Mathematics and numbers
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Be very afraid.
| −0 | Zero has a negative flavor in the worlds of computing, experimental science and statistical mechanics. |
| 0.999... | An infinitely long way to write 1. |
| 2 + 2 = 5 | ...or perhaps it equals 1984... |
| 616 (number) | The real number of the beast? |
| 65537-gon | This many-sided polygon can be constructed with a compass and straight edge... but then again, so can a circle, and it's not like you'd notice the 15 parts per billion of difference. |
| A Million Random Digits with 100,000 Normal Deviates | A pioneering book that does exactly what it says on the cover. Somehow, not the only random number book either. |
| All horses are the same color | Flawed mathematical induction proof that all horses are the same color. |
| Almost everywhere | Does not refer to advertising or corrupt corporate practices, but is instead a term in measure theory. |
| Almost integer | By a strange coincidence, - and that's just the tip of the iceberg. |
| Almost periodic function | Well, at least they tried. |
| Banach–Tarski paradox | Tutorial to make two spheres from one. |
| Belphegor's prime | 1 followed by 13 zeros followed by 666 followed by 13 zeros followed by 1. |
| Bertrand's postulate | Despite now being a theorem, still conventionally called a postulate. |
| Calculator spelling | 5318008! |
| The Complexity of Songs | A treatise on the computational complexity of songs by venerable computer scientist Donald Knuth. |
| Cox–Zucker machine | This machine does what?! |
| Homicidal chauffeur problem | What does a murderous driver have in common with a guided missile? |
| Erdős–Bacon number | A combination of the degrees of separation from actor Kevin Bacon and mathematician Paul Erdős. |
| Extravagant number | Don't take it shopping. Not very friendly with the frugal number either. |
| Gabriel's horn | A geometric figure with an infinite surface area but finite volume. So even if the horn was filled with paint, you could never paint its surface. |
| Graham's number | A number so large that the observable universe is not big enough to write it in full in decimal notation or even scientific notation. |
| Hairy ball theorem | Seriously... Couldn't you come up with a better name?! |
| Happy number | Not just a cheery song on the radio. |
| Hilbert's paradox of the Grand Hotel | A fully occupied hotel cannot accommodate any more guests. Or can it? Or, once it can, can it not? |
| Illegal number | Does the US government forbid knowledge of the existence of certain numbers? |
| Illumination Problem | A room with a bit of a shadow. |
| Indiana Pi Bill | A notorious attempt to legislate the value of pi as 3.2. |
| Infinite monkey theorem | An infinite number of monkeys typing on an infinite number of typewriters will (almost surely) produce all possible written texts. |
| Interesting number paradox | Either all natural numbers are interesting or else none of them are. |
| Kruskal's tree theorem | TREE(1) = 1; TREE(2) = 3; TREE(3) = ...wait, where did all my disk space go? |
| Legendre's constant | After 91 years and much effort, this legendary constant was found to be ... 1. Just 1. |
| Look-and-say sequence | Also known as the Cuckoo's Egg. |
| Mathematical fallacy | Trying to prove that 2 = 1 or that 1 < 0. |
| Mathematical joke | Complex numbers are all fun and games until someone loses an i. That's when things get real. |
| Minkowski's question-mark function | A function with an unusual notation and possessing unusual fractal properties. |
| Monty Hall problem | The counter-intuitive way to prevail when playing Let's Make a Deal. |
| Moving sofa problem | What is the largest area of a sofa that can be manoeuvred through an L-shaped corner? |
| Narcissistic number | The pluperfect digital invariant says "Count me in"! |
| Nothing-up-my-sleeve number | A number which is "above suspicion". |
| Number of the beast | For beastly people bored of the number of unluckiness. |
| Numbers station | [Six bars of The Lincolnshire Poacher play] "¡Atención! ¡Atención! One, four, seventeen, twenty-four..." |
| Pi is 3 | Did Japanese education guidelines shockingly redefine pi as exactly 3? No, they didn't, but where's the news story and public outcry in that? |
| Pointless topology | Not as useless as it sounds. |
| Potato paradox | If potatoes consisting of 99% water dry so that they are 98% water, they lose 50% of their weight. |
| Ramanujan summation | What do you get when you add all positive integers, up to infinity? You get a negative fraction. |
| Schizophrenic number | Can numbers have mental disorders? |
