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David Richeson: Division by Zero

divisbyzero.com

Professor of Mathematics and the John J. Curley '60 and Ann Conser Curley '63 Faculty Chair in the Liberal Arts at Dickinson College [Icons made by Freepik from www.flaticon.com.]

MathFun / humour

’10’15’20’25
Posts
396 from 11 Sep 2008 to 1 Feb 2026
Posting
now and then (3 in the last year)
Kind
personal blog
Feed
https://divisbyzero.com/feed/

2026 1

  1. 3D-Printed Slices and Traces of Functions of Two Variables

2025 6

  1. Constructing a Large 251-Point String Art Cardioid
  2. A Laser-Cut Tromino Puzzle
  3. The Unknotting Number is Not Additive
  4. Mathematics Departments at Liberal Arts Colleges
  5. Magic Eye Image as an Animated GIF
  6. Type Math on a Mac—Remapping My Keyboard

2024 6

  1. Make a “Magic Eye” image using Excel
  2. Making the Mandelbrot Set with Excel
  3. Fold-and-Cut Turtle Tile
  4. Fold-and-Cut Hat and Spectre Tiles
  5. Cardioid Drawbridges
  6. Folding a Cardioid

2023 4

  1. Connections with mathematical terms
  2. Drawing a Celtic Knot
  3. Math class technology policy: Fall 2023 edition
  4. 3D-Printable Aperiodic Monotiles and Aperiodic Monotile Coloring Book Pages

2022 2

  1. Halloween craft: FLEX-A-GHOUL
  2. Preorders and Finite Topological Spaces

2021 4

  1. My Two-Day Crash Course in PGFPlots and TikZ
  2. Essential Trigonometry for Calculus
  3. Puzzle: Let’s (Un)Do the Twist!
  4. Mathematicians’ Phone Passcodes

2020 3

  1. Make a Real Projective Plane (Boy’s Surface) out of Paper
  2. The Magnificent Möbius Band
  3. How to Present a Mathematical Proof or Problem

2019 6

  1. Homemade Klein Bottle
  2. Tales of Impossibility: Now Published!
  3. BraidTiles—A Mathematical Braid Puzzle
  4. A Numerical Crossword Puzzle
  5. The Most Imaginary Number… is Real!
  6. My First Crossword Puzzle

2018 9

  1. Make Your Own Pythagorean Cup
  2. A bad, but interesting, exam question
  3. Proof by Induction with a Difficult Base Case
  4. Proof Without Words: Gregory’s Theorem
  5. Möbius Band Ambigram
  6. Rubik’s Cube Tri-Hexaflexagon
  7. I Heart Cardioids
  8. Finite differences of polynomials
  9. Who was Pierre Wantzel? A translation crowdsourcing project

2017 7

  1. Math Crafts: Salt Designs, Newton Snowflakes, Fractal Christmas Trees, Paper Pentagons, and Flip Books
  2. The Division Symbol Goes Viral
  3. Advice for College Students
  4. The Math Behind a Reflected Double Rainbow
  5. Card Table Proof of the Pythagorean Theorem
  6. Seventeen Facts about 17
  7. A Geometric Proof of Brooks’s Trisection?

2016 9

  1. Two More Impossible Cylinders
  2. Make a Sugihara Circle/Square Optical Illusion Out of Paper
  3. Sugihara’s Circle/Square Optical Illusion
  4. Measuring Tapes for Circles and Spheres
  5. Zip-Apart Möbius Bands
  6. A Trig-free Proof of Crockett Johnson’s Theorem
  7. A Geometry Theorem Looking for a Geometric Proof
  8. A Trisectrix from a Carpenter’s Square
  9. Good activity for an Introduction to Proofs class

2015 1

  1. Editing a Very Poorly Written Proof

2014 3

  1. Tangent lines to the sine function with rational slope
  2. Gabriel’s paper horn
  3. Undergraduate Math Bloggers

2013 8

  1. 2013: the year of pi
  2. Using a kayak to measure the perimeter of a lake
  3. Countability of the rationals drawn using TikZ
  4. Greatest living mathematician and expositor?
  5. Bubble diagrams for functions in LaTeX using TikZ
  6. Circular reasoning: who first proved that C/d is a constant?
  7. Interview on Wild about Math!
  8. Editing mathematical writing

2012 11

  1. How I teach topology: an inquiry-based learning approach
  2. The Pigpen Cipher in Latex
  3. Online LaTeX editors
  4. How do you place incoming mathematics students?
  5. Ancient number systems in XeTeX
  6. Mathematics departments at liberal arts colleges
  7. Plato’s approximation of pi?
  8. Puzzler: a squarable region from Leonardo da Vinci
  9. Angle trisection using origami
  10. An interesting multivariable calculus example
  11. Parametric curve project for multivariable calculus

