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Olivia MFSK is an amateur radioteletype protocol, using multiple frequency-shift keying (MFSK) and designed to work in difficult (low signal-to-noise ratio plus multipath propagation) conditions on shortwave bands. The signal can be accurately received even if the surrounding noise is 10 dB stronger. It is commonly used by amateur radio operators to reliably transmit ASCII characters over noisy channels using the high frequency (3–30 MHz) spectrum. The effective data rate of the Olivia MFSK protocol is 150 characters/minute.

Olivia modes are commonly referred to as Olivia X / Y (or, alternatively, Olivia Y / X ), where X refers to the number of different audio tones transmitted and Y refers to the bandwidth in hertz over which these signals are spread. Examples of common Olivia modes are 16/500, 32/1000 and 8/250.

Gauss's Pythagorean right triangle proposal is an idea attributed to Carl Friedrich Gauss for a method to signal extraterrestrial beings by constructing an immense right triangle and three squares on the surface of the Earth. The shapes would be a symbolic representation of the Pythagorean theorem, large enough to be seen from the Moon or Mars.

Although credited in numerous sources as originating with Gauss, with exact details of the proposal set out, the specificity of detail, and even whether Gauss made the proposal, have been called into question. Many of the earliest sources do not actually name Gauss as the originator, instead crediting a "German astronomer" or using other nonspecific descriptors, and in some cases naming a different author entirely. The details of the proposal also change significantly upon different retellings. Nevertheless, Gauss's writings reveal a belief and interest in finding a method to contact extraterrestrial life, and that he did, at the least, propose using amplified light using a heliotrope, his own 1818 invention, to signal supposed inhabitants of the Moon.

The O-ring theory of economic development is a model of economic development put forward by Michael Kremer in 1993, which proposes that tasks of production must be executed proficiently together in order for any of them to be of high value. The key feature of this model is positive assortative matching, whereby people with similar skill levels work together.

The name comes from the 1986 Challenger shuttle disaster, a catastrophe caused by the failure of a single O-ring.

Kremer thinks that the O-ring development theory explains why rich countries produce more complicated products, have larger firms and much higher worker productivity than poor countries.

iPTF14hls is an unusual supernova star that erupted continuously for about 1,000 days beginning in September 2014 before becoming a remnant nebula. It had previously erupted in 1954. None of the theories nor proposed hypotheses fully explain all the aspects of the object.

The Mariko Aoki phenomenon (青木まりこ現象, Aoki Mariko genshō) is a Japanese expression referring to an urge to defecate that is suddenly felt after entering bookstores. The phenomenon's name derives from the name of the woman who mentioned the phenomenon in a magazine article in 1985. According to Japanese social psychologist Shozo Shibuya, the specific causes that trigger a defecation urge in bookstores are not yet clearly understood (as of 2014). There are also some who are skeptical about whether such a peculiar phenomenon really exists at all, and it is sometimes discussed as one type of urban myth.

The series of processes through which being in a bookstore leads to an awareness of a defecation urge is something that cannot be explained from a medical perspective as a single pathological concept, at least at present. According to a number of discussions on the topic, even if it can be sufficiently found that this phenomenon actually exists, it is a concept that would be difficult to be deemed a specific pathological entity (such as a "Mariko Aoki disease", for example). On the other hand, it is also a fact that a considerable number of the intellectuals (particularly clinicians) who discuss this phenomenon have adopted existing medical terminology such as from diagnostics and pathology. Borrowing from this approach, this article also uses expressions from existing medical terminology for convenience.

The Stonehenge Archer is the name given to a Bronze Age man whose body was discovered in the outer ditch of Stonehenge. Unlike most burials in the Stonehenge Landscape, his body was not in a barrow, although it did appear to have been deliberately and carefully buried in the ditch.

Examination of the skeleton indicated that the man was local to the area and aged about 30 when he died. Radiocarbon dating suggests that he died around 2300 BCE, making his death roughly contemporary with the Amesbury Archer and the Boscombe Bowmen buried 3 miles away in Amesbury.

He came to be known as an archer because of the stone wrist-guard and a number of flint arrowheads buried with him. In fact, several of the arrowheads' tips were located in the skeleton's bones, suggesting that the man had been killed by them.

His body was excavated in 1978 by Richard Atkinson and John G. Evans who had been re-examining an older trench in the ditch and bank of Stonehenge. His remains are now housed in the Salisbury Museum in Salisbury.

