Polarization identity


In mathematics, the polarization identity is any one of a family of formulas that express the inner product of two vectors in terms of the norm of a normed vector space. Let denote the norm of vector x and the inner product of vectors x and y. Then the underlying theorem, attributed to Fréchet, von Neumann and Jordan, is stated as:

Formula

The various forms given below are all related by the parallelogram law:
The polarization identity can be generalized to various other contexts in abstract algebra, linear algebra, and functional analysis.

For vector spaces with real scalars

If V is a real vector space, then the inner product is defined by the polarization identity

For vector spaces with complex scalars

If V is a complex vector space the inner product is given by the polarization identity
where is the imaginary unit. Note that this defines an inner product which is anti-linear in its first argument and linear in its second argument. For the convention using the reversed definition, one needs to take the complex conjugate:

Other forms for real vector spaces

The parallelogram law can be used to derive other forms:

Theorem

In a normed space, if the parallelogram law holds, then there is an inner product on V such that for all.

Proof

Considering the real case the inner product will be the one suggested by the polarization identity
We need to prove that this is actually an inner product and that the norm induced by this product is which defines.

For to be an inner product it must satisfy the following properties:
In our case with
Which is obvious because
First we show that :
remembering that
Now using the parallelogram law we can see that:
We will use these three form depending on what we need.
Considering we can show that using the homogeneity and the subadditivity of :
Because .
This property holds in particular with as. Using symmetry and the sign property already shown above
Therefore. Being at the same time greater and smaller than 0 we can be sure that
If then let be :
Now we show that
Because .
Again considering the just found relation with in the place of we can prove that. Therefore
Combining the equations we have found we can conclude that Concluding that is linear and therefore it is an inner product.
Now we complete the proof showing that induces :

Application to dot products

Relation to the law of cosines

The second form of the polarization identity can be written as
This is essentially a vector form of the law of cosines for the triangle formed by the vectors u, v, and u - v. In particular,
where θ is the angle between the vectors u and v.

Derivation

The basic relation between the norm and the dot product is given by the equation
Then
and similarly
Forms and of the polarization identity now follow by solving these equations for u · v, while form follows from subtracting these two equations.

Generalizations

Norms

In linear algebra, the polarization identity applies to any norm on a vector space defined in terms of an inner product by the equation
As noted for the dot product case above, for real vectors u and v, an angle θ can be introduced using:
which is acceptable due to the Cauchy–Schwarz inequality:
This inequality ensures that the magnitude of the above defined cosine ≤ 1. The choice of the cosine function ensures that when , the angle θ = π/2 or −π/2, where the sign is determined by an orientation on the vector space.
In this case, the identities become
Conversely, if a norm on a vector space satisfies the parallelogram law, then any one of the above identities can be used to define a compatible inner product. In functional analysis, introduction of an inner product norm like this often is used to make a Banach space into a Hilbert space.

Symmetric bilinear forms

The polarization identities are not restricted to inner products. If B is any symmetric bilinear form on a vector space, and Q is the quadratic form defined by
then
The so-called symmetrization map generalizes the latter formula, replacing Q by a homogeneous polynomial of degree k defined by Q = B, where B is a symmetric k-linear map.
The formulas above even apply in the case where the field of scalars has characteristic two, though the left-hand sides are all zero in this case. Consequently, in characteristic two there is no formula for a symmetric bilinear form in terms of a quadratic form, and they are in fact distinct notions, a fact which has important consequences in L-theory; for brevity, in this context "symmetric bilinear forms" are often referred to as "symmetric forms".
These formulas also apply to bilinear forms on modules over a commutative ring, though again one can only solve for B if 2 is invertible in the ring, and otherwise these are distinct notions. For example, over the integers, one distinguishes integral quadratic forms from integral symmetric forms, which are a narrower notion.
More generally, in the presence of a ring involution or where 2 is not invertible, one distinguishes ε-quadratic forms and ε-symmetric forms; a symmetric form defines a quadratic form, and the polarization identity from a quadratic form to a symmetric form is called the "symmetrization map", and is not in general an isomorphism. This has historically been a subtle distinction: over the integers it was not until the 1950s that relation between "twos out" and "twos in" was understood – see discussion at integral quadratic form; and in the algebraization of surgery theory, Mishchenko originally used symmetric L-groups, rather than the correct quadratic L-groups – see discussion at L-theory.

Complex numbers

In linear algebra over the complex numbers, it is customary to use a sesquilinear inner product, with the property that is the complex conjugate of. In this case the standard polarization identities only give the real part of the inner product:
Using , the imaginary part of the inner product can be retrieved as follows:

Homogeneous polynomials of higher degree

Finally, in any of these contexts these identities may be extended to homogeneous polynomials of arbitrary degree, where it is known as the polarization formula, and is reviewed in greater detail in the article on the polarization of an algebraic form.
The polarization identity can be stated in the following way: