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🔗 Computer science

The circle–ellipse problem in software development (sometimes called the square–rectangle problem) illustrates several pitfalls which can arise when using subtype polymorphism in object modelling. The issues are most commonly encountered when using object-oriented programming (OOP). By definition, this problem is a violation of the Liskov substitution principle, one of the SOLID principles.

The problem concerns which subtyping or inheritance relationship should exist between classes which represent circles and ellipses (or, similarly, squares and rectangles). More generally, the problem illustrates the difficulties which can occur when a base class contains methods which mutate an object in a manner which may invalidate a (stronger) invariant found in a derived class, causing the Liskov substitution principle to be violated.

The existence of the circle–ellipse problem is sometimes used to criticize object-oriented programming. It may also imply that hierarchical taxonomies are difficult to make universal, implying that situational classification systems may be more practical.

🔗 Biography 🔗 Aviation 🔗 Soviet Union 🔗 Russia 🔗 Russia/technology and engineering in Russia 🔗 Spaceflight 🔗 Military history 🔗 Military history/Military aviation 🔗 Biography/science and academia 🔗 Military history/Military biography 🔗 Biography/military biography 🔗 Aviation/aerospace biography 🔗 Military history/Cold War 🔗 Biography/sports and games 🔗 Russia/Russian, Soviet, and CIS military history 🔗 Military history/Russian, Soviet and CIS military history 🔗 Russia/history of Russia

Yuri Alekseyevich Gagarin  (9 March 1934 – 27 March 1968) was a Soviet Air Forces pilot and cosmonaut who became the first human to journey into outer space, achieving a major milestone in the Space Race; his capsule Vostok 1 completed one orbit of Earth on 12 April 1961. Gagarin became an international celebrity and was awarded many medals and titles, including Hero of the Soviet Union, his nation's highest honour.

Born in the village of Klushino near Gzhatsk (a town later renamed after him), in his youth Gagarin was a foundryman at a steel plant in Lyubertsy. He later joined the Soviet Air Forces as a pilot and was stationed at the Luostari Air Base, near the Norwegian border, before his selection for the Soviet space programme with five other cosmonauts. Following his spaceflight, Gagarin became deputy training director of the Cosmonaut Training Centre, which was later named after him. He was also elected as a deputy of the Soviet of the Union in 1962 and then to the Soviet of Nationalities, respectively the lower and upper chambers of the Supreme Soviet.

Vostok 1 was Gagarin's only spaceflight but he served as the backup crew to the Soyuz 1 mission, which ended in a fatal crash, killing his friend and fellow cosmonaut Vladimir Komarov. Fearing for his life, Soviet officials permanently banned Gagarin from further spaceflights. After completing training at the Zhukovsky Air Force Engineering Academy on 17 February 1968, he was allowed to fly regular aircraft. Gagarin died five weeks later when the MiG-15 training jet he was piloting with his flight instructor Vladimir Seryogin crashed near the town of Kirzhach.

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

Node.js is an open-source, cross-platform, JavaScript runtime environment that executes JavaScript code outside of a web browser. Node.js lets developers use JavaScript to write command line tools and for server-side scripting—running scripts server-side to produce dynamic web page content before the page is sent to the user's web browser. Consequently, Node.js represents a "JavaScript everywhere" paradigm, unifying web-application development around a single programming language, rather than different languages for server- and client-side scripts.

Though .js is the standard filename extension for JavaScript code, the name "Node.js" doesn't refer to a particular file in this context and is merely the name of the product. Node.js has an event-driven architecture capable of asynchronous I/O. These design choices aim to optimize throughput and scalability in web applications with many input/output operations, as well as for real-time Web applications (e.g., real-time communication programs and browser games).

The Node.js distributed development project was previously governed by the Node.js Foundation, and has now merged with the JS Foundation to form the OpenJS Foundation, which is facilitated by the Linux Foundation's Collaborative Projects program.

Corporate users of Node.js software include GoDaddy, Groupon, IBM, LinkedIn, Microsoft, Netflix, PayPal, Rakuten, SAP, Voxer, Walmart, and Yahoo!.

🔗 Internet 🔗 Computing 🔗 Computing/Software 🔗 Websites 🔗 Websites/Computing 🔗 Computing/Computer Security 🔗 Computing/Websites

Evercookie is a JavaScript-based application created by Samy Kamkar that produces zombie cookies in a web browser that are intentionally difficult to delete. In 2013, a top-secret NSA document was leaked by Edward Snowden, citing Evercookie as a method of tracking Tor users.

🔗 Geology

Geothermal gradient is the rate of increasing temperature with respect to increasing depth in Earth's interior. Away from tectonic plate boundaries, it is about 25–30 °C/km (72–87 °F/mi) of depth near the surface in most of the world. Strictly speaking, geo-thermal necessarily refers to Earth but the concept may be applied to other planets.

Earth's internal heat comes from a combination of residual heat from planetary accretion, heat produced through radioactive decay, latent heat from core crystallization, and possibly heat from other sources. The major heat-producing isotopes in Earth are potassium-40, uranium-238, uranium-235, and thorium-232. At the center of the planet, the temperature may be up to 7,000 K and the pressure could reach 360 GPa (3.6 million atm). Because much of the heat is provided by radioactive decay, scientists believe that early in Earth history, before isotopes with short half-lives had been depleted, Earth's heat production would have been much higher. Heat production was twice that of present-day at approximately 3 billion years ago, resulting in larger temperature gradients within the Earth, larger rates of mantle convection and plate tectonics, allowing the production of igneous rocks such as komatiites that are no longer formed.

