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In
geometry and
algebra, the triple product is a product of three 3-
dimensional vectors, usually
Euclidean vectors. The name "triple product" is used for two different products, the
scalar-valued scalar triple product and, less often, the
vector-valued vector triple product.
Scalar triple product
Three vectors defining a parallelepiped
The scalar triple product (also called the mixed product, box product, or triple scalar product) is defined as the
dot product of one of the vectors with the
cross product of the other two.
Geometric interpretation
Geometrically, the scalar triple product
is the (signed)
volume of the
parallelepiped defined by the three vectors given.
Properties
The scalar triple product is unchanged under a
circular shift of its three operands (a, b, c):
Swapping the positions of the operators without re-ordering the operands leaves the triple product unchanged. This follows from the preceding property and the commutative property of the dot product:
Swapping any two of the three operands
negates the triple product. This follows from the circular-shift property and the
anticommutativity of the cross product:
The scalar triple product can also be understood as the
determinant of the 3×3 matrix that has the three vectors either as its rows or its columns (a matrix has the same determinant as its
transpose):
If the scalar triple product is equal to zero, then the three vectors a, b, and c are
coplanar, since the parallelepiped defined by them would be flat and have no volume.
If any two vectors in the scalar triple product are equal, then its value is zero:
Also:
The
simple product of two triple products (or the square of a triple product), may be expanded in terms of dot products:[1]
This restates in vector notation that the product of the determinants of two 3×3 matrices equals the determinant of their matrix product. As a special case, the square of a triple product is a
Gram determinant.
The ratio of the triple product and the product of the three vector norms is known as a
polar sine:
which ranges between −1 and 1.
Scalar or pseudoscalar
Although the scalar triple product gives the volume of the parallelepiped, it is the signed volume, the sign depending on the
orientation of the frame or the
parity of the permutation of the vectors. This means the product is negated if the orientation is reversed, for example by a
parity transformation, and so is more properly described as a
pseudoscalar if the orientation can change.
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 pseudoscalar-valued.
Strictly speaking, a
scalar does not change at all under a coordinate transformation. (For example, the factor of 2 used for doubling a vector does not change if the vector is in spherical vs. rectangular coordinates.) However, if each vector is transformed by a matrix then the triple product ends up being multiplied by the determinant of the transformation matrix, which could be quite arbitrary for a non-rotation. That is, the triple product is more properly described as a
scalar density.
As an exterior product
The three vectors spanning a parallelepiped have triple product equal to its volume. (However, beware that the direction of the arrows in this diagram are incorrect.)
In
exterior algebra and
geometric algebra the exterior product of two vectors is a
bivector, while the exterior product of three vectors is a
trivector. A bivector is an oriented plane element and a trivector is an oriented volume element, in the same way that a vector is an oriented line element.
Given vectors a, b and c, the product
is a trivector with magnitude equal to the scalar triple product, i.e.
,
and is the
Hodge dual of the scalar triple product. As the exterior product is associative brackets are not needed as it does not matter which of a ∧ b or b ∧ c is calculated first, though the order of the vectors in the product does matter. Geometrically the trivector a ∧ b ∧ c corresponds to the parallelepiped spanned by a, b, and c, with bivectors a ∧ b, b ∧ c and a ∧ c matching the
parallelogram faces of the parallelepiped.
As a trilinear function
The triple product is identical to the
volume form of the Euclidean 3-space applied to the vectors via
interior product. It also can be expressed as a
contraction of vectors with a rank-3 tensor equivalent to the form (or a
pseudotensor equivalent to the volume pseudoform); see
below.
Vector triple product
The vector triple product is defined as the
cross product of one vector with the cross product of the other two. The following relationship holds:
.
This is known as triple product expansion, or Lagrange's formula,[2][3] although the latter name is also used for
several other formulas. Its right hand side can be remembered by using the
mnemonic "ACB − ABC", provided one keeps in mind which vectors are dotted together. A proof is provided
below. Some textbooks write the identity as such that a more familiar
mnemonic "BAC − CAB" is obtained, as in “back of the cab”.
Since the cross product is anticommutative, this formula may also be written (up to permutation of the letters) as:
From Lagrange's formula it follows that the vector triple product satisfies:
which is the
Jacobi identity for the cross product. Another useful formula follows:
These formulas are very useful in simplifying vector calculations in
physics. A related identity regarding
gradients and useful in
vector calculus is Lagrange's formula of vector cross-product identity:[4]
If geometric algebra is used the cross product b × c of vectors is expressed as their exterior product b∧c, a
bivector. The second cross product cannot be expressed as an exterior product, otherwise the scalar triple product would result. Instead a
left contraction[6] can be used, so the formula becomes[7]
The proof follows from the properties of the contraction.[6] The result is the same vector as calculated using a × (b × c).
referring to the -th component of the resulting vector. This can be simplified by performing a
contraction on the
Levi-Civita symbols,
where is the
Kronecker delta function ( when and when ) and is the
generalized Kronecker delta function. We can reason out this identity by recognizing that the index will be summed out leaving only and . In the first term, we fix and thus . Likewise, in the second term, we fix and thus .
Returning to the triple cross product,
Vector calculus
Consider the
flux integral of the vector field across the parametrically-defined surface : . The unit normal vector to the surface is given by , so the integrand is a scalar triple product.
This section needs expansion. You can help by
adding to it. (January 2014)
^Joseph Louis Lagrange did not develop the cross product as an algebraic product on vectors, but did use an equivalent form of it in components: see Lagrange, J-L (1773). "Solutions analytiques de quelques problèmes sur les pyramides triangulaires". Oeuvres. Vol. 3. He may have written a formula similar to the triple product expansion in component form. See also
Lagrange's identity and
Kiyosi Itô (1987). Encyclopedic Dictionary of Mathematics. MIT Press. p. 1679.
ISBN0-262-59020-4.