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.
Theorem. There exist irrational numbers x and y such that x^y is rational.
November 13, 2025 at 10:03 PM
Theorem. There exist irrational numbers x and y such that x^y is rational.
I'm about to start watching Death by Lightning, a four-episode drama about James Garfield. I hope they can squeeze in Garfield's proof of the Pythagorean theorem! www.youtube.com/watch?v=6jop...
Death by Lightning | Official Trailer | Netflix
YouTube video by Netflix
www.youtube.com
November 13, 2025 at 2:53 AM
I'm about to start watching Death by Lightning, a four-episode drama about James Garfield. I hope they can squeeze in Garfield's proof of the Pythagorean theorem! www.youtube.com/watch?v=6jop...
I brought my cardioid to campus.
November 11, 2025 at 7:57 PM
I brought my cardioid to campus.
Reposted by Dave Richeson
I really enjoyed the Scientific American article on this experiment: bsky.app/profile/moti...
Guys will tell you they know a color then take you to a darkened room, make you bite down on a bar, and shine a laser in your eyes so it only stimulates M cones.
Five people—three researchers and two test subjects—have just seen a color no human has ever seen before.
November 11, 2025 at 5:33 PM
I really enjoyed the Scientific American article on this experiment: bsky.app/profile/moti...
TIL about the color "olo," which only five humans have seen. Olo is short for (0,1,0), which means that the scientists figured out how to excite only the middle-wavelength M-cone in the eye. This hue below is most similar to olo. en.wikipedia.org/wiki/Olo_(co...
November 11, 2025 at 5:28 PM
TIL about the color "olo," which only five humans have seen. Olo is short for (0,1,0), which means that the scientists figured out how to excite only the middle-wavelength M-cone in the eye. This hue below is most similar to olo. en.wikipedia.org/wiki/Olo_(co...
My friend showed me this daily web game: Which Year? It shows five photos, and you have to guess the year for each one. You can get four free digits with no penalty, but each digit can be used only once (in other words, you can get the thousands, hundreds, tens, or ones each only once). whichyr.com
Which Year - Photo Year Guessing Game
Guess the year real-world photos were taken. Test your history knowledge with a daily challenge featuring a new set of photos each day.
whichyr.com
November 10, 2025 at 8:39 PM
My friend showed me this daily web game: Which Year? It shows five photos, and you have to guess the year for each one. You can get four free digits with no penalty, but each digit can be used only once (in other words, you can get the thousands, hundreds, tens, or ones each only once). whichyr.com
Giant cardioid: version 2.5. Version 1.0 was made using Pex tubing and grommet tape. Version 2.0 was made from laser-cut pieces. The wood was too fragile, and it broke during construction. In version 2.5, I made the pieces wider and I used thicker wood. Success, I'd say! The diameter is about 5 ft.
November 9, 2025 at 4:34 PM
Giant cardioid: version 2.5. Version 1.0 was made using Pex tubing and grommet tape. Version 2.0 was made from laser-cut pieces. The wood was too fragile, and it broke during construction. In version 2.5, I made the pieces wider and I used thicker wood. Success, I'd say! The diameter is about 5 ft.
I had a really nice couple of days at Oberlin College hanging out with @baabbbaash.bsky.social, his colleagues, and students! While walking around campus, I saw this unusual window. Assuming it is the unit circle, what's the equation of the ellipse?
November 8, 2025 at 12:10 AM
I had a really nice couple of days at Oberlin College hanging out with @baabbbaash.bsky.social, his colleagues, and students! While walking around campus, I saw this unusual window. Assuming it is the unit circle, what's the equation of the ellipse?
Reposted by Dave Richeson
*The Mathematics of Origami*.
Expected online publication date: December 2025. Print publication: 31 December 2025.
www.science.smith.edu/~jorourke/Ma...
#MathSky #Mathematics 🧪 #Geometry #Origami #MathArt
Expected online publication date: December 2025. Print publication: 31 December 2025.
www.science.smith.edu/~jorourke/Ma...
#MathSky #Mathematics 🧪 #Geometry #Origami #MathArt
November 5, 2025 at 3:01 PM
*The Mathematics of Origami*.
Expected online publication date: December 2025. Print publication: 31 December 2025.
www.science.smith.edu/~jorourke/Ma...
