyone, except the error maker, know about. Since he only knows about $k$ errors, one of the errors must be his own and he must resign.

In the case where there are 17 professors where all professors know about exactly 16 errors. All the professors will resign on the 16th meeting after Professor X made his statement.

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Oh... I think I get it now. But how do I go about answering 28? –  Lob Jan 8 '13 at 17:33
Before Professor X said anything, each professor knew nothing about what the other professors knew. After Professor X said something, then each professor knew that the other professors knew about at least 1 error. And subsequently after each meeting the professors gain more information when the other professor's do not resign. –  Danikar Jan 8 '13 at 17:38
But I thought each professor mentions the error to everyone except the guy who made the error? "his error had been mentioned to all other members of the department" or am I interpreting that wrong? –  Lob Jan 8 '13 at 18:18
You are correct. However, Professor X's statement provides more information about what the others know. For example in the 3 professor problem from A's perspective. A knows errors for B and C. A knows B knows C's error, and A knows C knows B's error. So A knows no one will resign immediately. However, once C does not resign, A knows that B knows that C knows some error. So unless B knows two errors, he knows C knows an error about him. Thus he must resign. If he does not resign, then A knows B knows 2 errors. And since B wouldn't know an error about himself, B must know an error about A. –  Danikar Jan 8 '13 at 18:33