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In three dimensions the cross product is invariant under the action of the rotation group, SO(3), so the cross product of x and y after they are rotated is the image of x × y under the rotation. But this invariance is not true in seven dimensions; that is, the cross product is not invariant under the group of rotations in seven dimensions, SO(7).
When n = 7, the set of all such locations is called 7-dimensional space. Often such a space is studied as a vector space, without any notion of distance. Seven-dimensional Euclidean space is seven-dimensional space equipped with a Euclidean metric, which is defined by the dot product. [disputed – discuss]
A cross product for 7-dimensional vectors can be obtained in the same way by using the octonions instead of the quaternions. The nonexistence of nontrivial vector-valued cross products of two vectors in other dimensions is related to the result from Hurwitz's theorem that the only normed division algebras are the ones with dimension 1, 2, 4, and 8.
To paraphrase Baylis: Given two polar vectors (that is, true vectors) a and b in three dimensions, the cross product composed from a and b is the vector normal to their plane given by c = a × b. Given a set of right-handed orthonormal basis vectors { e ℓ}, the cross product is expressed in terms of its components as:
This also relates to the handedness of the cross product; the cross product transforms as a pseudovector under parity transformations and so is properly described as a pseudovector. The dot product of two vectors is a scalar but the dot product of a pseudovector and a vector is a pseudoscalar, so the scalar triple product (of vectors) must be ...
That source is about the cross product in 3D: it deduces the Schwarz inequality in general, then shows that in 3D the result gives the expression above. Nowhere does it work out a 7D result, so it is irrelevant to this article which is on the seven-dimensional cross product.--JohnBlackburne words deeds 12:51, 18 April 2010 (UTC)
From it and the other properties it's shown that a product only exists in 3 and 7 dimensions. For other reasons there are multiple products in 7D. Meanwhile please look at the two lines above. One is the expression for z 1 from the 7D cross product page, the other is what you claim is z 1 2. It is clear that if the first equation is squared you ...
E.g. the product of two vectors in three dimensions the binary product in three dimensions, the cross product, so to generate additional products we only need consider dimensions higher than three. The product of three vectors in four dimensions is an instance of n-1 in n dimensions, so is covered by the second point.--