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8 (eight ) is the natural number following 7 and preceding 9. ==In mathematics== 8 is a composite number, its proper divisors being 1, 2, and 4. It is twice 4 or four times 2. Eight is a power of two, being (two cubed), and is the first number of the form , p being an integer greater than 1. It is also the first number which is neither prime nor semiprime. It has an aliquot sum of 7 in the 4 member aliquot sequence (8,7,1,0) being the first member of 7-aliquot tree. All powers of 2 ;(), have an aliquot sum of one less than themselves. A number is divisible by 8 if its last 3 digits are also divisible by 8. Eight is the first number to be the aliquot sum of two numbers other than itself; the discrete biprime 10, and the square number 49. 8 is the base of the octal number system, which is mostly used with computers. In octal, one digit represents 3 bits. In modern computers, a byte is a grouping of eight bits, also called an octet. The number 8 is a Fibonacci number, being 3 plus 5. The next Fibonacci number is 13. 8 is the only positive Fibonacci number, aside from 1, that is a perfect cube.〔Bryan Bunch, ''The Kingdom of Infinite Number''. New York: W. H. Freeman & Company (2000): 88〕 8 is the only nonzero perfect power that is one less than another perfect power, by Mihăilescu's Theorem. 8 is the order of the smallest non-abelian group all of whose subgroups are normal. 8 and 9 form a Ruth–Aaron pair under the second definition in which repeated prime factors are counted as often as they occur. There are a total of eight convex deltahedra. A polygon with eight sides is an octagon. Figurate numbers representing octagons (including eight) are called octagonal numbers. A polyhedron with eight faces is an octahedron. A cuboctahedron has as faces six equal squares and eight equal regular triangles. A cube has eight vertices. Sphenic numbers always have exactly eight divisors. 8 is the dimension of the octonions and is the highest possible dimension of a normed division algebra. The number 8 is involved with a number of interesting mathematical phenomena related to the notion of Bott periodicity. For example, if is the direct limit of the inclusions of real orthogonal groups then . Clifford algebras also display a periodicity of 8. For example, the algebra is isomorphic to the algebra of 16 by 16 matrices with entries in . We also see a period of 8 in the K-theory of spheres and in the representation theory of the rotation groups, the latter giving rise to the 8 by 8 spinorial chessboard. All of these properties are closely related to the properties of the octonions. The spin group is the unique such group that exhibits the phenomenon of triality. The lowest-dimensional even unimodular lattice is the 8-dimensional E8 lattice. Even positive definite unimodular lattice exist only in dimensions divisible by 8. A figure 8 is the common name of a geometric shape, often used in the context of sports, such as skating. Figure-eight turns of a rope or cable around a cleat, pin, or bitt are used to belay something. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「8 (number)」の詳細全文を読む スポンサード リンク
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