#MathSky #Mathematics 🧪 #Geometry #Origami #MathArt
Expected online publication date: December 2025. Print publication: 31 December 2025.
www.science.smith.edu/~jorourke/Ma...
#MathSky #Mathematics 🧪 #Geometry #Origami #MathArt
Reposted by Dave Richeson
[Edited] On behalf of the Oberlin Mathematics Department, I am thrilled to say that Dave Richeson will be delivering this year's Tamura/Lilly Lecture, "A Romance of Many (and Fractional) Dimensions," on 11.6.25. The reception will be at 3:30pm in King 203. The talk will be at 4:30pm in King 337.
I'm excited to give this year's Tamura/Lily lecture at Oberlin College (this Thursday, November 6, 4:30 pm). It will be nice to see @baabbbaash.bsky.social, his colleagues, and his students. If you are at Oberlin, stop by! www.oberlin.edu/events/mathe...
Mathematics Tamura/Lilly Lecture Series with David Richeson | Oberlin College and Conservatory
Title: A Romance of Many (and Fractional) Dimensions
www.oberlin.edu
November 3, 2025 at 8:17 PM
[Edited] On behalf of the Oberlin Mathematics Department, I am thrilled to say that Dave Richeson will be delivering this year's Tamura/Lilly Lecture, "A Romance of Many (and Fractional) Dimensions," on 11.6.25. The reception will be at 3:30pm in King 203. The talk will be at 4:30pm in King 337.
I'm excited to give this year's Tamura/Lily lecture at Oberlin College (this Thursday, November 6, 4:30 pm). It will be nice to see @baabbbaash.bsky.social, his colleagues, and his students. If you are at Oberlin, stop by! www.oberlin.edu/events/mathe...
Mathematics Tamura/Lilly Lecture Series with David Richeson | Oberlin College and Conservatory
Title: A Romance of Many (and Fractional) Dimensions
www.oberlin.edu
November 3, 2025 at 3:18 PM
I'm excited to give this year's Tamura/Lily lecture at Oberlin College (this Thursday, November 6, 4:30 pm). It will be nice to see @baabbbaash.bsky.social, his colleagues, and his students. If you are at Oberlin, stop by! www.oberlin.edu/events/mathe...
My son sent this to me: How to play the Severnce theme song in Desmos. www.youtube.com/watch?v=qdzH...
The Severance Theme... but it's Desmos
YouTube video by Marc Evanstein / music․py
www.youtube.com
November 1, 2025 at 8:45 PM
My son sent this to me: How to play the Severnce theme song in Desmos. www.youtube.com/watch?v=qdzH...
In case anyone wants to make some of these boards, I put the laser-cutter templates on my blog. divisbyzero.com/2025/11/01/a...
November 1, 2025 at 6:10 PM
In case anyone wants to make some of these boards, I put the laser-cutter templates on my blog. divisbyzero.com/2025/11/01/a...
I just taught induction in my intro-to-proofs class. I told them the base case is usually easy and the inductive step is more challenging. Here's a blog post I wrote giving an example of the opposite. TL;DR: the product rule for n functions. divisbyzero.com/2018/11/07/p...
October 30, 2025 at 5:48 PM
I just taught induction in my intro-to-proofs class. I told them the base case is usually easy and the inductive step is more challenging. Here's a blog post I wrote giving an example of the opposite. TL;DR: the product rule for n functions. divisbyzero.com/2018/11/07/p...
I met with my knot theory independent study students today. The topic: braids. I broke out these "BraidTiles" that I made several years ago. You can download a printable pdf here (recommendation: print them on cardstock): divisbyzero.com/2019/05/01/b...
October 29, 2025 at 7:09 PM
I met with my knot theory independent study students today. The topic: braids. I broke out these "BraidTiles" that I made several years ago. You can download a printable pdf here (recommendation: print them on cardstock): divisbyzero.com/2019/05/01/b...
In my intro-to-proofs class, I like to assign induction problems that aren't only sum/product formulas. For this problem, I made and laser cut this puzzle: Begin with a 2ⁿx2ⁿ grid of squares. Black out a single square. Prove that it is possible to tile the rest of the grid with trominos.
