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This article documents notable spaceflight events during the year 2019.

In mathematical logic, Goodstein's theorem is a statement about the natural numbers, proved by Reuben Goodstein in 1944, which states that every Goodstein sequence eventually terminates at 0. Laurence Kirby and Jeff Paris showed that it is unprovable in Peano arithmetic (but it can be proven in stronger systems, such as second-order arithmetic). This was the third example of a true statement about natural numbers that is unprovable in Peano arithmetic, after the examples provided by Gödel's incompleteness theorem and Gerhard Gentzen's 1943 direct proof of the unprovability of ε₀-induction in Peano arithmetic. The Paris–Harrington theorem gave another example.

Kirby and Paris introduced a graph-theoretic hydra game with behavior similar to that of Goodstein sequences: the "Hydra" (named for the mythological multi-headed Hydra of Lerna) is a rooted tree, and a move consists of cutting off one of its "heads" (a branch of the tree), to which the hydra responds by growing a finite number of new heads according to certain rules. Kirby and Paris proved that the Hydra will eventually be killed, regardless of the strategy that Hercules uses to chop off its heads, though this may take a very long time. Just like for Goodstein sequences, Kirby and Paris showed that it cannot be proven in Peano arithmetic alone.

In probability theory and intertemporal portfolio choice, the Kelly criterion (or Kelly strategy or Kelly bet), also known as the scientific gambling method, is a formula for bet sizing that leads almost surely to higher wealth compared to any other strategy in the long run (i.e. approaching the limit as the number of bets goes to infinity). The Kelly bet size is found by maximizing the expected value of the logarithm of wealth, which is equivalent to maximizing the expected geometric growth rate. The Kelly Criterion is to bet a predetermined fraction of assets, and it can seem counterintuitive. It was described by J. L. Kelly Jr, a researcher at Bell Labs, in 1956.

For an even money bet, the Kelly criterion computes the wager size percentage by multiplying the percent chance to win by two, then subtracting one-hundred percent. So, for a bet with a 70% chance to win the optimal wager size is 40% of available funds.

The practical use of the formula has been demonstrated for gambling and the same idea was used to explain diversification in investment management. In the 2000s, Kelly-style analysis became a part of mainstream investment theory and the claim has been made that well-known successful investors including Warren Buffett and Bill Gross use Kelly methods. William Poundstone wrote an extensive popular account of the history of Kelly betting.

The High Arctic relocation (French: La délocalisation du Haut-Arctique, Inuktitut syllabics: ᖁᑦᑎᒃᑐᒥᐅᑦᑕ ᓅᑕᐅᓂᖏᑦ Inuktitut: Quttiktumut nuutauningit) took place during the Cold War in the 1950s, when 92 Inuit were moved by the Government of Canada to the High Arctic.

The relocation has been a source of controversy: on one hand being described as a humanitarian gesture to save the lives of starving indigenous people and enable them to continue a subsistence lifestyle; and on the other hand, said to be a forced migration instigated by the federal government to assert its sovereignty in the Far North by the use of "human flagpoles", in light of both the Cold War and the disputed territorial claims to the Canadian Arctic Archipelago. Both sides acknowledge that the relocated Inuit were not given sufficient support to prevent extreme privation during their first years after the move. The story was the subject of a book called The Long Exile, published by Melanie McGrath in 2006.

The Water Integrator (Russian: Гидравлический интегратор) was an early analog computer built in the Soviet Union in 1936 by Vladimir Sergeevich Lukyanov. It functioned by careful manipulation of water through a room full of interconnected pipes and pumps. The water level in various chambers (with precision to fractions of a millimeter) represented stored numbers, and the rate of flow between them represented mathematical operations. This machine was capable of solving inhomogeneous differential equations.

The first versions of Lukyanov's integrators were rather experimental, made of tin and glass tubes, and each integrator could be used to solve only one problem. In the 1930s it was the only computer in the Soviet Union for solving partial differential equations.

In 1941, Lukyanov created a hydraulic integrator of modular design, which made it possible to assemble a machine for solving various problems. Two-dimensional and three-dimensional hydraulic integrators were designed.