2011 27

  1. Cantor set applet
  2. Applet to illustrate the epsilon-delta definition of limit
  3. The danger of false positives
  4. A neat probability rule-of-thumb
  5. Advice for new college students
  6. A quick guide to LaTeX
  7. Highlights from MathFest 2011
  8. The Japanese theorem for nonconvex polygons
  9. Some LaTeX odds and ends
  10. Extreme examples and counterexamples
  11. Hankel on Diophantus
  12. What shape are the golden arches?
  13. Auden: minus times minus equals plus, the reason for this we need not discuss
  14. A pyramidologist’s value for pi
  15. What do you want on your tombstone?
  16. Happy birthday Uncle Leonhard, I hope you enjoy your new home
  17. Math books for young children
  18. Albrecht Dürer’s ruler and compass constructions
  19. A picture of frustration: Sam Loyd’s 15 puzzle
  20. Millay’s Euclid looks on Beauty bare
  21. Lincoln and squaring the circle
  22. Counting triangles on a tin ceiling (solution, take 2)
  23. Counting triangles on a tin ceiling
  24. Inspiration is for amateurs
  25. Music is math: ten songs about mathematics
  26. Irving Kaplansky’s “A Song about Pi”
  27. Beautiful theorems about dynamical systems on the plane

2010 75

  1. Three geometric theorems
  2. The Edmonton Eulers
  3. Google Translate now knows Latin
  4. An amazing paragraph from Euler’s Introductio
  5. Odds and ends: the genius of Euler, Bulgarian solitaire, and mathematical surprises
  6. Top ten transcendental numbers
  7. Trigonometric functions and rational multiples of pi
  8. Pumpkin pi
  9. Circle squaring limerick
  10. Exceptional MathReviews
  11. The hitchhiker’s guide to infinity
  12. Musicians with PhD’s
  13. The transcendence of e (part 3)
  14. The transcendence of e (part 2)
  15. The transcendence of e
  16. Mathematics in Moby-Dick
  17. Furstenberg’s topological proof of the infinitude of primes
  18. Irrational rotations of the circle and Benford’s law
  19. A game for budding knot theorists
  20. Mathematical surprises
  21. Goodstein’s unprovable theorem
  22. An island on an island on an island
  23. The left-handed boy problem
  24. Mathematical magic tricks for kids
  25. Thirteen mathematically-inspired products
  26. Tricks for easily creating BibTeX files
  27. Algebra: the Faustian bargain
  28. Neat facts from Euler 2010
  29. Nested nations
  30. Coffee stains and the Simpsons in your LaTeX document
  31. Water and land, nested
  32. Happy tau day!
  33. 12 scholarly hoaxes, randomly generated articles, and other tricky fun
  34. What’s in a name?
  35. The earth is big
  36. Catching up on some reading: Dyson’s birds and frogs
  37. Turing’s topological proof that every written alphabet is finite
  38. Mathematics in novels and Martin Gardner RIP
  39. Volumes of n-dimensional balls
  40. Movie day in topology class: the Poincaré conjecture
  41. Interview on Strongly Connected Components
  42. Nice surprise
  43. Topological maps or topographic maps?
  44. John D. Cook’s daily Twitter tips
  45. Mathematical cousins and collaboration numbers
  46. An Euler line geogebra applet
  47. Everyday geometry: hat size
  48. Woo hoo! A power of 10! Hooray! Here’s to an arbitrary milestone!
  49. Making a hyperboloid out of skewers and rubber bands
  50. Perelman to be awarded the Clay Millennium Prize
  51. A card trick that will probably amaze your friends (solution)
  52. A card trick that will probably amaze your friends
  53. An application of graph theory to architecture
  54. What do Augustus De Morgan, Chelsea Clinton, Samuel Adams, and Caligula have in common?
  55. Readers’ response: Euler’s greatest hits
  56. Polya on Euler
  57. Using wikis in mathematics classes
  58. More about the neat calculator trick
  59. The math behind a neat calculator trick
  60. Interesting approximations using trigonometry
  61. A tale of why you (U, that is) needs a tail
  62. Posting items to my blog using wplatex
  63. What is the cardinality of the Euclidean topology?
  64. Proof that pi is irrational
  65. Three applets illustrating parametric curves
  66. Why do mirrors reverse right and left but not up and down?
  67. Three applets for linear algebra or multivariable calculus
  68. Division by Zero listed as one of 50 Best Blogs for Education Leaders
  69. Steven Strogatz writes about the elements of mathematics in the NY Times
  70. How to email all the students in your class with only two clicks of your mouse
  71. The relative sizes of the stars and planets
  72. Playing the probabilities in Settlers of Catan
  73. Odds and ends: the 2010 Joint Mathematics Meeting and Euler’s Gem
  74. New applets page
  75. Geogebra applet for families of discrete dynamical systems

