Dave Richeson
@divbyzero.bsky.social
Mathematician. John J. & Ann Curley Chair in Liberal Arts at Dickinson College. Author of Tales of Impossibility and Euler's Gem. Coffee drinker. [Everything in the timeline before October 2024 was imported from my Twitter/X feed 2008-24.]
Pinned
Dave Richeson
@divbyzero.bsky.social
· Oct 21
Since I'm new here, I'll introduce myself. I'm a math prof at Dickinson College. I'm author of Euler's Gem and Tales of Impossibility. I was editor of Math Horizons. I am interested in topology, dynamical systems, geometry, history of math, recreational math, math & art, and expository math writing.
Still playing with these impossible cylinders. Here I have an ellipse with the "Batman curve" in the reflection.
February 11, 2026 at 3:09 PM
Still playing with these impossible cylinders. Here I have an ellipse with the "Batman curve" in the reflection.
How long do I cook for a crispier crust?
Instructions: Wait until the pizza looks perfect. Nope! You're wrong. Close the oven door and cook one more minute.
Math version: What is [12,15] +1?
(Oh, no! I added peppers: what's [12,15] +1 + [2,3]???)
Instructions: Wait until the pizza looks perfect. Nope! You're wrong. Close the oven door and cook one more minute.
Math version: What is [12,15] +1?
(Oh, no! I added peppers: what's [12,15] +1 + [2,3]???)
February 10, 2026 at 1:28 AM
How long do I cook for a crispier crust?
Instructions: Wait until the pizza looks perfect. Nope! You're wrong. Close the oven door and cook one more minute.
Math version: What is [12,15] +1?
(Oh, no! I added peppers: what's [12,15] +1 + [2,3]???)
Instructions: Wait until the pizza looks perfect. Nope! You're wrong. Close the oven door and cook one more minute.
Math version: What is [12,15] +1?
(Oh, no! I added peppers: what's [12,15] +1 + [2,3]???)
One more impossible shape. This one is a moon/star.
February 9, 2026 at 11:35 PM
One more impossible shape. This one is a moon/star.
Here are a couple more impossible cylinders: A heart/infinity symbol and a cardioid whose reflection is not reversed by the mirror.
February 9, 2026 at 3:57 PM
Here are a couple more impossible cylinders: A heart/infinity symbol and a cardioid whose reflection is not reversed by the mirror.
A fun Sunday activity: designing and printing some more impossible cylinders. Here is one I call “Division by Zero.” 🤣
February 8, 2026 at 7:55 PM
A fun Sunday activity: designing and printing some more impossible cylinders. Here is one I call “Division by Zero.” 🤣
I just realized that when I'm in Ireland this summer for the Bridges conference, I'll be able to visit Hamilton's Broom Bridge when I go through Dublin! en.wikipedia.org/wiki/Broom_B...
February 5, 2026 at 10:09 PM
I just realized that when I'm in Ireland this summer for the Bridges conference, I'll be able to visit Hamilton's Broom Bridge when I go through Dublin! en.wikipedia.org/wiki/Broom_B...
I created some OpenSCAD code to print these slices/traces of surfaces for a multivariable calculus class. I wrote about it on my blog and included a link to my GitHub page with the code.
divisbyzero.com/2026/02/01/3...
divisbyzero.com/2026/02/01/3...
February 1, 2026 at 7:08 PM
I created some OpenSCAD code to print these slices/traces of surfaces for a multivariable calculus class. I wrote about it on my blog and included a link to my GitHub page with the code.
divisbyzero.com/2026/02/01/3...
divisbyzero.com/2026/02/01/3...
That situation where you run into an acquaintance in the produce section of a grocery store. And then in the next aisle. And then the next one. And then the next one...
February 1, 2026 at 4:44 PM
That situation where you run into an acquaintance in the produce section of a grocery store. And then in the next aisle. And then the next one. And then the next one...
Game my daughter showed me: Redactle. Each game is a Wikipedia article with all non-filler words redacted. You type words at random. If you find a word in the article, it tells you, and all instances are unredacted (as well as related words such as plurals and -ing forms). The goal is to find the
redactle.net
January 30, 2026 at 2:06 PM
Game my daughter showed me: Redactle. Each game is a Wikipedia article with all non-filler words redacted. You type words at random. If you find a word in the article, it tells you, and all instances are unredacted (as well as related words such as plurals and -ing forms). The goal is to find the
An inauspicious start to the day: I dropped a full box of pushpins, and my office is now fully booby-trapped for the Home Alone burglars.
Good thing I don’t believe in omens or signs.
Good thing I don’t believe in omens or signs.
January 29, 2026 at 1:43 PM
An inauspicious start to the day: I dropped a full box of pushpins, and my office is now fully booby-trapped for the Home Alone burglars.
Good thing I don’t believe in omens or signs.
