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Seven-dimensional cross product : ウィキペディア英語版
Seven-dimensional cross product
In mathematics, the seven-dimensional cross product is a bilinear operation on vectors in seven-dimensional Euclidean space. It assigns to any two vectors a, b in R7 a vector also in R7.〔
〕 Like the cross product in three dimensions, the seven-dimensional product is anticommutative and is orthogonal both to a and to b. Unlike in three dimensions, it does not satisfy the Jacobi identity. And while the three-dimensional cross product is unique up to a sign, there are many seven-dimensional cross products. The seven-dimensional cross product has the same relationship to octonions as the three-dimensional product does to quaternions.
The seven-dimensional cross product is one way of generalising the cross product to other than three dimensions, and it is the only other non-trivial bilinear product of two vectors that is vector valued, anticommutative and orthogonal.〔 In other dimensions there are vector-valued products of three or more vectors that satisfy these conditions, and binary products with bivector results.
==Multiplication table==

The product can be given by a multiplication table, such as the one here. This table, due to Cayley,〔
〕〔

〕 gives the product of basis vectors e''i'' and e''j'' for each ''i'', ''j'' from 1 to 7. For example from the table
:\mathbf_1 \times \mathbf_2 = \mathbf_3 =-\mathbf_2 \times \mathbf_1
The table can be used to calculate the product of any two vectors. For example to calculate the e1 component of x × y the basis vectors that multiply to produce e1 can be picked out to give
:\left( \mathbf\right)_1 = x_2y_3 - x_3y_2 +x_4y_5-x_5y_4 + x_7y_6-x_6y_7.
This can be repeated for the other six components.
There are 480 such tables, one for each of the products satisfying the definition.〔
〕 This table can be summarized by the relation〔
:\mathbf_i \mathbf \mathbf_j = \varepsilon _ \mathbf_k,
where \varepsilon _ is a completely antisymmetric tensor with a positive value +1 when ''ijk'' = 123, 145, 176, 246, 257, 347, 365.
The top left 3 × 3 corner of this table gives the cross product in three 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「Seven-dimensional cross product」の詳細全文を読む



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