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๐ Sexagesimal
Sexagesimal (also known as base 60 or sexagenary) is a numeral system with sixty as its base. It originated with the ancient Sumerians in the 3rd millennium BC, was passed down to the ancient Babylonians, and is still usedโin a modified formโfor measuring time, angles, and geographic coordinates.
The number 60, a superior highly composite number, has twelve factors, namely 1, 2, 3, 4, 5, 6, 10, 12, 15, 20, 30, and 60, of which 2, 3, and 5 are prime numbers. With so many factors, many fractions involving sexagesimal numbers are simplified. For example, one hour can be divided evenly into sections of 30 minutes, 20 minutes, 15 minutes, 12 minutes, 10 minutes, 6 minutes, 5 minutes, 4 minutes, 3 minutes, 2 minutes, and 1 minute. 60 is the smallest number that is divisible by every number from 1 to 6; that is, it is the lowest common multiple of 1, 2, 3, 4, 5, and 6.
- In this article, all sexagesimal digits are represented as decimal numbers, except where otherwise noted. For example, 10 means the number ten and 60 means the number sixty.
Discussed on
- "Sexagesimal" | 2020-10-31 | 31 Upvotes 10 Comments
๐ Cleo, the mathematician that tricked Stack Exchange
Cleo was the pseudonym of an anonymous mathematician active on the mathematics Stack Exchange from 2013 to 2015, who became known for providing precise answers to complex mathematical integration problems without showing any intermediate steps. Due to the extraordinary accuracy and speed of the provided solutions, mathematicians debated whether Cleo was an individual genius, a collective pseudonym, or even an early artificial intelligence system.
During the poster's active period, Cleo posted 39 answers to advanced mathematical questions, primarily focusing on complex integration problems that had stumped other users. Cleo's answers were characterized by being consistently correct while providing no explanation of methodology, often appearing within hours of the original posts. The account claimed to be limited in interaction due to an unspecified medical condition.
The mystery surrounding Cleo's identity and mathematical abilities generated significant interest in the mathematical community, with users attempting to analyze solution patterns and writing style for clues. Some compared Cleo to historical mathematical figures like Srinivasa Ramanujan, known for providing solutions without conventional proofs. In 2025, Cleo was revealed to be Vladimir Reshetnikov, a software developer originally from Uzbekistan.
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- "Cleo (Mathematician)" | 2026-10-06 | 256 Upvotes 58 Comments
- "Cleo, the mathematician that tricked Stack Exchange" | 2025-05-20 | 69 Upvotes 9 Comments
๐ Extreme Value Theory
Extreme value theory or extreme value analysis (EVA) is a branch of statistics dealing with the extreme deviations from the median of probability distributions. It seeks to assess, from a given ordered sample of a given random variable, the probability of events that are more extreme than any previously observed. Extreme value analysis is widely used in many disciplines, such as structural engineering, finance, earth sciences, traffic prediction, and geological engineering. For example, EVA might be used in the field of hydrology to estimate the probability of an unusually large flooding event, such as the 100-year flood. Similarly, for the design of a breakwater, a coastal engineer would seek to estimate the 50-year wave and design the structure accordingly.
๐ Laconic Phrase
A laconic phrase or laconism is a concise or terse statement, especially a blunt and elliptical rejoinder. It is named after Laconia, the region of Greece including the city of Sparta, whose ancient inhabitants had a reputation for verbal austerity and were famous for their blunt and often pithy remarks.
Discussed on
- "Laconic Phrase" | 2015-04-24 | 12 Upvotes 3 Comments
๐ Turin Papyrus Map
The Turin Papyrus Map is an ancient Egyptian map, generally considered the oldest surviving map of topographical interest from the ancient world. It is drawn on a papyrus reportedly discovered at Deir el-Medina in Thebes, collected by Bernardino Drovetti (known as Napoleon's Proconsul) in Egypt sometime before 1824 AD and now preserved in Turin's Museo Egizio. The map was drawn about 1150 BC by the well-known Scribe-of-the-Tomb Amennakhte, son of Ipuy. It was prepared for Ramesses IV's quarrying expedition to the Wadi Hammamat in the Eastern Desert, which exposes Precambrian rocks of the Arabian-Nubian Shield. The purpose of the expedition was to obtain blocks of bekhen-stone (metagraywacke sandstone) to be used for statues of the king.
