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.
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
Worth watching. (It's also available on the NY Times Daily podcast feed.) www.youtube.com/watch?v=9zDA...
John Green on Anxiety, Hope and His Next Book | The Interview
YouTube video by The Interview
www.youtube.com
November 23, 2025 at 5:12 PM
Worth watching. (It's also available on the NY Times Daily podcast feed.) www.youtube.com/watch?v=9zDA...
For anyone interested, I wrote a "how-to" guide for making this large string art cardioid. There's a link on my blog. I have also included the PDF files for laser-cutting the pieces. Enjoy! divisbyzero.com/2025/11/22/c...
Constructing a Large 251-Point String Art Cardioid
I recently designed and made a string art cardioid sculpture, which is now hanging in our mathematics department’s window. It is made from laser-cut plywood and yarn. I wrote a “how-to&…
divisbyzero.com
November 22, 2025 at 4:01 PM
For anyone interested, I wrote a "how-to" guide for making this large string art cardioid. There's a link on my blog. I have also included the PDF files for laser-cutting the pieces. Enjoy! divisbyzero.com/2025/11/22/c...
"Over the past five years, the report said, the number of incoming students whose math skills fall below middle-school standards increased nearly thirtyfold — representing roughly one in eight freshmen — despite the fact that they had strong high-school grades."
www.chronicle.com/article/peop...
www.chronicle.com/article/peop...
A ‘Steep Decline’ in Students’ Academic Preparation at UC-San Diego Struck a Nerve
Faculty members published a new report documenting the subpar math and writing skills of the first-year class at one of the nation’s most selective institutions.
www.chronicle.com
November 21, 2025 at 2:09 PM
"Over the past five years, the report said, the number of incoming students whose math skills fall below middle-school standards increased nearly thirtyfold — representing roughly one in eight freshmen — despite the fact that they had strong high-school grades."
www.chronicle.com/article/peop...
www.chronicle.com/article/peop...
Play this classic Martin Gardner puzzle: The Red-Faced Cube.
Rules: The cube has one red face. "Roll" it around this board so it visits every square once, ends at the green square, and never has its red face up (except in the starting and ending squares). erkal.github.io/RedFacedCube...
Rules: The cube has one red face. "Roll" it around this board so it visits every square once, ends at the green square, and never has its red face up (except in the starting and ending squares). erkal.github.io/RedFacedCube...
November 20, 2025 at 10:21 PM
Play this classic Martin Gardner puzzle: The Red-Faced Cube.
Rules: The cube has one red face. "Roll" it around this board so it visits every square once, ends at the green square, and never has its red face up (except in the starting and ending squares). erkal.github.io/RedFacedCube...
Rules: The cube has one red face. "Roll" it around this board so it visits every square once, ends at the green square, and never has its red face up (except in the starting and ending squares). erkal.github.io/RedFacedCube...
NY Times Spelling Bee from a few days ago. DOMAIN was accepted, but I'm disappointed that CODOMAIN wasn't. It would have been a panagram.
November 19, 2025 at 4:18 PM
NY Times Spelling Bee from a few days ago. DOMAIN was accepted, but I'm disappointed that CODOMAIN wasn't. It would have been a panagram.
Today, in class, we discussed "the barber paradox" and "Russell's paradox." As I do every time I talk about this, I plugged this excellent graphic novel (really, nonfiction with slight embellishments, all acknowledged at the end of the book): Logicomix by Doxiadis and Papadimitriou.
November 18, 2025 at 8:14 PM
Today, in class, we discussed "the barber paradox" and "Russell's paradox." As I do every time I talk about this, I plugged this excellent graphic novel (really, nonfiction with slight embellishments, all acknowledged at the end of the book): Logicomix by Doxiadis and Papadimitriou.
We're talking about one-to-one and onto functions in our intro-to-proofs class. Here's a video I made a few years ago in which I introduce these definitions, talk about how to prove that functions have the properties, and discuss some common pitfalls. www.youtube.com/watch?v=pMBR...
Proving that Functions are Injective and Surjective (One-to-One and Onto)
YouTube video by David Richeson
www.youtube.com
November 18, 2025 at 7:28 PM
We're talking about one-to-one and onto functions in our intro-to-proofs class. Here's a video I made a few years ago in which I introduce these definitions, talk about how to prove that functions have the properties, and discuss some common pitfalls. www.youtube.com/watch?v=pMBR...
Question for higher ed colleagues: (Undergraduate) student attendance numbers at events like our department colloquia have not returned to pre-pandemic levels. Are other schools seeing the same thing? If so, do you have any theories about why?
It may have nothing to do with the pandemic; that's
It may have nothing to do with the pandemic; that's
November 18, 2025 at 6:36 PM
Question for higher ed colleagues: (Undergraduate) student attendance numbers at events like our department colloquia have not returned to pre-pandemic levels. Are other schools seeing the same thing? If so, do you have any theories about why?
It may have nothing to do with the pandemic; that's
It may have nothing to do with the pandemic; that's
I love our Japanese maple tree. The orange is spectacular this year.
November 15, 2025 at 8:44 PM
I love our Japanese maple tree. The orange is spectacular this year.
I'm playing around with Caleb Madrigal's "Fuzzy Graph" graphing "nonbinary" calculator. Rather than plotting where y equals f(x) (a binary condition), it shades the plot according to how close y is to f(x). fuzzygraph.com
November 14, 2025 at 6:12 PM
I'm playing around with Caleb Madrigal's "Fuzzy Graph" graphing "nonbinary" calculator. Rather than plotting where y equals f(x) (a binary condition), it shades the plot according to how close y is to f(x). fuzzygraph.com
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...