Good thing I don’t believe in omens or signs.
I'm excited for this new book by my friend and colleague John MacCormick! He's an amazing scholar and writer. I'm sure this will be great.
Kirkus Reviews calls Thinking AI (publishing 4/21) a "clear-eyed guide to modern artificial intelligence. . . . The book moves briskly through core ideas in AI and philosophy of mind, without oversimplifying." Check out the full review:
THINKING AI | Kirkus Reviews
Grappling with a deceptively simple question: How can computer programs appear to think like humans?
www.kirkusreviews.com
January 29, 2026 at 1:58 AM
I'm excited for this new book by my friend and colleague John MacCormick! He's an amazing scholar and writer. I'm sure this will be great.
I printed four n=7th iterate dragon curves. You can fit two together to get n=8 dragon curve, and four together to get n=9 dragon curve. You can also fit them together in a way that tiles the plane.
January 28, 2026 at 6:23 PM
I printed four n=7th iterate dragon curves. You can fit two together to get n=8 dragon curve, and four together to get n=9 dragon curve. You can also fit them together in a way that tiles the plane.
I've been playing with the new 3D printer I got over winter break (Bambu P2S). I designed and printed these three (approximate) space-filling curves: Peano's (blue), Hilbert's (purple), and Moore's (orange). The walls are 1 mm wide and 6 mm high, so they flex when you hold them. I was going to
January 28, 2026 at 2:05 PM
I've been playing with the new 3D printer I got over winter break (Bambu P2S). I designed and printed these three (approximate) space-filling curves: Peano's (blue), Hilbert's (purple), and Moore's (orange). The walls are 1 mm wide and 6 mm high, so they flex when you hold them. I was going to
I always find it so interesting how different classes of students can be. I walked into the first meeting of my 9:00 AM calculus class a few minutes early, expecting the usual room of sleepy students all on their phones. But they were almost all there, happily chatting with each other!
January 21, 2026 at 4:06 PM
I always find it so interesting how different classes of students can be. I walked into the first meeting of my 9:00 AM calculus class a few minutes early, expecting the usual room of sleepy students all on their phones. But they were almost all there, happily chatting with each other!
You know you are a mathematician when… you are invited to a holiday party at a fancy mansion, and you mostly take pictures of the beautiful tiling patterns in the wood floor!
January 13, 2026 at 2:07 AM
You know you are a mathematician when… you are invited to a holiday party at a fancy mansion, and you mostly take pictures of the beautiful tiling patterns in the wood floor!
Reposted by Dave Richeson
For what it's worth, I tangled with the same year-of-publication question until I came across this (fascinating!) footnote in Copeland's book "The Essential Turing", and it sounded authoritative enough for me to take it as gospel.
![A footnote from Copeland's book "The Essential Turing". It reads:
"Proceedings of the London Mathematical Society, 42 (1936–7), 230–65. The publication date of ‘On Computable Numbers’ is sometimes cited, incorrectly, as 1937. The article was published in two parts, both parts appearing in 1936. The break between the two parts occurred, rather inelegantly, in the middle of Section 5, at the bottom of p. 240 (p. 67 in the present volume). Pages 230–40 appeared in part 3 of volume 42, issued on 30 Nov. 1936, and the remainder of the article appeared in part 4, issued on 23 Dec. 1936. This information is given on the title pages of parts 3 and 4 of volume 42, which show the contents of each part and their dates of issue. (I am grateful to Robert Soare for sending me these pages. See R. I. Soare, ‘Computability and Recursion’, Bulletin of Symbolic Logic, 2 (1996), 284–321.)
The article was published bearing the information ‘Received 28 May, 1936.—Read 12 November, 1936.’ However, Turing was in the United States on 12 November, having left England in September 1936 for what was to be a stay of almost two years (see the introductions to Chapters 3 and 4). Although papers were read at the meetings of the London Mathematical Society, many of those published in the Proceedings were ‘taken as read’, the author not necessarily being present at the meeting in question. Mysteriously, the minutes of the meeting held on 18 June 1936 list ‘On Computable Numbers, with an Application to the Entscheidungs- problem’ as one of 22 papers taken as read at that meeting. The minutes of an Annual General Meeting held [...]"](https://cdn.bsky.app/img/feed_thumbnail/plain/did:plc:f6tvcgb4p6fmvinz23kus77k/bafkreicnvavdeuakfijlzm5z5t3pv7p6ppou3lohgligl7jumyvi3czwh4@jpeg)
![The continuation of a footnote from Copeland's book "The Essential Turing". It reads:
"[...] on 12 Nov. 1936 contain no reference to the paper. (I am grateful to Janet Foster, Archives Consultant to the London Mathematical Society, for information.)"](https://cdn.bsky.app/img/feed_thumbnail/plain/did:plc:f6tvcgb4p6fmvinz23kus77k/bafkreicbeixwjoo5ceveokhzdu3tw7zqmdmg77l2deuhgqxtkfibr4snda@jpeg)
January 5, 2026 at 11:10 PM
For what it's worth, I tangled with the same year-of-publication question until I came across this (fascinating!) footnote in Copeland's book "The Essential Turing", and it sounded authoritative enough for me to take it as gospel.