October 29, 2025 at 1:56 PM
In my intro-to-proofs class, I like to assign induction problems that aren't only sum/product formulas. For this problem, I made and laser cut this puzzle: Begin with a 2ⁿx2ⁿ grid of squares. Black out a single square. Prove that it is possible to tile the rest of the grid with trominos.
I don't believe in omens, but dropping a full box of thumbtacks on the floor was an inauspicious start to the day. 😬
October 29, 2025 at 1:34 PM
I don't believe in omens, but dropping a full box of thumbtacks on the floor was an inauspicious start to the day. 😬
🎻😭🎻
(Cheating service for students shedding jobs because of new, more powerful, and convenient ways to cheat.)
(Cheating service for students shedding jobs because of new, more powerful, and convenient ways to cheat.)
Hit by AI, edtech firm Chegg slashes jobs and names new CEO in major overhaul reut.rs/4ntryLL
Hit by AI, edtech firm Chegg slashes jobs and names new CEO in major overhaul
Educational technology company Chegg said on Monday it would cut 388 roles globally, or about 45% of the workforce, to reduce costs and streamline operations as it works to adapt to the growing shift toward AI-powered tools.
reut.rs
October 28, 2025 at 6:46 PM
🎻😭🎻
(Cheating service for students shedding jobs because of new, more powerful, and convenient ways to cheat.)
(Cheating service for students shedding jobs because of new, more powerful, and convenient ways to cheat.)
If you're looking for a Halloween activity to do with your kids or students, you can make a "Flex-a-ghoul"—this is a Halloween-themed hexaflexagon that I made. There's a printable PDF on my blog: divisbyzero.com/2022/11/01/h...
October 27, 2025 at 3:02 PM
If you're looking for a Halloween activity to do with your kids or students, you can make a "Flex-a-ghoul"—this is a Halloween-themed hexaflexagon that I made. There's a printable PDF on my blog: divisbyzero.com/2022/11/01/h...
I am doing some laser cutting outside. It is producing a lot of smoke! I am hoping the neighbors don’t call the fire department.
October 27, 2025 at 1:42 PM
I am doing some laser cutting outside. It is producing a lot of smoke! I am hoping the neighbors don’t call the fire department.
I met with my independent study knot theory students today. We discussed Seifert surfaces (orientable surfaces whose boundaries are knots and links), and I showed them @mathforge.org's Seifert surface pieces. They had fun playing with them. github.com/loopspace/Se...
October 22, 2025 at 7:12 PM
I met with my independent study knot theory students today. We discussed Seifert surfaces (orientable surfaces whose boundaries are knots and links), and I showed them @mathforge.org's Seifert surface pieces. They had fun playing with them. github.com/loopspace/Se...
Another Martin Gardner problem from yesterday's G4G talk: I've dealt someone 13 cards. They say, "I have an ace." What is the probability that they have another ace?
What if, instead, they say, "I have the ace of spades."
Same probability? Different?
What if, instead, they say, "I have the ace of spades."
Same probability? Different?
October 22, 2025 at 3:55 PM
Another Martin Gardner problem from yesterday's G4G talk: I've dealt someone 13 cards. They say, "I have an ace." What is the probability that they have another ace?
What if, instead, they say, "I have the ace of spades."
Same probability? Different?
What if, instead, they say, "I have the ace of spades."
Same probability? Different?
It is fall break here. I asked for volunteers who happened to be on campus to test my idea for making a giant string-art cardioid. It worked pretty well, and I have some concrete ideas for improvement in version 2.0. (This circle has about a 20 ft. circumference and 239 holes.)
October 21, 2025 at 8:43 PM
It is fall break here. I asked for volunteers who happened to be on campus to test my idea for making a giant string-art cardioid. It worked pretty well, and I have some concrete ideas for improvement in version 2.0. (This circle has about a 20 ft. circumference and 239 holes.)
Here's a neat puzzle from Martin Gardner's January 1958 column. It is surprising (but true) that you can turn a punctured torus ("inner tube") inside out. Suppose we drew a circle on the outside and the inside of the tube; they are linked. Turn the torus inside out. Now they are not linked. Explain!
October 21, 2025 at 4:59 PM
Here's a neat puzzle from Martin Gardner's January 1958 column. It is surprising (but true) that you can turn a punctured torus ("inner tube") inside out. Suppose we drew a circle on the outside and the inside of the tube; they are linked. Turn the torus inside out. Now they are not linked. Explain!