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π List of virtual communities with more than 1 million users
This is a list of notable active virtual communities with more than 1 million registered members.
- Defunct
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- "List of virtual communities with more than 1 million users" | 2016-12-18 | 68 Upvotes 24 Comments
π R/K selection theory
In ecology, r/K selection theory relates to the selection of combinations of traits in an organism that trade off between quantity and quality of offspring. The focus on either an increased quantity of offspring at the expense of individual parental investment of r-strategists, or on a reduced quantity of offspring with a corresponding increased parental investment of K-strategists, varies widely, seemingly to promote success in particular environments.
The terminology of r/K-selection was coined by the ecologists Robert MacArthur and E. O. Wilson in 1967 based on their work on island biogeography; although the concept of the evolution of life history strategies has a longer history (see e.g. plant strategies).
The theory was popular in the 1970s and 1980s, when it was used as a heuristic device, but lost importance in the early 1990s, when it was criticized by several empirical studies. A life-history paradigm has replaced the r/K selection paradigm but continues to incorporate many of its important themes.
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- "R/K selection theory" | 2014-03-31 | 17 Upvotes 3 Comments
π Citizenship in a Republic
Citizenship in a Republic is the title of a speech given by the former President of the United States, Theodore Roosevelt, at the Sorbonne in Paris, France on April 23, 1910.
One notable passage on page seven of the 35-page speech is referred to as "The Man in the Arena":
It is not the critic who counts; not the man who points out how the strong man stumbles, or where the doer of deeds could have done them better. The credit belongs to the man who is actually in the arena, whose face is marred by dust and sweat and blood; who strives valiantly; who errs, who comes short again and again, because there is no effort without error and shortcoming; but who does actually strive to do the deeds; who knows great enthusiasms, the great devotions; who spends himself in a worthy cause; who at the best knows in the end the triumph of high achievement, and who at the worst, if he fails, at least fails while daring greatly, so that his place shall never be with those cold and timid souls who neither know victory nor defeat.
Someone who is heavily involved in a situation that requires courage, skill, or tenacity (as opposed to someone sitting on the sidelines and watching), is sometimes referred to as "the man in the arena".
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- "Citizenship in a Republic" | 2013-02-27 | 21 Upvotes 14 Comments
π Autostereogram
An autostereogram is a single-image stereogram (SIS), designed to create the visual illusion of a three-dimensional (3D) scene from a two-dimensional image. In order to perceive 3D shapes in these autostereograms, one must overcome the normally automatic coordination between accommodation (focus) and horizontal vergence (angle of one's eyes). The illusion is one of depth perception and involves stereopsis: depth perception arising from the different perspective each eye has of a three-dimensional scene, called binocular parallax.
The simplest type of autostereogram consists of horizontally repeating patterns (often separate images) and is known as a wallpaper autostereogram. When viewed with proper convergence, the repeating patterns appear to float above or below the background. The well-known Magic Eye books feature another type of autostereogram called a random dot autostereogram. One such autostereogram is illustrated above right. In this type of autostereogram, every pixel in the image is computed from a pattern strip and a depth map. A hidden 3D scene emerges when the image is viewed with the correct convergence.
Autostereograms are similar to normal stereograms except they are viewed without a stereoscope. A stereoscope presents 2D images of the same object from slightly different angles to the left eye and the right eye, allowing us to reconstruct the original object via binocular disparity. When viewed with the proper vergence, an autostereogram does the same, the binocular disparity existing in adjacent parts of the repeating 2D patterns.
There are two ways an autostereogram can be viewed: wall-eyed and cross-eyed. Most autostereograms (including those in this article) are designed to be viewed in only one way, which is usually wall-eyed. Wall-eyed viewing requires that the two eyes adopt a relatively parallel angle, while cross-eyed viewing requires a relatively convergent angle. An image designed for wall-eyed viewing if viewed correctly will appear to pop out of the background, while if viewed cross-eyed it will instead appear as a cut-out behind the background and may be difficult to bring entirely into focus.
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- "Autostereogram" | 2020-01-13 | 17 Upvotes 4 Comments
π Geohash: text representation allows you to sort locations by proximity
Geohash is a public domain geocode system invented in 2008 by Gustavo Niemeyer and (similar work in 1966) G.M. Morton, which encodes a geographic location into a short string of letters and digits. It is a hierarchical spatial data structure which subdivides space into buckets of grid shape, which is one of the many applications of what is known as a Z-order curve, and generally space-filling curves.
