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.]
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!
Thanks for the information and conversation! The funny thing is, I had 1936 in the blog post. But yesterday, before I shared the link, I went to the journal's website and saw 1937, so I changed it! I appreciate your sleuthing. I'll change it back! londmathsoc.onlinelibrary.wiley.com/doi/abs/10.1...
January 6, 2026 at 2:32 PM
Thanks for the information and conversation! The funny thing is, I had 1936 in the blog post. But yesterday, before I shared the link, I went to the journal's website and saw 1937, so I changed it! I appreciate your sleuthing. I'll change it back! londmathsoc.onlinelibrary.wiley.com/doi/abs/10.1...
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! 🤣🤣🤣
to gearing choices. So, I've been reading up on gearing concepts and relevant terminology. Very cool stuff! Here's a @geogebra.org applet I made while I was playing around with the concepts. www.geogebra.org/m/bbqgu4ws
December 30, 2025 at 1:43 PM
to gearing choices. So, I've been reading up on gearing concepts and relevant terminology. Very cool stuff! Here's a @geogebra.org applet I made while I was playing around with the concepts. www.geogebra.org/m/bbqgu4ws
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
That looks familiar! Here’a a photo I took in Detroit in 2023!
December 28, 2025 at 11:36 PM
That looks familiar! Here’a a photo I took in Detroit in 2023!
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.
We got a bird feeder with a camera in it for my parents for Christmas. I had an idea for how it could be the basis of a New Yorker comic. Here’s what I came up with (artwork by ChatGPT).
December 26, 2025 at 3:00 AM
We got a bird feeder with a camera in it for my parents for Christmas. I had an idea for how it could be the basis of a New Yorker comic. Here’s what I came up with (artwork by ChatGPT).
Went for a post-meal walk today and saw a vehicle had driven through paint. My phone measured the distance between the marks as 8 ft. 8 in. = 104 in. So, the diameter of the tires were 104/π=33 in. It must have been a large-wheeled vehicle like a truck or jeep.
December 26, 2025 at 2:55 AM
Went for a post-meal walk today and saw a vehicle had driven through paint. My phone measured the distance between the marks as 8 ft. 8 in. = 104 in. So, the diameter of the tires were 104/π=33 in. It must have been a large-wheeled vehicle like a truck or jeep.
My son was telling me about how, when you copy something, several versions are saved for pasting. This website allows you to see what's on the pasteboard. evercoder.github.io/clipboard-in... I copied some Excel cells, and it saved the cells as HTML, plaintext, and an image of the grid. Who knew?
December 23, 2025 at 2:09 PM
My son was telling me about how, when you copy something, several versions are saved for pasting. This website allows you to see what's on the pasteboard. evercoder.github.io/clipboard-in... I copied some Excel cells, and it saved the cells as HTML, plaintext, and an image of the grid. Who knew?
Ben Sparks made this wonderful GeoGebra app that lets you create a twisted band (Möbius strip, etc.) and cut it in half or into thirds. As someone who uses GeoGebra quite a bit: Wow! www.geogebra.org/m/v5z33vth
December 17, 2025 at 4:15 PM
Ben Sparks made this wonderful GeoGebra app that lets you create a twisted band (Möbius strip, etc.) and cut it in half or into thirds. As someone who uses GeoGebra quite a bit: Wow! www.geogebra.org/m/v5z33vth
My colleague shared this with me: Regexle (a daily RegEx hexagonal crossword puzzle). regexle.com
December 14, 2025 at 12:02 AM
My colleague shared this with me: Regexle (a daily RegEx hexagonal crossword puzzle). regexle.com
As an exercise, I drew a random K_6 and highlighted all pairs of triangles. Five pairs of triangles are linked (the number under the figure is the absolute value of the linking number).
December 11, 2025 at 8:54 PM
As an exercise, I drew a random K_6 and highlighted all pairs of triangles. Five pairs of triangles are linked (the number under the figure is the absolute value of the linking number).
