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🔗 Mathematics

The dangerous bend or caution symbol ☡ (U+2621 ☡ CAUTION SIGN) was created by the Nicolas Bourbaki group of mathematicians and appears in the margins of mathematics books written by the group. It resembles a road sign that indicates a "dangerous bend" in the road ahead, and is used to mark passages tricky on a first reading or with an especially difficult argument.

🔗 Novels 🔗 Novels/Science fiction 🔗 Novels/Short story

Axiomatic (ISBN 0-7528-1650-0) is a 1995 collection of short science fiction stories by Greg Egan. The stories all delve into different aspects of self and identity.

The Guardian described it as "Wonderful mind-expanding stuff, and well-written too."

🔗 Biography 🔗 Philosophy 🔗 Middle Ages 🔗 Islam 🔗 Middle Ages/History 🔗 Philosophy/Philosophers 🔗 Philosophy/Philosophy of religion 🔗 Syria 🔗 Philosophy/Ancient philosophy

Abū al-ʿAlāʾ al-Maʿarrī (Arabic: أبو العلاء المعري‎‎, full name أبو العلاء أحمد بن عبد الله بن سليمان التنوخي المعري‎ Abū al-ʿAlāʾ Aḥmad ibn ʿAbd Allāh ibn Sulaymān al-Tanūkhī al-Maʿarrī; December 973 – May 1057) was a blind Arab philosopher, poet, and writer. Despite holding a controversially irreligious worldview, he is regarded as one of the greatest classical Arabic poets.

Born in the city of Ma'arra during the Abbasid era, he studied in nearby Aleppo, then in Tripoli and Antioch. Producing popular poems in Baghdad, he nevertheless refused to sell his texts. In 1010, he returned to Syria after his mother began declining in health, and continued writing which gained him local respect.

Described as a "pessimistic freethinker", al-Ma'arri was a controversial rationalist of his time, citing reason as the chief source of truth and divine revelation. He was pessimistic about life, describing himself as "a double prisoner" of blindness and isolation. He attacked religious dogmas and practices, was equally critical and sarcastic about Judaism, Christianity, Islam and Zoroastrianism, and became a Deist.

He advocated social justice and lived a secluded, ascetic lifestyle. He was a vegan, known in his time as moral vegetarianism, entreating: "do not desire as food the flesh of slaughtered animals / Or the white milk of mothers who intended its pure draught / for their young". Al-Ma'arri held an antinatalist outlook, in line with his general pessimism, suggesting that children should not be born to spare them of the pains and suffering of life.

Al-Ma'arri wrote three main works that were popular in his time. Among his works are The Tinder Spark, Unnecessary Necessity, and The Epistle of Forgiveness. Al-Ma'arri never married and died at the age of 83 in the city where he was born, Ma'arrat al-Nu'man. In 2013, a statue of al-Ma'arri located in his Syrian hometown was beheaded by jihadists from the al-Nusra Front.

🔗 Military history 🔗 Military history/Military science, technology, and theory 🔗 Military history/Weaponry 🔗 Skepticism 🔗 Law Enforcement 🔗 Iraq 🔗 British crime 🔗 Explosives

The ADE 651 is a fake bomb detector produced by the British company Advanced Tactical Security & Communications Ltd (ATSC). Its manufacturer claimed it could detect bombs, guns, ammunition, and more from kilometers away. However, it was a scam, and the device was little more than a dowsing rod. The device was sold for up to US$60,000 each, despite costing almost nothing to produce. It was widely used in the Middle East, and may have led to numerous deadly bombings in Iraq due to its inability to detect explosives. Its inventor, James McCormick, was sentenced to 10 years in prison in 2013 for fraud.

🔗 Cryptography 🔗 Cryptography/Computer science 🔗 Citizendium Porting

Kerckhoffs's principle (also called Kerckhoffs's desideratum, assumption, axiom, doctrine or law) of cryptography was stated by Netherlands born cryptographer Auguste Kerckhoffs in the 19th century: A cryptosystem should be secure even if everything about the system, except the key, is public knowledge.

Kerckhoffs's principle was reformulated (or possibly independently formulated) by American mathematician Claude Shannon as "the enemy knows the system", i.e., "one ought to design systems under the assumption that the enemy will immediately gain full familiarity with them". In that form, it is called Shannon's maxim. This concept is widely embraced by cryptographers, in contrast to "security through obscurity", which is not.

🔗 Linguistics 🔗 New York (state) 🔗 New York (state)/Western New York

"Buffalo buffalo Buffalo buffalo buffalo buffalo Buffalo buffalo" is a grammatically correct sentence in American English, often presented as an example of how homonyms and homophones can be used to create complicated linguistic constructs through lexical ambiguity. It has been discussed in literature in various forms since 1967, when it appeared in Dmitri Borgmann's Beyond Language: Adventures in Word and Thought.

The sentence employs three distinct meanings of the word buffalo:

• as a proper noun to refer to a specific place named Buffalo, the city of Buffalo, New York, being the most notable;
• as a verb (uncommon in regular usage) to buffalo, meaning "to bully, harass, or intimidate" or "to baffle"; and
• as a noun to refer to the animal, bison (often called buffalo in North America). The plural is also buffalo.

An expanded form of the sentence which preserves the original word order is: "Buffalo bison, that other Buffalo bison bully, also bully Buffalo bison."

Alternative Title: Just because you have some money, that doesn't mean that you have to spend it.

In biology, the hallmarks of an aggressive cancer include limitless multiplication of ordinarily beneficial cells, even when the body signals that further multiplication is no longer needed. The Wikipedia page on the wheat and chessboard problem explains that nothing can keep growing forever. In biology, the unwanted growth usually terminates with the death of the host. Ever-increasing spending can often lead to the same undesirable result in organizations.

🔗 Computing 🔗 Computer science 🔗 Lists 🔗 Science Fiction

Computers have often been used as fictional objects in literature, movies and in other forms of media. Fictional computers tend to be considerably more sophisticated than anything yet devised in the real world.

This is a list of computers that have appeared in notable works of fiction. The work may be about the computer, or the computer may be an important element of the story. Only static computers are included. Robots and other fictional computers that are described as existing in a mobile or humanlike form are discussed in a separate list of fictional robots and androids.

🔗 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.

🔗 Geology

The Chesapeake Bay impact crater was formed by a bolide that impacted the eastern shore of North America about 35.5 ± 0.3 million years ago, in the late Eocene epoch. It is one of the best-preserved "wet-target" impact craters in the world.

Continued slumping of sediments over the rubble of the crater has helped shape the Chesapeake Bay.