Geohashes offer properties like arbitrary precision and the possibility of gradually removing characters from the end of the code to reduce its size (and gradually lose precision). As a consequence of the gradual precision degradation, nearby places will often (but not always) present similar prefixes. While in rare cases nearby places may have very short shared prefixes, the longer their shared prefix is, the closer two places are guaranteed to be.
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- "Geohash" | 2023-11-12 | 44 Upvotes 15 Comments
- "Geohash: text representation allows you to sort locations by proximity" | 2014-08-23 | 39 Upvotes 8 Comments
π BrouwerβHilbert controversy
In a foundational controversy in twentieth-century mathematics, L. E. J. Brouwer, a supporter of intuitionism, opposed David Hilbert, the founder of formalism.
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- "BrouwerβHilbert controversy" | 2018-11-08 | 95 Upvotes 32 Comments
π Wikipedia ordered by Portuguese courts to censor articles and provide user data
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- "Wikipedia ordered by Portuguese courts to censor articles and provide user data" | 2025-08-05 | 38 Upvotes 9 Comments
π 65537-gon
In geometry, a 65537-gon is a polygon with 65,537 (216 + 1) sides. The sum of the interior angles of any nonβself-intersecting 65537-gon is 11796300Β°.
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- "65537-gon" | 2025-04-15 | 14 Upvotes 3 Comments
π Manned Orbiting Laboratory
The Manned Orbiting Laboratory (MOL), originally referred to as the Manned Orbital Laboratory, was a never-flown part of the United States Air Force's human spaceflight program, a successor to the canceled Boeing X-20 Dyna-Soar military reconnaissance space plane project. The project was developed from several early Air Force and NASA concepts of crewed space stations to be used for reconnaissance purposes. MOL evolved into a single-use laboratory, with which crews would be launched on 40-day missions and return to Earth using a Gemini B spacecraft, derived from NASA's Project Gemini.
The MOL program was announced to the public on 10 December 1963 as an inhabited platform to prove the utility of putting people in space for military missions. Astronauts selected for the program were later told of the reconnaissance mission for the program. The contractor for the MOL was the Douglas Aircraft Company. The Gemini B was externally similar to NASA's Gemini spacecraft, although it underwent several modifications, including the addition of a circular hatch through the heat shield, which allowed passage between the spacecraft and the laboratory.
MOL was canceled in 1969, during the height of the Apollo program, when it was shown that uncrewed reconnaissance satellites could achieve the same objectives much more cost-effectively. U.S. space station development was instead pursued with the civilian NASA Skylab (Apollo Applications Program) which flew in the mid-1970s.
In the 1970s, the Soviet Union launched three Almaz military space stations, similar in intent to the MOL, but cancelled the program in 1977 for the same reasons.
There is a MOL space suit on display at the Oklahoma City Science Museum, presumably never used.
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- "Manned Orbiting Laboratory" | 2019-06-05 | 12 Upvotes 4 Comments
π F-Law
Management f-Laws are subversive epigrams about common management practices. Based on observation and experience, they are used to draw attention to entrenched ways of thinking about management and business that are often at odds with common sense or our actual experience.
Systems theorist Russell L. Ackoff, his co-author Herbert J. Addison and Sally Bibb invented the term in 2006 to describe their series of over 100 distilled observations of bad leadership and the misplaced wisdom that often surrounds management in organizations. Ackoff and Addison's f-Laws might seem counter-intuitive. They are designed to challenge organizations' unquestioning adherence to established management habits or beliefs. Many of the f-Laws describe a relationship of inverse proportionality, in example: "The lower the rank of managers, the more they know about fewer things."
The f-Laws advocate adopting a positive, forward-looking and interactive approach to structural or systematic change within organizations, following the principles of idealized design. This is a process that "involves redesigning the organization on the assumption that it was destroyed last night... The most effective way of creating the future is by closing or reducing the gap between the current state and the idealized design".
Three collections of f-Laws entitled A Little Book of f-Laws: 13 Common Sins of Management, Management f-Laws: How Organizations Really Work and Systems Thinking for Curious Managers have been published. While, if read in isolation, each f-Law is a witty and thought-provoking axiom, the books provide a context that draws upon systems thinking and the debate over the importance of developing soft skills in business environments.
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- "F-Law" | 2020-11-07 | 19 Upvotes 2 Comments