Years ago (15?) I must have written my name and email address somewhere, and someone typed it as "Dale" instead of "Dave." It is interesting to observe the waves of Dale-email I get when some spammer buys that list of addresses. I hadn't for a while until a week or so ago, and now I'm flooded again.
January 5, 2026 at 5:09 PM
Years ago (15?) I must have written my name and email address somewhere, and someone typed it as "Dale" instead of "Dave." It is interesting to observe the waves of Dale-email I get when some spammer buys that list of addresses. I hadn't for a while until a week or so ago, and now I'm flooded again.
Here's an old blog post I wrote about a footnote in Turing's 1937 article "On computable numbers with an application to the Entscheidungsproblem," in which he gave a topological argument that any alphabet must be finite. divisbyzero.com/2010/05/27/t...
Turing’s topological proof that every written alphabet is finite
Recently one of my colleagues was reading Alan Turing’s groundbreaking 1936 article “On Computable Numbers with an Application to the Entscheidungsproblem.” This is the article in…
divisbyzero.com
January 5, 2026 at 4:48 PM
Here's an old blog post I wrote about a footnote in Turing's 1937 article "On computable numbers with an application to the Entscheidungsproblem," in which he gave a topological argument that any alphabet must be finite. divisbyzero.com/2010/05/27/t...
Reposted by Dave Richeson
Super-ambitious project by Jonatan Nielavitzky: a 3D printed Babbage Difference Engine! He says it's a work-in-progress but it looks pretty polished to me.
Show him some appreciation on youtube if you have a moment! (I'm trying to encourage him to keep going)
www.youtube.com/watch?v=NvOR...
Show him some appreciation on youtube if you have a moment! (I'm trying to encourage him to keep going)
www.youtube.com/watch?v=NvOR...
3D Printed Difference Engine
YouTube video by Jonatan Nielavitzky
www.youtube.com
January 5, 2026 at 2:26 PM
Super-ambitious project by Jonatan Nielavitzky: a 3D printed Babbage Difference Engine! He says it's a work-in-progress but it looks pretty polished to me.
Show him some appreciation on youtube if you have a moment! (I'm trying to encourage him to keep going)
www.youtube.com/watch?v=NvOR...
Show him some appreciation on youtube if you have a moment! (I'm trying to encourage him to keep going)
www.youtube.com/watch?v=NvOR...
My year in books: Goodreads said I read 49 books in 2025. However, six were DNFs (a record for me in one year?). As in previous years, I gravitated toward novels, but it is the nonfiction books that really stuck with me. Here are my favorite reads of the year, in no particular order.
January 2, 2026 at 8:07 PM
My year in books: Goodreads said I read 49 books in 2025. However, six were DNFs (a record for me in one year?). As in previous years, I gravitated toward novels, but it is the nonfiction books that really stuck with me. Here are my favorite reads of the year, in no particular order.
My son was **very** pleased with this gift he bought me: a Klein bottle opener! 🤣🤣🤣
January 1, 2026 at 10:36 PM
My son was **very** pleased with this gift he bought me: a Klein bottle opener! 🤣🤣🤣
I saw my college friend Josh Hudak last week. He makes stunning orreries (mechanical models of the solar system), including the glass planets, all the gears, etc. (His entry point was via glass blowing.) Check out his work here www.instagram.com/planetarymac.... He had some questions for me related
December 30, 2025 at 1:43 PM
I saw my college friend Josh Hudak last week. He makes stunning orreries (mechanical models of the solar system), including the glass planets, all the gears, etc. (His entry point was via glass blowing.) Check out his work here www.instagram.com/planetarymac.... He had some questions for me related
I was listening to an interview with Johnny Marr. He shared this quote by Pablo Picasso, which I think is great:
“Inspiration exists, but it has to find you working.”
“Inspiration exists, but it has to find you working.”
December 28, 2025 at 11:32 PM
I was listening to an interview with Johnny Marr. He shared this quote by Pablo Picasso, which I think is great:
“Inspiration exists, but it has to find you working.”
“Inspiration exists, but it has to find you working.”
Another winter season, another example of four and eight pointed snowflakes.
December 28, 2025 at 5:54 PM
Another winter season, another example of four and eight pointed snowflakes.
Computer scientists probably know this, but TIL that the Python programming language was named after Monty Python.
December 26, 2025 at 4:20 PM
Computer scientists probably know this, but TIL that the Python programming language was named after Monty Python.