The Binet–Cauchy identity and exterior algebra[edit]
When n = 3, the first and second terms on the right hand side become the squared magnitudes of dot and cross products respectively; in n dimensions these become the magnitudes of the dot and wedge products. We may write it
where a, b, c, and d are vectors. It may also be written as a formula giving the dot product of two wedge products, as
which can be written as
in the n = 3 case.
In the special case a = c and b = d, the formula yields
When both a and b are unit vectors, we obtain the usual relation
where φ is the angle between the vectors.
This is a special case of the inner product on the exterior algebra of a vector space, which is defined on wedge-decomposable elements as the Gram determinant of their components.
A general form, also known as the Cauchy–Binet formula, states the following:
Suppose A is an m×nmatrix and B is an n×m matrix. If S is a subset of {1, ..., n} with m elements, we write AS for the m×m matrix whose columns are those columns of A that have indices from S. Similarly, we write BS for the m×m matrix whose rows are those rows of B that have indices from S.
Then the determinant of the matrix product of A and B satisfies the identity
where the sum extends over all possible subsets S of {1, ..., n} with m elements.
We get the original identity as special case by setting