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.
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.
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?
Reposted by Dave Richeson
It’s always a highlight of the year.
The 2025 Headline of the Year Nominees
🧵
🧵
December 23, 2025 at 1:02 AM
It’s always a highlight of the year.
I'm rereading Cliff Stoll's 1989 book _The Cuckoo's Egg._ It's the true story of a 1986 incident in which Stoll, an astronomer working at Lawrence Berkeley National Laboratory, tracks down a hacker who gained access to their computer network.
In it, he discusses the logical implications of
In it, he discusses the logical implications of
Louis Armstrong - Everybody Loves My Baby
YouTube video by Okmusix
www.youtube.com
December 22, 2025 at 12:32 AM
I'm rereading Cliff Stoll's 1989 book _The Cuckoo's Egg._ It's the true story of a 1986 incident in which Stoll, an astronomer working at Lawrence Berkeley National Laboratory, tracks down a hacker who gained access to their computer network.
In it, he discusses the logical implications of
In it, he discusses the logical implications of
One of my least favorite words to see in a proof: "where." It can mean either "for all" or "for some." An example:
Suppose f(n)=0, where n∊A.
Suppose f(n)=0 for all n∊A.
Suppose f(n)=0 for some n∊A.
Similarly, just using the word "for":
Suppose f(n)=0 for n∊A.
Suppose f(n)=0, where n∊A.
Suppose f(n)=0 for all n∊A.
Suppose f(n)=0 for some n∊A.
Similarly, just using the word "for":
Suppose f(n)=0 for n∊A.
December 19, 2025 at 4:36 PM
One of my least favorite words to see in a proof: "where." It can mean either "for all" or "for some." An example:
Suppose f(n)=0, where n∊A.
Suppose f(n)=0 for all n∊A.
Suppose f(n)=0 for some n∊A.
Similarly, just using the word "for":
Suppose f(n)=0 for n∊A.
Suppose f(n)=0, where n∊A.
Suppose f(n)=0 for all n∊A.
Suppose f(n)=0 for some n∊A.
Similarly, just using the word "for":
Suppose f(n)=0 for n∊A.
I made a Spotify playlist of 50 songs from 2025 that I enjoyed (with my favorites at the top). One song per artist. It was a GOOD YEAR for music! (I know Spotify isn't a great company, but I'm locked in to a family plan with my kids and wife.)
open.spotify.com/playlist/7d3...
open.spotify.com/playlist/7d3...
50 Songs from 2025
open.spotify.com
December 17, 2025 at 6:10 PM
I made a Spotify playlist of 50 songs from 2025 that I enjoyed (with my favorites at the top). One song per artist. It was a GOOD YEAR for music! (I know Spotify isn't a great company, but I'm locked in to a family plan with my kids and wife.)
open.spotify.com/playlist/7d3...
open.spotify.com/playlist/7d3...
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
I saw Gwenifer Raymond's album _Last Night I Heard the Dog Star Bark_ on a "best of 2025" list. I'm loving it! But I also wonder whether she studied set theory and whether her song "Banjo Players Of Aleph One" is about the cardinality of the real numbers. gweniferraymond.bandcamp.com/track/banjo-...
Banjo Players of Aleph One, by Gwenifer Raymond
from the album Last Night I Heard The Dog Star Bark
gweniferraymond.bandcamp.com
December 12, 2025 at 3:25 PM
I saw Gwenifer Raymond's album _Last Night I Heard the Dog Star Bark_ on a "best of 2025" list. I'm loving it! But I also wonder whether she studied set theory and whether her song "Banjo Players Of Aleph One" is about the cardinality of the real numbers. gweniferraymond.bandcamp.com/track/banjo-...
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...
Today's the last day of my intro-to-proofs class. I decided to tell them about several ways to show that a set is countably infinite. Here's an ancient blog post with a few nonstandard methods. divisbyzero.com/2009/09/11/c...
Cardinality of infinite sets, part 1: four nonstandard proofs of countability
The study of cardinalities of infinite sets is one of the most intriguing areas of mathematics that an undergraduate mathematics major will encounter. It never fails to bring crooked smiles of joy,…
divisbyzero.com
December 11, 2025 at 3:22 PM
Today's the last day of my intro-to-proofs class. I decided to tell them about several ways to show that a set is countably infinite. Here's an ancient blog post with a few nonstandard methods. divisbyzero.com/2009/09/11/c...
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.”
Reposted by Dave Richeson
I was folding origami stars with this same technique this morning. bookzoompa.wordpress.com/2017/11/09/p...
November 30, 2025 at 5:42 PM
I was folding origami stars with this same technique this morning. bookzoompa.wordpress.com/2017/11/09/p...
Reposted by Dave Richeson
This seems to call it a square twist: origametry.net/ast/ast.html
The Augmented Square Twist
origametry.net
November 30, 2025 at 5:56 PM
This seems to call it a square twist: origametry.net/ast/ast.html
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.
Reposted by Dave Richeson
just upset the entire family by saying that a fork and spoon are topologically equivalent
November 29, 2025 at 5:42 PM
just upset the entire family by saying that a fork and spoon are topologically equivalent
Mathematical veggies in today’s CSA box.
November 26, 2025 at 4:30 AM
Mathematical veggies in today’s CSA box.
New blog post from @mrhonner.com.web.brid.gy: He says that Naviance, the go-to portal for high school students applying to colleges, now has a "Compose with AI" option for teachers writing letters of recommendation. 😢 mrhonner.com/archives/21796
AI-Generated Letters of Recommendation
I write around 30 recommendation letters a year. These are mostly for students applying to college, but increasingly I’m asked to write recs for competitive summer programs, private schools, schola…
mrhonner.com
November 25, 2025 at 3:39 AM
New blog post from @mrhonner.com.web.brid.gy: He says that Naviance, the go-to portal for high school students applying to colleges, now has a "Compose with AI" option for teachers writing letters of recommendation. 😢 mrhonner.com/archives/21796