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Congratulations to Joel David Hamkins who on (I think!) the 27th November 2022, was the first MathOverflow user to pass 200k reputation! Since Joel joined on the 19th November 2009, he has supplied 1746 answers, and asked 93 questions.

On the occasion of passing 100k reputation, Joel wrote an essay about how he views MathOverflow.

Since such an announcement is not really a question, how about this:

What is your favourite post by JDH? It could be an Answer or a Question.

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    $\begingroup$ I see that crossing 100k was mentioned among the milestones here: History of MathOverflow. So maybe this could be added too - perhaps with a link to this post. $\endgroup$ Dec 1, 2022 at 5:27
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    $\begingroup$ I seem to recall MathOverflow has deliberately disabled the display of reputation next to usernames not too long ago… how do posts like this square with the general desire to shun the ostentatiousness of Internet points? $\endgroup$ Dec 1, 2022 at 22:57
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    $\begingroup$ @user3840170 Because I as an individual am not the same as MathOverflow the website/organisation, and I thought it worth celebrating, even though it's an arbitrary threshold. I did not do this in my official capacity as a mod, or in consultation with anyone else, and so take responsibility for this post. $\endgroup$
    – David Roberts Mod
    Dec 2, 2022 at 0:55
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    $\begingroup$ Thank you very much, David, for the question here. I am touched by the attention. $\endgroup$ Dec 2, 2022 at 2:38
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    $\begingroup$ @DavidRoberts Using the featured tag is a moderator privilege, which means you are doing this in an official capacity. $\endgroup$ Dec 2, 2022 at 7:24
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    $\begingroup$ " the general desire to shun the ostentatiousness of Internet points?" Why does general mean here? There is a poll somewhere? It seems to me that disabling the display of reputation was a decision taken by the moderators, but it does not reflect (as far as I know) a general consensus. $\endgroup$ Dec 2, 2022 at 11:45
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    $\begingroup$ A decision was made by moderators to hide reputation points, for the express purpose of stopping calling attention to them. With this post calling attention to someone’s reputation, advertised using moderators’ privileges, this decision now comes off as hypocritical. $\endgroup$ Dec 2, 2022 at 16:47
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    $\begingroup$ @user3840170 it might, but for the fact I joined the mod team between the two events. And one can celebrate contributions of a prolific user while supporting the general de-emphasis of rep in day-to-day operations, making sure it isn't in everyone's view constantly. $\endgroup$
    – David Roberts Mod
    Dec 2, 2022 at 22:28
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    $\begingroup$ It would be more accurate to say that an option to hide reputation on the main site has been provided; reputation isn't really disabled anywhere, and I don't think there's any contradiction in having a "hide reputation" feature and celebrating a prolific user's contributions through such a post. $\endgroup$ Dec 9, 2022 at 9:40
  • $\begingroup$ I don’t see where I can enable reputation display. It was disabled site-wide for everyone. And even if it were possible to re-enable, defaults still matter. $\endgroup$ Jan 3 at 15:31
  • $\begingroup$ @user3840170 you may see my answer to "Hiding reputation" for how to toggle the display of reputation points. Agreed that defaults matter, so my earlier comment should have referred to the "show reputation" feature, not the "hide reputation" feature. $\endgroup$ Jan 3 at 15:52
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    $\begingroup$ In my opinion, the "show reputation" feature should be more clearly displayed. $\endgroup$ Jan 7 at 10:17

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My favorite JDH paragraph, from Have you solved problems in your sleep?, is worth quoting in full.