In 1949–1955, an integrator in the form of standard unified units was developed at the NIISCHETMASH Institute. In 1955, the Ryazan plant of calculating and analytical machines began the serial production of integrators with the factory brand name “IGL” (russian: Интегратор Гидравлический Лукьянова - integrator of the Lukyanov hydraulic system). Integrators were widely distributed, delivered to Czechoslovakia, Poland, Bulgaria and China.

A water integrator was used in the design of the Karakum Canal in the 1940s, and the construction of the Baikal–Amur Mainline in the 1970s. Water analog computers were used in the Soviet Union until the 1980s for large-scale modelling. They were used in geology, mine construction, metallurgy, rocket production and other fields.

Currently, two hydraulic integrators are kept in the Polytechnic Museum in Moscow.

In philosophy and psychology, an alief is an automatic or habitual belief-like attitude, particularly one that is in tension with a person's explicit beliefs.

For example, a person standing on a transparent balcony may believe that they are safe, but alieve that they are in danger. A person watching a sad movie may believe that the characters are completely fictional, but their aliefs may lead them to cry nonetheless. A person who is hesitant to eat fudge that has been formed into the shape of feces, or who exhibits reluctance in drinking from a sterilized bedpan may believe that the substances are safe to eat and drink, but may alieve that they are not.

The term alief was introduced by Tamar Gendler, a professor of philosophy and cognitive science at Yale University, in a pair of influential articles published in 2008. Since the publication of these original articles, the notion of alief has been utilized by Gendler and others — including Paul Bloom and Daniel Dennett — to explain a range of psychological phenomena in addition to those listed above, including the pleasure of stories, the persistence of positive illusions, certain religious beliefs, and certain psychiatric disturbances, such as phobias and obsessive–compulsive disorder.

In computing, tree shaking is a dead code elimination technique that is applied when optimizing code written in ECMAScript dialects like JavaScript or TypeScript into a single bundle that is loaded by a web browser. Often contrasted with traditional single-library dead code elimination techniques common to minifiers, tree shaking eliminates unused functions from across the bundle by starting at the entry point and only including functions that may be executed. It is succinctly described as "live code inclusion".

An exotic atom is an otherwise normal atom in which one or more sub-atomic particles have been replaced by other particles of the same charge. For example, electrons may be replaced by other negatively charged particles such as muons (muonic atoms) or pions (pionic atoms). Because these substitute particles are usually unstable, exotic atoms typically have very short lifetimes and all currently observed atoms cannot persist under normal conditions.

Elon Musk's Tesla Roadster is an electric sports car that served as the dummy payload for the February 2018 Falcon Heavy test flight and became an artificial satellite of the Sun. "Starman", a mannequin dressed in a spacesuit, occupies the driver's seat. The car and rocket are products of Tesla and SpaceX, respectively, both companies founded by Elon Musk. The 2008-model Roadster was previously used by Musk for commuting to work and is the only production car in space.

The car, mounted on the rocket's second stage, acquired enough velocity to escape Earth's gravity and enter an elliptical heliocentric orbit crossing the orbit of Mars. The orbit reaches a maximum distance from the Sun at aphelion of 1.66 astronomical units (au). During the early portion of the voyage outside the Earth's atmosphere, live video was transmitted back to the mission control center and live-streamed for slightly over four hours.

Advertising analysts noted Musk's sense of brand management and use of new media for his decision to launch a Tesla into space. While some commenters voiced concern that the car contributed to space debris, others saw it as a work of art. Musk explained he wanted to inspire the public about the "possibility of something new happening in space" as part of his larger vision for spreading humanity to other planets.

Who put Bella down the Wych Elm? is graffiti that appeared in 1944 following the 1943 discovery by four children of the skeletonised remains of a woman inside a wych elm in Hagley Wood, Hagley (located in the estate of Hagley Hall), in Worcestershire, England. The victim—whose murder is approximated to have occurred in 1941—remains unidentified, and the current location of her skeleton and autopsy report is unknown.