2009 131

  1. Math Midway at the Da Vinci Science center
  2. Showing two expressions are equal: stop the madness
  3. Twitter and Origami
  4. Edublog award nomination
  5. Mathematical holiday cookies at Starbucks
  6. Euler’s formula in a children’s book
  7. Cobweb plots for the logistic map: a Geogebra applet
  8. An applet illustrating a continuous, nowhere differentiable function
  9. Applet for discrete dynamical systems
  10. The geometry of meandering rivers
  11. Legendre who?
  12. Oops
  13. Math in literature
  14. Thoughts on teaching induction
  15. Kindergarten Mathematics (part 2): a report
  16. Mathematical art by Kevin Van Aelst
  17. Kindergarten mathematics
  18. Tennenbaum’s proof of the irrationality of the square root of 2
  19. A new way to collaborate: DropBox
  20. Bolzano-Weierstrass Rap
  21. Is or an inclusive or or an exclusive or?
  22. An applet for teaching the limit of a sequence
  23. LaTeX now available in Google Docs
  24. The nuts and bolts of writing mathematics
  25. Cardinality of infinite sets, part 1: four nonstandard proofs of countability
  26. Casting out nines
  27. Mathematical spellcheck fail
  28. Symbolic dynamics for nonhyperbolic systems
  29. Tom Lehrer’s math songs
  30. More prime phone numbers
  31. Look who majored in mathematics
  32. A wiki about blogs
  33. The 56th Carnival of Mathematics is now online
  34. More on twin primes and Pythagorean triples
  35. 867-5309; more than Jenny’s phone number
  36. Applets for multivariable calculus
  37. A musical lecture on the twin prime conjecture
  38. What is a howicide?
  39. Photos of Family Matters panel
  40. Delicious pie chart
  41. Calculating miles per gallon for electric cars
  42. Infinite hat problems (solutions)
  43. Infinite hat problems
  44. I need to learn how to say no
  45. New journal publishes inaugural issue
  46. What is the LaTeX code for that?
  47. Videos of past meetings of the ICM
  48. Quantifier soup
  49. Ambigram: pie≠3.14
  50. Last Sunday was a perfect day
  51. Putting CVs online
  52. Indeterminate form in The New Yorker
  53. The famous trick donkeys: a Sam Loyd puzzle
  54. Carnot’s Theorem
  55. Euler’s Gem is the #2 best-selling mathematics book for libraries
  56. Three cool facts about rotations of the circle
  57. Keith Devlin totally looks like Graham Nash
  58. Recommended readings 6/5/09
  59. Parabolas and focal points II
  60. Short talks at conferences
  61. Parabolas and focal points
  62. The Japanese Theorem
  63. Pentagrams and quartic polynomials
  64. The NSF totally looks like the UAW
  65. Computing integer sums using l’Hôpital’s rule
  66. Recommended readings (5/11/09)
  67. Let’s go to the carnival
  68. Symmetry groups of viral doilies
  69. Maypole braid group (solutions)
  70. The maypole braid group
  71. Mathematical Google logos
  72. Recommended readings (4/27/09)
  73. Happy birthday Michael Atiyah
  74. How the arch got its shape
  75. PAR scoring in squash
  76. xkcd in the New York Times
  77. Musings about traffic on the highway
  78. Arthur Benjamin: Lightning Calculation and Other “Mathemagic”
  79. Featured MAA review
  80. Happy birthday!
  81. Freeman Dyson and a mathematical puzzle in the NY Times
  82. My virtual seminar goes live
  83. Recommended readings (3/29/09)
  84. Great conference on a rainy Saturday
  85. Ambigrams
  86. Mythematics: a call for participation
  87. Photo in Focus
  88. Recommended readings (3/22/09)
  89. Bubbles with knotted boundaries
  90. Congress designates 3/14 National Pi Day
  91. The area versus the circumference of a circle
  92. Putting mathematical scholarship in perspective
  93. Recommended readings (3/12/09)
  94. Flashlights and conic sections
  95. Sines and cosines (part 2)
  96. Sines and cosines (part 1)
  97. New graduation requirement
  98. Top 8 most popular polygons
  99. Happy square root day!
  100. Recommended readings (2/27/09)
  101. Multiplication of multi-digit numbers: a visual approach
  102. Students as Facebook friends?
  103. Recommended readings for 2/19/09
  104. Make your own polyhedral calendar
  105. Happy Valentine’s Day
  106. What’s special about tomorrow?
  107. Is this the Cayley table of a group? (Part 2)
  108. Is this the Cayley table of a group? (Part 1)
  109. LaTeX bugs in WordPress
  110. Fibonachos
  111. Math on a bridge
  112. The Great British Venn Diagram
  113. Units are important
  114. Gibbs’ phase rule and Euler’s formula
  115. 100th post
  116. Clearance price fail
  117. What is wrong with this picture?
  118. Six degrees of separation
  119. Photos from the PUP booth at the Joint Meetings
  120. John Stallings’ obit in NYT
  121. Why do mirrors reverse right and left but not up and down?
  122. How to use KnotPlot
  123. Can you hear me now?
  124. Tetragon fail
  125. 24 is the highest number
  126. Calculus and sustainability
  127. Publish or perish…?
  128. Mathematician: top job
  129. Asking for a letter of recommendation
  130. Happy New Year!
  131. Definition of continuity video

