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McGraw Hill Financial, Inc. is an American publicly traded corporation headquartered in Rockefeller Center in New York City. Its primary areas of business are financial, publishing, and business services.
An attempt to formulate the grammar of a language in mathematical terms. Language theory is an important area of linguistics and computer science. Formal language theory was initiated in the mid-1950s in an attempt to develop theories on natural language acquisition. This theory, and in particular context-free grammars, was found to be relevant to the languages used in computers. Interest in the relationship between abstract languages and automata theory began with a seminal paper by S. C. Kleene in 1956, in which he characterized the languages in which membership of a sentence could be decided by a finite-state machine. Formal or abstract languages are based on the mathematical notion of a language as defined by Noam Chomsky around 1956. To understand this concept, we may begin by defining what a language is. The <i>New Oxford American Dictionary</i> defines it as “the method of human communication, either spoken or written, consisting of the use of words in a structured and conventional way.” However, this definition is too vague to use as a building block of a language theory. To formalize the notion of an abstract language, it is necessary to introduce some preliminary definitions. An alphabet, vocabulary, or the set of terminals, denoted by Σ, is any finite, nonempty set of indivisible symbols. For example, the binary alphabet has only two symbols. This set is generally represented as Σ = (0, 1). A word or string, over a particular alphabet, is a finite sequence of symbols of the alphabet. In mathematical terms, a typical word, x, can be written as x = a<sub>1</sub>, a<sub>2</sub>, a<sub>3</sub>, …,a<sub>k</sub> where k ≥ 0, a<sub>i</sub> ∊ Σ for 1 ≤ i ≤ k. Notice that if k = 0, the word is called the null word or empty word and is denoted by Λ. For example, using the binary alphabet we can form the words x = 0010 and y = 010. Given a word, x, the number of occurrences of symbols of a given alphabet in the word is called the length of the word and is denoted by
Industry:Science
An audible sound wave generated by an object that moves faster than the speed of sound (supersonic object). The sonic boom forms because the air is pushed away faster than the air molecules can move. The displaced air becomes highly compressed and creates a very strong sound wave, referred to as a compressional head shock or bow shock. At the back of the supersonic object the air has to fill the void left as the object moves forward; in this case, the gas becomes rarefied and a rarefractional tail shock develops. These shock waves are the main components of a sonic boom, and they are generated the entire time that an object flies faster than the speed of sound, not just when it breaks the sonic barrier.
Industry:Science
An automatic means for steering an aircraft or other vehicle. The original use of an autopilot, or automatic pilot, was to provide pilot relief during cruise modes. Autopilots now perform functions more rapidly and with greater precision than the human pilot. The functions, designs, and uses of autopilots vary widely depending on the type of vehicle. In addition to controlling various types of aircraft and spacecraft, autopilots are used to control ships or sea-based vehicles and in some cases land-based vehicles. This article discusses autopilots used in aircraft and space vehicles.
Industry:Science
An autonomous undersea vehicle that can be launched from submarines, surface ships, or aircraft to attack enemy submarines and surface ships. An acoustic torpedo is a sophisticated weapon. Its main components are a guidance and control system, a power plant to provide propulsive and electrical energy, a propulsor to control speed and direction, and a warhead. The launching platform performs the function of determining the approximate location of the target and launching the torpedo in the proper direction. The torpedo typically utilizes an acoustic sensor in its nose and is controlled by an on-board computer. During its operation, the torpedo searches the volume of the ocean determined by the launch platform. It progresses through the following phases: detection (an object is present), classification (the object is a target of interest), homing (steer at the object), and detonation of the warhead.
Industry:Science
An autotransformer that has its winding divided into a number of equal-turn sections (usually 10) so that when an alternating voltage <i>V</i> is applied to the whole winding, the voltage across each section is nominally <i>V</i>/10. The progressive voltages from one end to the section junctions are thus <i>V</i>/10, 2<i>V</i>/10, 3<i>V</i>/10, … , and 9<i>V</i>/10. This voltage division can be realized with errors considerably less than 1 part per million of <i>V</i>, and such units therefore find wide use as standards of ac voltage ratio in the discipline of electrical measurements.
Industry:Science
An early listing of nebulae and star clusters. Although Charles Messier (1730–1817) was primarily a comet hunter, he is remembered for his catalog of objects. As the first serious and systematic comet hunter (his comet discoveries spanned from 1760 to 1801), Messier learned about the hazards of being misled by the galaxies, nebulae, and star clusters all over the sky that look like comets. In 1758, when Messier found a fuzzy patch near the star beta Tauri that he first thought was a comet but that never moved as a comet would, he began to build a catalog of these objects. “This nebula had such a resemblance to a comet, in its form and brightness,” Messier wrote, “that I endeavored to find others, so that astronomers would not confuse these same nebulae with comets just beginning to shine.” This first entry is the Crab Nebula, the remnant of the supernova, a near-total destruction of a star, that was observed on July 4, 1054, to be as bright as Venus. (The Crab Nebula was previously discovered by John Bevis.)
Industry:Science
An early mechanical calculator whose design has evolved through the centuries, with two styles in use today. Both the Chinese and the Japanese styles consist of a frame with a crossbeam. They may be made from many different materials, such as wood or brass. Rods or wires carrying sliding beads extend vertically through the crossbeam. The Chinese <i>suan pan</i> has two beads above the beam on each rod and five beads below. Each rod of the Japanese <i>soroban</i> carries one bead above and four below. Similar to the abacus in both construction and use, but much larger, are the counting frames used in elementary schools. Braille versions of the abacus are available for use by those without sight.
Industry:Science
An eclectic discipline comprising the technologies used for the exploitation of crude oil and natural gas reservoirs. It is usually subdivided into the branches of petrophysical, geological, reservoir drilling, production, and construction engineering. After an oil or gas accumulation is discovered, technical supervision of the reservoir is transferred to the petroleum engineering group, although in the exploration phase the drilling and petrophysical engineers have played a role in the completion and evaluation of the discovery.
Industry:Science
An economically important, multisystemic viral disease of cattle that is common worldwide. The causal agent is bovine viral diarrhea virus (BVDV). Cattle of all ages may be infected with this virus, and a variety of acute or chronic disease processes occur that may affect the digestive, respiratory, and reproductive systems.
Industry:Science
An effect arising from the relation between the angular momentum and the magnetization of a magnetic substance. It is the effect which is exploited in the measurement of the gyromagnetic ratio of magnetic materials. The gyromagnetic effect is demonstrated by a simple experiment in which a freely suspended magnetic substance is subjected to a magnetic field. Upon a change in direction of the magnetic field, the magnetization of the substance must change. In order for this to happen, the atoms must change their angular momentum. Since there are no external torques acting on the system, the total angular momentum must remain constant. Thus the sample must acquire a mass rotation which may be measured. In this way, the gyromagnetic ratio may be determined. Two common methods of determination are the Einstein-de Haas method and the Barnett method.
Industry:Science