๐ โซโฌโโฎtxet lanoitcerid-iB
A bidirectional text contains both text directionalities, right-to-left (RTL or dextrosinistral) and left-to-right (LTR or sinistrodextral). It generally involves text containing different types of alphabets, but may also refer to boustrophedon, which is changing text direction in each row.
Some writing systems including the Arabic and Hebrew scripts or derived systems such as the Persian, Urdu, and Yiddish scripts, are written in a form known as right-to-left (RTL), in which writing begins at the right-hand side of a page and concludes at the left-hand side. This is different from the left-to-right (LTR) direction used by the dominant Latin script. When LTR text is mixed with RTL in the same paragraph, each type of text is written in its own direction, which is known as bidirectional text. This can get rather complex when multiple levels of quotation are used.
Many computer programs fail to display bidirectional text correctly. For example, the Hebrew name Sarah (ืฉืจื) is spelled: sin (ืฉ) (which appears rightmost), then resh (ืจ), and finally heh (ื) (which should appear leftmost).
Note: Some web browsers may display the Hebrew text in this article in the opposite direction.
Discussed on
- "โซโฌโโฎtxet lanoitcerid-iB" | 2014-11-27 | 105 Upvotes 14 Comments
๐ Luhn Algorithm for validating credit cards
The Luhn algorithm or Luhn formula, also known as the "modulus 10" or "mod 10" algorithm, named after its creator, IBM scientist Hans Peter Luhn, is a simple checksum formula used to validate a variety of identification numbers, such as credit card numbers, IMEI numbers, National Provider Identifier numbers in the United States, Canadian Social Insurance Numbers, Israel ID Numbers, South African ID Numbers, Greek Social Security Numbers (ฮฮฮฮ), and survey codes appearing on McDonald's, Taco Bell, and Tractor Supply Co. receipts. It is described in U.S. Patent No. 2,950,048, filed on January 6, 1954, and granted on August 23, 1960.
The algorithm is in the public domain and is in wide use today. It is specified in ISO/IEC 7812-1. It is not intended to be a cryptographically secure hash function; it was designed to protect against accidental errors, not malicious attacks. Most credit cards and many government identification numbers use the algorithm as a simple method of distinguishing valid numbers from mistyped or otherwise incorrect numbers.
Discussed on
- "Luhn Algorithm for validating credit cards" | 2015-07-02 | 53 Upvotes 31 Comments
๐ Tell HN: Perseids: "Last Chance" for Best Meteor Viewing Tonight
The Perseids are a prolific meteor shower associated with the comet SwiftโTuttle. The meteors are called the Perseids because the point from which they appear to hail (called the radiant) lies in the constellation Perseus.
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- "Tell HN: Perseids: "Last Chance" for Best Meteor Viewing Tonight" | 2009-08-12 | 20 Upvotes 13 Comments
๐ Surface-conduction electron-emitter display
A surface-conduction electron-emitter display (SED) is a display technology for flat panel displays developed by a number of companies. SEDs use nanoscopic-scale electron emitters to energize colored phosphors and produce an image. In a general sense, an SED consists of a matrix of tiny cathode ray tubes, each "tube" forming a single sub-pixel on the screen, grouped in threes to form red-green-blue (RGB) pixels. SEDs combine the advantages of CRTs, namely their high contrast ratios, wide viewing angles and very fast response times, with the packaging advantages of LCD and other flat panel displays. They also use much less power than an LCD television of the same size.
After considerable time and effort in the early and mid-2000s, SED efforts started winding down in 2009 as LCD became the dominant technology. In August 2010, Canon announced they were shutting down their joint effort to develop SEDs commercially, signalling the end of development efforts. SEDs are closely related to another developing display technology, the field emission display, or FED, differing primarily in the details of the electron emitters. Sony, the main backer of FED, has similarly backed off from their development efforts.
Discussed on
- "Surface-conduction electron-emitter display" | 2019-11-18 | 26 Upvotes 9 Comments
๐ The man who singlehandedly carved a road through a mountain
Dashrath Manjhi (1934 โ 17 August 2007), also known as Mountain Man, was a laborer in Gehlaur village, near Gaya in Bihar, India, who carved a path 110 m long (360 ft), 9.1 m (30 ft) wide and 7.7 m (25 ft) deep through a ridge of hills using only a hammer and chisel. After 22 years of work, Dashrath shortened travel between the Atri and Wazirganj blocks of Gaya town from 55ย km to 15ย km.
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- "The man who singlehandedly carved a road through a mountain" | 2012-09-22 | 483 Upvotes 96 Comments