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🔗 Game theory

Game theory studies strategic interaction between individuals in situations called games. Classes of these games have been given names. This is a list of the most commonly studied games

🔗 Military history/Early Muslim military history 🔗 Games

The two envelopes problem, also known as the exchange paradox, is a brain teaser, puzzle, or paradox in logic, probability, and recreational mathematics. It is of special interest in decision theory, and for the Bayesian interpretation of probability theory. Historically, it arose as a variant of the necktie paradox. The problem typically is introduced by formulating a hypothetical challenge of the following type:

It seems obvious that there is no point in switching envelopes as the situation is symmetric. However, because you stand to gain twice as much money if you switch while risking only a loss of half of what you currently have, it is possible to argue that it is more beneficial to switch. The problem is to show what is wrong with this argument.

🔗 Buses

A bus trap (car trap in the UK) is a metal grate placed over a ditch or pit in the road with tines (sides) spaced far enough apart that small (shorter axle) vehicles fall between the tines but close enough that larger-diameter-wheeled vehicles, such as buses, may pass. Cycles may cross broader sided examples.

Different versions exist for restricting access. The tines cross the path of the road, not parallel with the road direction. Small-wheeled vehicles bottom out in between the tines, preventing the vehicle from continuing over the obstacle.

🔗 Computing 🔗 Computing/Software 🔗 Computing/Free and open-source software

netcat (often abbreviated to nc) is a computer networking utility for reading from and writing to network connections using TCP or UDP. The command is designed to be a dependable back-end that can be used directly or easily driven by other programs and scripts. At the same time, it is a feature-rich network debugging and investigation tool, since it can produce almost any kind of connection its user could need and has a number of built-in capabilities.

Its list of features includes port scanning, transferring files, and port listening, and it can be used as a backdoor.

🔗 Human rights

🔗 China 🔗 Mythology 🔗 Taoism

Erlang Shen (二郎神), or Erlang is a Chinese God with a third truth-seeing eye in the middle of his forehead.

Er-lang Shen may be a deified version of several semi-mythical folk heroes who help regulate China's torrential floods dating variously from the Qin, Sui, and Jin dynasties. A later Buddhist source identifies him as the second son of the Northern Heavenly King Vaishravana.

In the Ming semi-mythical novels Creation of the Gods and Journey to the West, Erlang Shen is the nephew of the Jade Emperor. In the former, he assists the Zhou army in defeating the Shang. In the latter, he is the second son of a mortal and Jade emperor's sister. In the legend, he is known as the greatest warrior god of heaven.

🔗 France 🔗 Middle Ages 🔗 Middle Ages/History

The Bal des Ardents (Ball of the Burning Men) or Bal des Sauvages (Ball of the Wild Men) was a masquerade ball held on 28 January 1393 in Paris at which Charles VI of France performed in a dance with five members of the French nobility. Four of the dancers were killed in a fire caused by a torch brought in by a spectator, Charles's brother Louis I, Duke of Orléans. Charles and another of the dancers survived. The ball was one of a number of events intended to entertain the young king, who the previous summer had suffered an attack of insanity. The event undermined confidence in Charles's capacity to rule; Parisians considered it proof of courtly decadence and threatened to rebel against the more powerful members of the nobility. The public's outrage forced the king and his brother Orléans, whom a contemporary chronicler accused of attempted regicide and sorcery, to offer penance for the event.

Charles's wife, Isabeau of Bavaria, held the ball to honor the remarriage of a lady-in-waiting. Scholars believe the dance performed at the ball had elements of traditional charivari, with the dancers disguised as wild men, mythical beings often associated with demonology, that were commonly represented in medieval Europe and documented in revels of Tudor England. The event was chronicled by contemporary writers such as the Monk of St Denis and Jean Froissart, and illustrated in a number of 15th-century illuminated manuscripts by painters such as the Master of Anthony of Burgundy. The incident later provided inspiration for Edgar Allan Poe's short story Hop-Frog.

🔗 Computing

Macintosh Common Lisp (MCL) is an implementation and IDE for the Common Lisp programming language. Various versions of MCL run under the classic Mac OS (m68k and PPC) and Mac OS X.

Versions of MCL up to and including 5.1 are proprietary. Version 5.2 has been open sourced.

In 2009 a new different version of MCL has been open sourced: RMCL. RMCL is based on MCL 5.1 and does run under Rosetta on Intel-based Macs.

🔗 Computer science

TLA⁺ is a formal specification language developed by Leslie Lamport. It is used to design, model, document, and verify programs, especially concurrent systems and distributed systems. TLA⁺ has been described as exhaustively-testable pseudocode, and its use likened to drawing blueprints for software systems; TLA is an acronym for Temporal Logic of Actions.

For design and documentation, TLA⁺ fulfills the same purpose as informal technical specifications. However, TLA⁺ specifications are written in a formal language of logic and mathematics, and the precision of specifications written in this language is intended to uncover design flaws before system implementation is underway.

Since TLA⁺ specifications are written in a formal language, they are amenable to finite model checking. The model checker finds all possible system behaviours up to some number of execution steps, and examines them for violations of desired invariance properties such as safety and liveness. TLA⁺ specifications use basic set theory to define safety (bad things won't happen) and temporal logic to define liveness (good things eventually happen).

TLA⁺ is also used to write machine-checked proofs of correctness both for algorithms and mathematical theorems. The proofs are written in a declarative, hierarchical style independent of any single theorem prover backend. Both formal and informal structured mathematical proofs can be written in TLA⁺; the language is similar to LaTeX, and tools exist to translate TLA⁺ specifications to LaTeX documents.

TLA⁺ was introduced in 1999, following several decades of research into a verification method for concurrent systems. A toolchain has since developed, including an IDE and distributed model checker. The pseudocode-like language PlusCal was created in 2009; it transpiles to TLA⁺ and is useful for specifying sequential algorithms. TLA⁺² was announced in 2014, expanding language support for proof constructs. The current TLA⁺ reference is The TLA⁺ Hyperbook by Leslie Lamport.

🔗 Environment 🔗 Meteorology 🔗 Astronomy 🔗 Middle Ages 🔗 Middle Ages/History 🔗 Geology 🔗 Glaciers 🔗 Solar System

The 774–775 carbon-14 spike is an observed increase of 1.2% in the concentration of carbon-14 isotope in tree rings dated to 774 or 775, which is about 20 times as high as the normal background rate of variation. It was discovered during a study of Japanese cedar trees, with the year of occurrence determined through dendrochronology. A surge in beryllium isotope ¹⁰
Be, detected in Antarctic ice cores, has also been associated with the 774–775 event. It is known as the Miyake event or the Charlemagne event and it produced the largest and most rapid rise in carbon-14 ever recorded.

The event appears to have been global, with the same carbon-14 signal found in tree rings from Germany, Russia, the United States, Finland and New Zealand.

The signal exhibits a sharp increase of around 1.2% followed by a slow decline (see Figure 1), which is typical for an instant production of carbon-14 in the atmosphere, indicating that the event was short in duration. The globally averaged production of carbon-14 for this event is calculated as Q = 1.3×10⁸ ± 0.2×10⁸ atoms/cm².