At times, naturally, it is an interesting (or obsessive) MathOverflow question that I set myself to thinking about as I lay myself down. But let me say categorically that it has never been the case (ahem, cough, cough) that an hour or two after going to bed, I would wake with an answer and crawl out to my computer to type up an MO answer in the dark, while the rest of the household is sleeping, only to realize at that point, right before clicking "Post Your Answer" that the solution was totally flawed or wrong. What a downer that would be, to be sitting in the dark in the middle of the night, tired, with nothing to show for it but a wrong mathematical idea. That has NEVER happened... :-)

That I felt reassured that I was not the world's worst MO addict has NOTHING to do with why I like this paragraph. :-)

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A lot of Joel David Hamkins best questions are things which are really elementary in statement, but quickly lead to thorny logical issues. Examples here are this question about a game involving chocolate eating or this question about Hex on an infinite board. At the same time, many of his answers to more traditional logic and set theory questions are really fantastic, helping (at least for me but I suspect for others as well) understand a lot of subtle points in set theory and model theory. I made a joke to someone a few days ago that "I'm not a logician but I read JDH's Mathoverflow answers regularly" but it really was only halfway a joke.

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    $\begingroup$ Thanks so much, Joshua, that is indeed my favorite kind of mathematical situation, where something simple leads one on to a logical labyrinth. $\endgroup$ Dec 8, 2022 at 14:07
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My favorite answer by Joel was to the fourth question I ever asked on MathOverflow, six years ago.

I was just recently wishing there was an 'MO appreciation thread' on meta where users could express thanks to one another, specifically so I could address this post -- thank you for the opportunity, David!

When I asked that question, I was an undergraduate student who had just been given his first taste of mathematics research. The university I was attending didn't offer any classes in set theory or mathematical logic, but they were topics central to my emerging research interests -- I'd had a brief exposure to MO as an awestruck lurker, but was still uncomfortable with the prospect of posting.

Joel's answer was not only informative, his tone was welcoming in a way that made me more confident in my future interactions with the site. I wouldn't be where I am today mathematically without his support, and the support of the community here more broadly. Cheers to 200k Joel!

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    $\begingroup$ I am so glad that my answer had such a welcome effect! Thank you so much for mentioning it. $\endgroup$ Dec 8, 2022 at 14:05
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His answer, explaining different ways to think about the continuum hypothesis is a really good one.

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    $\begingroup$ Thank you so much for mentioning it. $\endgroup$ Dec 8, 2022 at 14:13
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As an amateur mathematician with an interest in set theory and computability, I have to say JDH has probably advanced my mathematical knowledge in those areas more than anyone else (there are some close seconds!). Perhaps this is due to his prolific use of the site, but more likely I think it is because his writing is clear and he has the ability to provide concise examples that really illustrate a point.

For example, his answer on a question about Goldbach, where some unique insight is added about the arithmetic hierarchy, and a number of examples are provided. Perhaps this is all obvious for anyone that covers this rigorously in a formal education, but I remember this post giving me a sudden understanding of what the arithmetic hierarchy is "used" for (previously only understanding some vague concepts as outlined on the wikipedia page).

And to venture slightly out of bounds, I want to add this clear writing style extends outside of MO, such as some recent lecture notes "A Review of the Godel Fixed Point Theorem" which I will be making use of in a project of my own.

So I'd like to thank JDH for taking the time to answer questions in a clear and accessible manner. Without such kind souls I would be lost still trying to decipher, e.g. Kleene or Davis.


Honorable mention: "The Set theoretic multiverse" was really a joy to read.

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    $\begingroup$ I am so glad to have been helpful, Ben, and thank you so much for your kind words about my writing. $\endgroup$ Dec 8, 2022 at 14:08
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Congratulations to professor Hamkins for this achievement! I hope I am not off the rails here, since his answers are mostly above my level of understanding. But I wanted to mention his great book which is a joy to read: Proof and the Art of Mathematics.

Though I've just read six chapters and still should find time for the rest, but I wanted to use this opportunity to express my appreciation. Thank you, JDH!

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    $\begingroup$ I am so glad that you have enjoyed my proof-writing book! $\endgroup$ Dec 8, 2022 at 14:12
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As for favorites (or "favourites", if you like), I will go with this one for narrow selfish reasons. It is his answer to a question of mine that I intended less than fully literally, but he decided to construe it literally.

