−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? |
| 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 | Remember these from school? |
| The Complexity of Songs | A treatise on the computational complexity of songs by venerable computer scientist Donald Knuth. |
| 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. |
| Graham's number | A number so large that the observable universe is not big enough to write it in full in decimal notation. And that is a gross understatement. |
| Happy number | Not just a cheery song on the radio. |
| Hexakosioihexekontahexaphobia | For beastly people bored of triskaidekaphobia. |
| 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. |
| 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. |
| Monty Hall problem | The counter-intuitive way to prevail when playing Let's Make a Deal. |
| Narcissistic number | The pluperfect digital invariant says "Count me in"! |
| Nothing up my sleeve number | A number which is "above suspicion". |
| Numbers station | [Six bars of The Lincolnshire Poacher play] "¡Atención! ¡Atención! One, four, seventeen, twenty-four..." |
| Minkowski's question mark function | A function with an unusual notation and possessing unusual fractal properties. |
| Ramanujan summation | Numberphile made a real Parker Square of it, on . |
| 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. |
| Spaghetti sort | An algorithm for sorting rods of spaghetti. |
| 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". |
| 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. |
| Ulam spiral | A bored mathematician discovers an unusual numerical pattern while doodling. |
| Undecimber | In Java, the thirteenth month of the year. |
| Vampire number | Integers with real bite; some even have multiple pairs of fangs. |
| 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
A soon-to-be bye-bye pi pie.
Don't panic – it's Towel Day.
11:11 (numerology) The time where all 4 digits are 1s 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. 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. Phantom time hypothesis A theory by Heribert Illig that the Early Middle Ages (614–911) never occurred. Therefore, it is now 1723 rather than 2020. Pi Day The day – March 14 – on which the constant π is celebrated. Tau Day The day – June 28 – on which the constant 𝞃 is celebrated. 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. Towel Day Don't forget to bring a towel, terrible or otherwise. Winterval A word created as an alternative name for all the holidays at the end of a calendar year. It came to prominence after Birmingham City Council (the English city) used it in 1998. Year 2000 problem A possible computing problem in the 1990's that may occur when the 21st century and 3rd millennium has arisen. Year 2038 problem The computing problem that will arise due to the Unix time representation used in many computers. Year 10,000 problem The collective name for all potential software bugs that will emerge as the need to express years with five digits arises. Year zero Was there a year between 1 BC and AD 1?