🔗 Numismatics 🔗 Numismatics/Cryptocurrency 🔗 Cryptocurrency

A proof-of-work (PoW) system (or protocol, or function) is a consensus mechanism. It deters denial-of-service attacks and other service abuses such as spam on a network by requiring some work from the service requester, usually meaning processing time by a computer. The concept was invented by Cynthia Dwork and Moni Naor as presented in a 1993 journal article. The term "proof of work" was first coined and formalized in a 1999 paper by Markus Jakobsson and Ari Juels.

A key feature of these schemes is their asymmetry: the work must be moderately hard (yet feasible) on the requester side but easy to check for the service provider. This idea is also known as a CPU cost function, client puzzle, computational puzzle, or CPU pricing function. It is distinct from a CAPTCHA, which is intended for a human to solve quickly, while being difficult to solve for a computer.

🔗 Philosophy 🔗 Skepticism 🔗 Philosophy/Logic 🔗 Psychology

Magical thinking is a term used in anthropology, philosophy and psychology, denoting the causal relationships between thoughts, actions and events. There are subtle differences in meaning between individual theorists as well as amongst fields of study.

In anthropology, it denotes the attribution of causality between entities grouped with one another (coincidence) or similar to one another.

In psychology, the entities between which a causal relation has to be posited are more strictly delineated; here it denotes the belief that one's thoughts by themselves can bring about effects in the world or that thinking something corresponds with doing it. In both cases, the belief can cause a person to experience fear, seemingly not rationally justifiable to an observer outside the belief system, of performing certain acts or having certain thoughts because of an assumed correlation between doing so and threatening calamities.

In psychiatry, magical thinking is a disorder of thought content; here it denotes the false belief that one's thoughts, actions, or words will cause or prevent a specific consequence in some way that defies commonly understood laws of causality.

🔗 Economics

In finance, the rule of 72, the rule of 70 and the rule of 69.3 are methods for estimating an investment's doubling time. The rule number (e.g., 72) is divided by the interest percentage per period (usually years) to obtain the approximate number of periods required for doubling. Although scientific calculators and spreadsheet programs have functions to find the accurate doubling time, the rules are useful for mental calculations and when only a basic calculator is available.

These rules apply to exponential growth and are therefore used for compound interest as opposed to simple interest calculations. They can also be used for decay to obtain a halving time. The choice of number is mostly a matter of preference: 69 is more accurate for continuous compounding, while 72 works well in common interest situations and is more easily divisible. There is a number of variations to the rules that improve accuracy. For periodic compounding, the exact doubling time for an interest rate of r percent per period is

${\displaystyle t={\frac {\ln(2)}{\ln(1+r/100)}}\approx {\frac {72}{r}}}$,

where t is the number of periods required. The formula above can be used for more than calculating the doubling time. If one wants to know the tripling time, for example, replace the constant 2 in the numerator with 3. As another example, if one wants to know the number of periods it takes for the initial value to rise by 50%, replace the constant 2 with 1.5.

🔗 United States/U.S. Government 🔗 United States 🔗 Mass surveillance 🔗 Espionage 🔗 Military history 🔗 Military history/North American military history 🔗 Military history/United States military history 🔗 United States/Military history - U.S. military history 🔗 Military history/Military science, technology, and theory 🔗 Military history/Intelligence 🔗 United Kingdom 🔗 Military history/Australia, New Zealand and South Pacific military history 🔗 Military history/Canadian military history 🔗 Military history/European military history 🔗 Military history/British military history

ECHELON, originally a secret government code name, is a surveillance program (signals intelligence/SIGINT collection and analysis network) operated by the United States with the aid of four other signatory states to the UKUSA Security Agreement: Australia, Canada, New Zealand, and the United Kingdom, also known as the Five Eyes.

Created in the late 1960s to monitor the military and diplomatic communications of the Soviet Union and its Eastern Bloc allies during the Cold War, the ECHELON project became formally established in 1971.

By the end of the 20th century, the system referred to as "ECHELON" had evolved beyond its military and diplomatic origins into "a global system for the interception of private and commercial communications" (mass surveillance and industrial espionage).

🔗 Physics

In stellar physics, the Jeans instability causes the collapse of interstellar gas clouds and subsequent star formation, named after James Jeans. It occurs when the internal gas pressure is not strong enough to prevent gravitational collapse of a region filled with matter. For stability, the cloud must be in hydrostatic equilibrium, which in case of a spherical cloud translates to:

${\displaystyle {\frac {dp}{dr}}=-{\frac {G\rho (r)M_{enc}(r)}{r^{2}}}}$,

where ${\displaystyle M_{enc}(r)}$ is the enclosed mass, ${\displaystyle p}$ is the pressure, ${\displaystyle \rho (r)}$ is the density of the gas (at radius ${\displaystyle r}$), ${\displaystyle G}$ is the gravitational constant, and ${\displaystyle r}$ is the radius. The equilibrium is stable if small perturbations are damped and unstable if they are amplified. In general, the cloud is unstable if it is either very massive at a given temperature or very cool at a given mass; under these circumstances, the gas pressure cannot overcome gravity, and the cloud will collapse.