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    $\begingroup$ What a fun project that was, Michael! Your question led directly to published work (by us and several other co-authors), as well as several senior theses projects by students at Oxford and elsewhere. $\endgroup$ Dec 8, 2022 at 14:11
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I think that a good question from him (in my view, and I'm not a professor I will not be able to comment anything that will be interesting about his excellent posts on the main site MathOverflow) is a post on Meta with identifier 1186 and title MathOverflow at the Joint Mathematics Meetings, Baltimore, January 2014, because the post is proactive and this looks like the initiative to build a community.

I do not know him but he evokes the best feeling to me in this post and other simple and succinct comment edited in the post of Meta MathOverflow conferences, that's "Wow, sounds great!" It's is simple and succinct comment, but it was generous and elegant adding these words for the proposal of the author of the post on Meta 5446 (I hope this project can one day come true) from the user who has the highest reputation in MathOverflow. Wow!

Many thanks for your question on Meta 1186 and your love for this site MathOverflow adding your comment in the post Meta 5446. Thank you very much for your presence on the site MathOverflow and good posts, congratulations!

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    $\begingroup$ In addition, in this post Meta 1186 he edited an answer with information regarding a conference by a great mathematician and user of the site MathOverflow who passed away in recent months. $\endgroup$
    – user142929
    Dec 1, 2022 at 17:33
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    $\begingroup$ What is the relevance of Mark Sapir's passing in 2022 to an answer that JDH posted on meta.MO in 2013? $\endgroup$
    – Yemon Choi
    Dec 2, 2022 at 20:20
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    $\begingroup$ I'm not sure mere edits to a question or answer fit the bill of what I was envisaging. $\endgroup$
    – David Roberts Mod
    Dec 3, 2022 at 2:37
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    $\begingroup$ Please @DavidRoberts the post with title MathOverflow at the Joint Mathematics Meetings, Baltimore, January 2014 was edited (he is the original poster) by the professor. It fits in your Question: "What is your favourite post by JDH? It could be an Answer or a Question." $\endgroup$
    – user142929
    Dec 3, 2022 at 17:22
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    $\begingroup$ My mistake. But the comment meta.mathoverflow.net/questions/5446/… ? $\endgroup$
    – David Roberts Mod
    Dec 3, 2022 at 20:37
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    $\begingroup$ I've added the comment as companion of my answer. My answer in this post of Meta is the post edited on Meta with identifier 1186 by the professor that is owner of this post. If you feel that my post doesn't fit with your idea about this thread of posts I can to delete this my answer @DavidRoberts . I think that the contributions of professors on Meta and the building of the MathOverflow's community are also important, but really this is your website (the website of professors of mathematics in Internet), I invited myself years ago. As you decide, many thanks. $\endgroup$
    – user142929
    Dec 4, 2022 at 17:55
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    $\begingroup$ I emphasize that I don't want bother to professors or users here, if my post gets more downvotes I'm going to delete. Isn't required a response of this message, I hope that you've a good week @DavidRoberts $\endgroup$
    – user142929
    Dec 5, 2022 at 16:57
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    $\begingroup$ Please consider not deleting this. I do think pointing out Joel's non-mathematical contributions here is worthwhile. The style of the answer, giving id numbers to refer to posts, is perhaps unusual for some. $\endgroup$
    – David Roberts Mod
    Dec 5, 2022 at 19:55
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    $\begingroup$ If my comment isn't suitable in this comment thread I'm sorry, I apologize. My intention was remember the work of professor with simple words. Many thanks @YemonChoi and I wish you the best week. $\endgroup$
    – user142929
    Dec 7, 2022 at 16:46
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    $\begingroup$ Thank you @user142929. $\endgroup$ Dec 8, 2022 at 17:33
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    $\begingroup$ Thank you very much, this website is important thanks to professors as you @JoelDavidHamkins , PhD students and other mathematicians who dedicate their lives and time to edit mathematical content in their posts. $\endgroup$
    – user142929
    Dec 8, 2022 at 17:44

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