2008 83

  1. 2009 Times Square New Year’s Eve Ball
  2. Materials for a knot theory class
  3. 46th Carnival of Mathematics
  4. A 10-adic number that is a zero divisor
  5. The importance of definitions: chance of rain
  6. How to curve an exam and assign grades
  7. Multiple choice questions in mathematics
  8. Exam-time humor
  9. Fallacies, Flaws, and Flimflam it’s been good to know ya
  10. Sharkovsky’s theorem
  11. Facebook page for Euler’s Gem
  12. Is π the right constant?
  13. Möbius bubble wrap
  14. Kenken
  15. A new continued fraction for pi
  16. Four color theorem applets
  17. Twittering Division by Zero
  18. Secrets of my success
  19. A solution for tight budgets?
  20. Phone number fail
  21. Euathlus and Protagoras
  22. Thoughts on how to teach induction
  23. The prime number theorem in Calculus II
  24. Graph theory is funny
  25. What are p-adic numbers?
  26. Jenga mathematics
  27. Do you give partial credit? How to grade Venn diagrams
  28. Weapons of math instruction
  29. A correlation: 19th century cotton production and Obama votes
  30. Four shuffles suffice
  31. Judges, sonar, and innumeracy
  32. The sum of kth powers
  33. Google beats the CDC at their own game
  34. Google scholar trick makes it more useful
  35. Red and blue election maps
  36. How did the pollsters do?
  37. Google Books replaces the index
  38. Honda Civic plays the William Tell Overture
  39. Folding a golden rectangle
  40. Fractals on NOVA
  41. Knuth stops writing checks
  42. Princeton University Press blog
  43. Job opening: Lucasian Chair of Mathematics
  44. Calculus in politics
  45. Advice for the budding mathematician of any age
  46. E-Z Pass, speeding tickets, and the mean value theorem
  47. David Foster Wallace’s commencement address
  48. Wooden binary adding machine
  49. Measuring an angle with a ruler
  50. Shameless self-promotion
  51. The eyeballing game
  52. Lipson’s mathematical LEGO sculptures
  53. Tao on political polls
  54. Einstein’s math
  55. The US does not produce enough mathematical stars
  56. Create your own word clouds
  57. Flatland and other videos about dimension
  58. Cutting and folding paper
  59. Flash cards are a good idea
  60. DARPA’s 23 mathematical questions
  61. Math quotes on Freakonomics blog
  62. Kuratowski’s closure-complement theorem (solution)
  63. Kuratowski’s closure-complement theorem
  64. Not so elementary my dear Doyle
  65. Technology of today’s students
  66. Königsberg today
  67. What is the difference between a theorem, a lemma, and a corollary?
  68. Aggregating political polls
  69. They Might be Giants sing about polygons
  70. Learn the Greek alphabet
  71. Banach Obama
  72. Joke of the day
  73. Prime number worth $100K
  74. Cloud computing makes me nervous
  75. The nuts and bolts of writing mathematics
  76. Doodle replaces sign-up sheet on the door
  77. Advice for new faculty
  78. Does gold tarnish? The golden spiral and nautilus shells.
  79. Student podcasts in a history of mathematics course
  80. Wobbly tables and the intermediate value theorem
  81. Line Rider Calculus
  82. Topological claymation
  83. Hello world!