Here are a couple of neat theorems due to John Conway and Cameron Gordon (1983):
Any embedding of K_6 in 3D has two linked triangles (red and blue below).
Any embedding of K_7 in 3D has a knotted Hamiltonian circuit (red below).
Article: people.reed.edu/~ormsbyk/138...
Any embedding of K_6 in 3D has two linked triangles (red and blue below).
Any embedding of K_7 in 3D has a knotted Hamiltonian circuit (red below).
Article: people.reed.edu/~ormsbyk/138...
December 11, 2025 at 8:16 PM
Here are a couple of neat theorems due to John Conway and Cameron Gordon (1983):
Any embedding of K_6 in 3D has two linked triangles (red and blue below).
Any embedding of K_7 in 3D has a knotted Hamiltonian circuit (red below).
Article: people.reed.edu/~ormsbyk/138...
Any embedding of K_6 in 3D has two linked triangles (red and blue below).
Any embedding of K_7 in 3D has a knotted Hamiltonian circuit (red below).
Article: people.reed.edu/~ormsbyk/138...
One of my independent study students in knot theory made me this crocheted Möbius band (using Shiying Dong's video tutorial)!
December 10, 2025 at 8:08 PM
One of my independent study students in knot theory made me this crocheted Möbius band (using Shiying Dong's video tutorial)!
I had a great time seeing The Beths last night at Union Transfer! I love that they opened the set with a song about topology, "The Straight Line Was a Lie." 😂😂😂
I thought I was getting better
But I'm back to where I started
And the straight line was a circle
Yeah, the straight line was a lie
I thought I was getting better
But I'm back to where I started
And the straight line was a circle
Yeah, the straight line was a lie
December 8, 2025 at 2:49 PM
I had a great time seeing The Beths last night at Union Transfer! I love that they opened the set with a song about topology, "The Straight Line Was a Lie." 😂😂😂
I thought I was getting better
But I'm back to where I started
And the straight line was a circle
Yeah, the straight line was a lie
I thought I was getting better
But I'm back to where I started
And the straight line was a circle
Yeah, the straight line was a lie
In order to make this origami star, you need a piece of paper in the shape of a regular pentagon. There's a standard method of cutting a pentagon from a square piece of paper. The interesting thing is: it is only an approximation! www.youtube.com/watch?v=ezQV...
December 5, 2025 at 5:04 PM
In order to make this origami star, you need a piece of paper in the shape of a regular pentagon. There's a standard method of cutting a pentagon from a square piece of paper. The interesting thing is: it is only an approximation! www.youtube.com/watch?v=ezQV...
From my daughter, but could be for any of my math followers. Today's title: “It all adds up.”
December 3, 2025 at 9:40 PM
From my daughter, but could be for any of my math followers. Today's title: “It all adds up.”
I finally got around to making these. It also took me several tries, but now I think I have it down so I could do it again without as much trouble. Thanks for pointing me to @paulakrieg.bsky.social's blog post!
December 3, 2025 at 3:42 AM
I finally got around to making these. It also took me several tries, but now I think I have it down so I could do it again without as much trouble. Thanks for pointing me to @paulakrieg.bsky.social's blog post!
By the way, the back side is cool too! I just whipped up this image showing the fold lines, in case you want to make your own. I was given a folded version of this design. I'd love to know the story behind it or the name if it has one.
November 30, 2025 at 5:06 PM
By the way, the back side is cool too! I just whipped up this image showing the fold lines, in case you want to make your own. I was given a folded version of this design. I'd love to know the story behind it or the name if it has one.
I learned this folding technique yesterday, so I had to make something with it.
November 30, 2025 at 4:39 PM
I learned this folding technique yesterday, so I had to make something with it.
Mathematical veggies in today’s CSA box.
November 26, 2025 at 4:30 AM
Mathematical veggies in today’s CSA box.