| Sexy prime | Prime numbers that differ from each other by sex. Er... six. |
| Six nines in pi | A mathematical coincidence, the sequence "999999" appears a mere 762 digits into the decimal expansion of pi. |
| Tarski's circle-squaring problem | How to square the circle for real. |
| Spaghetti sort | An algorithm for sorting rods of spaghetti. |
| Squircle | Not quite a square, not quite a circle, definitely not a Pokémon either. |
| Taxicab number | Never tell a Numberphile that a number is uninteresting. |
| Tetraphobia | Sometimes found in conjunction with triskaidekaphobia (see below) in East Asian cultures. More prevalent in Japan, where 49 is associated with "suffering until death". |
| Titanic prime | Surprisingly, not discovered by Leonardo DiCaprio. |
| Tits group | The perfect sporadic group doesn't exi- |
| Triskaidekaphobia | No, it's not related to the Code of Hammurabi. No, it's not always considered unlucky. Yes, space exploration has been touched by it. |
| Tupper's self-referential formula | A formula that draws itself! |
| Ulam spiral | A bored mathematician discovers an unusual numerical pattern while doodling. |
| Umbral calculus | A mathematical method successfully used for over 100 years, despite the notable limitation of no one on Earth knowing exactly how or why it worked. |
| Unexpected hanging paradox | If you're told you'll be hanged on a day you'll never expect it, you can prove logically that there's no day you can be hanged at all. Which, of course, means you won't expect it when the hanging does happen as planned. |
| Unknot | The least knotted of all mathematical knots. |
| Vacuous truth | All pigs with wings speak Chinese. |
| Vampire number | Integers with real bite; some even have multiple pairs of fangs. |
| Wheat and chessboard problem | Do not mess with exponential growth, especially while agreeing to a suspiciously-low reward for a commoner. |
| Will Rogers phenomenon | When moving an element from one set to another set raises – counter-intuitively – the average values of both sets. Also known as the Will Rogers paradox. |
| Zenzizenzizenzic | You know how x3 is called "x cubed"? Well, x8 is called... |
| Zeroth | An ordinal number popular in computing and related cultures. |
Dates and timekeeping
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| Abolition of time zones | No more asking "So what time is it there?" |
| Ruth Belville | She followed her parents in the business of selling people Greenwich Mean Time. |
| Chrismukkah | A fictional Christmas-Hanukkah hybrid, popularized by the television show The O.C.. |
| Cinnamon Roll Day | A day too good for this world, too pure. |
| Festivus | December 23: Holiday celebrated by the Costanza family on the television show Seinfeld, since appropriated by many. |
| International Talk Like a Pirate Day | Shiver my timbers (a-harrr!) every September 19. |
| List of non-standard dates | Including, among other things, January 0, February 30, and May 35. |
| Manhattanhenge | Twice every year, the setting sun aligns with Manhattan's street grid. |
| Mole Day | The Avogadro constant is celebrated on October 23rd starting at exactly 6:02 am. |
| Pi Day | The day – March 14 – on which the constant π is celebrated. |
| Pocky & Pretz Day | A day in Japan celebrating long, thin biscuits. Due to their shape, it is celebrated on 11/11. |
| Singles' Day | One is the loneliest number. 11/11 makes an appropriate date to celebrate being single. |
| Square Root Day | Any date when the day and month are both the square root of the last two digits of the year (the next being 5th May 2025). |
| Star Wars Day | May the 4th be with you. |
| Steak and Blowjob Day Cake and Cunnilingus Day |
Male alternative to Valentine's Day and female response to that day. |
| Swatch Internet Time | In 1998, Swatch tried to reshape our timing system. |
| Thanksgivukkah | A Thanksgiving-Hanukkah hybrid when the two overlap in November in the US; maybe your Hanukkah present can be a Thanksgiving Dinner. |
| Towel Day | Don't forget to bring a towel, terrible or otherwise. |
| Undecimber | In Java, the thirteenth month of the year. |
| Winterval | An attempt to erase Christmas? No, just a word for Birmingham City Council's collective festive plans, but that didn't stop the UK media from going wild. |
| Year 2000 problem | A possible computing problem in the 1990's that was supposed to have occurred when the 21st century and 3rd millennium arrived. Of course, that never happened. |
| Year 2038 problem | The computing problem that will arise due to the Unix time representation used in many computers. |
| Year zero | Was there a year between 1 BC and AD 1? |