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In mathematics, the quadruple product is a product of four vectors in three-dimensional Euclidean space. The name "quadruple product" is used for two different products, the scalar-valued scalar quadruple product and the vector-valued vector quadruple product.
Scalar quadruple product
or using the determinant:
Vector quadruple product
The vector quadruple product is defined as the cross product of two cross products:
using the notation for the triple product:
where the last two forms are determinants with denoting unit vectors along three mutually orthogonal directions.
Equivalent forms can be obtained using the identity:
The quadruple products are useful for deriving various formulas in spherical and plane geometry. For example, if four points are chosen on the unit sphere, A, B, C, D, and unit vectors drawn from the center of the sphere to the four points, a, b, c, d respectively, the identity:
in conjunction with the relation for the magnitude of the cross product:
and the dot product:
where a = b = 1 for the unit sphere, results in the identity among the angles attributed to Gauss:
where x is the angle between a × b and c × d, or equivalently, between the planes defined by these vectors.
- Gibbs, Josiah Willard; Wilson, Edwin Bidwell (1901). Vector analysis: a text-book for the use of students of mathematics. Scribner.