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In mathematics, particularly in linear algebra, tensor analysis, and differential geometry, the Levi-Civita symbol represents a collection of numbers; defined from the sign of a permutation of the natural numbers 1, 2, …, ''n'', for some positive integer ''n''. It is named after the Italian mathematician and physicist Tullio Levi-Civita. Other names include the permutation symbol, antisymmetric symbol, or alternating symbol, which refer to its antisymmetric property and definition in terms of permutations. The standard letters to denote the Levi-Civita symbol are the Greek lower case epsilon ''ε'' or ''ϵ'', or less commonly the Latin lower case ''e''. Index notation allows one to display permutations in a way compatible with tensor analysis: : where ''each'' index ''i''1, ''i''2, …, ''i''''n'' takes values 1, 2, …, ''n''. There are ''nn'' indexed values of , which can be arranged into an ''n''-dimensional array. The key definitive property of the symbol is ''total antisymmetry'' in all the indices. When any two indices are interchanged, equal or not, the symbol is negated: : If any two indices are equal, the symbol is zero. When all indices are unequal, we have: : where ''p'' (called the parity of the permutation) is the number of interchanges of indices necessary to unscramble ''i''1, ''i''2, …, ''i''''n'' into the order 1, 2, …, ''n'', and the factor (−1)''p'' is called the sign or signature of the permutation. The value ''ε''12…''n'' must be defined, else the particular values of the symbol for all permutations are indeterminate. Most authors choose , which means the Levi-Civita symbol equals the sign of a permutation when the indices are all unequal. This choice is used throughout this article. The term "''n''-dimensional Levi-Civita symbol" refers to the fact that the number of indices on the symbol ''n'' matches the dimensionality of the relevant vector space in question, which may be Euclidean or non-Euclidean, pure space or spacetime. The values of the Levi-Civita symbol are independent of any metric tensor and coordinate system. Also, the specific term "symbol" emphasizes that it is not a tensor because of how it transforms between coordinate systems, however it can be interpreted as a tensor density. The Levi-Civita symbol allows the determinant of a square matrix, and the cross product of two vectors in 3d Euclidean space, to be expressed in index notation. ==Definition== The common dimensionalities of the Levi-Civita symbol are in 3d and 4d, and to some extent 2d, so it is useful to see these definitions before the general one in any number of dimensions. 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「Levi-Civita symbol」の詳細全文を読む スポンサード リンク
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