A prime number (or a prime) is a natural number greater than 1 that cannot be formed by multiplying two smaller natural numbers. A natural number greater than 1 that is not prime is called a composite number. For example, 5 is prime because the only ways of writing it as a product, 1 × 5 or 5 × 1, involve 5 itself. However, 6 is composite because it is the product of two numbers (2 × 3) that are both smaller than 6. Primes are central in number theory because of the fundamental theorem of arithmetic: every natural number greater than 1 is either a prime itself or can be factorized as a product of primes that is unique up to their order.

The property of being prime is called primality. A simple but slow method of checking the primality of a given number ${\displaystyle n}$, called trial division, tests whether ${\displaystyle n}$ is a multiple of any integer between 2 and ${\displaystyle {\sqrt {n}}}$. Faster algorithms include the Miller–Rabin primality test, which is fast but has a small chance of error, and the AKS primality test, which always produces the correct answer in polynomial time but is too slow to be practical. Particularly fast methods are available for numbers of special forms, such as Mersenne numbers. As of December 2018 the largest known prime number has 24,862,048 decimal digits.

There are infinitely many primes, as demonstrated by Euclid around 300 BC. No known simple formula separates prime numbers from composite numbers. However, the distribution of primes within the natural numbers in the large can be statistically modelled. The first result in that direction is the prime number theorem, proven at the end of the 19th century, which says that the probability of a randomly chosen number being prime is inversely proportional to its number of digits, that is, to its logarithm.

Several historical questions regarding prime numbers are still unsolved. These include Goldbach's conjecture, that every even integer greater than 2 can be expressed as the sum of two primes, and the twin prime conjecture, that there are infinitely many pairs of primes having just one even number between them. Such questions spurred the development of various branches of number theory, focusing on analytic or algebraic aspects of numbers. Primes are used in several routines in information technology, such as public-key cryptography, which relies on the difficulty of factoring large numbers into their prime factors. In abstract algebra, objects that behave in a generalized way like prime numbers include prime elements and prime ideals.

A planimeter, also known as a platometer, is a measuring instrument used to determine the area of an arbitrary two-dimensional shape.

A solar pond is a pool of saltwater which collects and stores solar thermal energy. The saltwater naturally forms a vertical salinity gradient also known as a "halocline", in which low-salinity water floats on top of high-salinity water. The layers of salt solutions increase in concentration (and therefore density) with depth. Below a certain depth, the solution has a uniformly high salt concentration.

The language of the court and government of the Ottoman Empire was Ottoman Turkish, but many other languages were in contemporary use in parts of the empire. Although the minorities of the Ottoman Empire were free to use their language amongst themselves, if they needed to communicate with the government they had to use Ottoman Turkish.

The Ottomans had three influential languages: Turkish, spoken by the majority of the people in Anatolia and by the majority of Muslims of the Balkans except in Albania, Bosnia, and various Aegean Sea islands; Persian, initially used by the educated in northern portions of the Ottoman Empire before being displaced by Ottoman Turkish; and Arabic, used in southern portions of the Ottoman Empire; Arabic was spoken mainly in Arabia, North Africa, Mesopotamia and the Levant. Throughout the vast Ottoman bureaucracy Ottoman Turkish language was the official language, a version of Turkish, albeit with a vast mixture of both Arabic and Persian grammar and vocabulary.

Virtually all intellectual and literate pursuits were taken in Turkish language. Some ordinary people had to hire special "request-writers" (arzuhâlcis) to be able to communicate with the government. The ethnic groups continued to speak within their families and neighborhoods (mahalles) with their own languages (e.g., Jews, Greeks, Armenians, etc.) In villages where two or more populations lived together, the inhabitants would often speak each other's language. In cosmopolitan cities, people often spoke their family languages, many non-ethnic Turks spoke Turkish as a second language. Educated Ottoman Turks spoke Arabic and Persian, as these were the main foreign languages in the pre-Tanzimat era, with the former being used for science and the latter for literary affairs.

In the last two centuries, French and English emerged as popular languages, especially among the Christian Levantine communities. The elite learned French at school, and used European products as a fashion statement. The use of Ottoman Turkish for science and literature grew steadily under the Ottomans, while Persian declined in those functions. Ottoman Turkish, during the period, gained many loanwords from Arabic and Persian. Up to 88% of the vocabulary of a particular work would be borrowed from those two languages.

Linguistic groups were varied and overlapping. In the Balkan Peninsula, Slavic, Greek and Albanian speakers were the majority, but there were substantial minorities of Turks and Romance-speaking Vlachs. In most of Anatolia, Turkish was the majority language, but Greek, Armenian and, in the east and southeast, Kurdish were also spoken. In Syria, Iraq, Arabia, Egypt and north Africa, most of the population spoke varieties of Arabic with, above them, a Turkish-speaking elite. However, in no province of the Empire was there a unique language.