# Bilinear form

In mathematics, more specifically in abstract algebra and linear algebra, a **bilinear form** on a vector space *V* is a bilinear map *V* × *V* → *K*, where *K* is the field of scalars. In other words, a bilinear form is a function *B* : *V* × *V* → *K* which is linear in each argument separately:

*B*(**u**+**v**,**w**) =*B*(**u**,**w**) +*B*(**v**,**w**)*B*(**u**,**v**+**w**) =*B*(**u**,**v**) +*B*(**u**,**w**)*B*(*λ***u**,**v**) =*B*(**u**,*λ***v**) =*λB*(**u**,**v**)

The definition of a bilinear form can be extended to include modules over a commutative ring, with linear maps replaced by module homomorphisms.

When *K* is the field of complex numbers **C**, one is often more interested in sesquilinear forms, which are similar to bilinear forms but are conjugate linear in one argument.

## Coordinate representation

Let *V* ≅ *K ^{n}* be an

*n*-dimensional vector space with basis {

**e**

_{1}, ...,

**e**

_{n}}. Define the

*n*×

*n*matrix

*A*by

*A*=

_{ij}*B*(

**e**

_{i},

**e**

_{j}). If the

*n*× 1 matrix

*x*represents a vector

**v**with respect to this basis, and analogously,

*y*represents

**w**, then:

Suppose {**f**_{1}, ..., **f**_{n}}Template:Void is another basis for *V*, such that:

- [
**f**_{1}, ...,**f**_{n}] = [**e**_{1}, ...,**e**_{n}]*S*

where *S* ∈ GL(*n*, *K*). Now the new matrix representation for the bilinear form is given by: *S*^{T}*AS*.

## Maps to the dual space

Every bilinear form *B* on *V* defines a pair of linear maps from *V* to its dual space *V*^{∗}. Define *B*_{1}, *B*_{2}: *V* → *V*^{∗} by

*B*_{1}(**v**)(**w**) =*B*(**v**,**w**)*B*_{2}(**v**)(**w**) =*B*(**w**,**v**)

This is often denoted as

*B*_{1}(**v**) =*B*(**v**, ⋅)*B*_{2}(**v**) =*B*(⋅,**v**)

where the dot ( ⋅ ) indicates the slot into which the argument for the resulting linear functional is to be placed.

For a finite-dimensional vector space *V*, if either of *B*_{1} or *B*_{2} is an isomorphism, then both are, and the bilinear form *B* is said to be nondegenerate. More concretely, for a finite-dimensional vector space, non-degenerate means that every non-zero element pairs non-trivially with some other element:

The corresponding notion for a module over a ring is that a bilinear form is **Template:Visible anchor** if *V* → *V*^{∗} is an isomorphism. Given a finite-dimensional module over a commutative ring, the pairing may be injective (hence "nondegenerate" in the above sense) but not unimodular. For example, over the integers, the pairing *B*(*x*, *y*) = 2*xy* is nondegenerate but not unimodular, as the induced map from *V* = **Z** to *V*^{∗} = **Z** is multiplication by 2.

If *V* is finite-dimensional then one can identify *V* with its double dual *V*^{∗∗}. One can then show that *B*_{2} is the transpose of the linear map *B*_{1} (if *V* is infinite-dimensional then *B*_{2} is the transpose of *B*_{1} restricted to the image of *V* in *V*^{∗∗}). Given *B* one can define the *transpose* of *B* to be the bilinear form given by

*B*^{∗}(**v**,**w**) =*B*(**w**,**v**).

The **left radical** and **right radical** of the form *B* are the kernels of *B*_{1} and *B*_{2} respectively;^{[1]} they are the vectors orthogonal to the whole space on the left and on the right.^{[2]}

If *V* is finite-dimensional then the rank of *B*_{1} is equal to the rank of *B*_{2}. If this number is equal to dim(*V*) then *B*_{1} and *B*_{2} are linear isomorphisms from *V* to *V*^{∗}. In this case *B* is nondegenerate. By the rank–nullity theorem, this is equivalent to the condition that the left and equivalently right radicals be trivial. For finite-dimensional spaces, this is often taken as the *definition* of nondegeneracy:

Definition:Bis nondegenerate if and only ifB(v,w) = 0 for allwimpliesv=0.

Given any linear map *A* : *V* → *V*^{∗} one can obtain a bilinear form *B* on *V* via

*B*(**v**,**w**) =*A*(**v**)(**w**).

This form will be nondegenerate if and only if *A* is an isomorphism.

If *V* is finite-dimensional then, relative to some basis for *V*, a bilinear form is degenerate if and only if the determinant of the associated matrix is zero. Likewise, a nondegenerate form is one for which the determinant of the associated matrix is non-zero (the matrix is non-singular). These statements are independent of the chosen basis. For a module over a ring, a unimodular form is one for which the determinant of the associate matrix is a unit (for example 1), hence the term; note that a form whose matrix is non-zero but not a unit will be nondegenerate but not unimodular, for example *B*(*x*, *y*) = 2*xy* over the integers.

## Symmetric, skew-symmetric and alternating forms

We define a form to be

**symmetric**if*B*(**v**,**w**) =*B*(**w**,**v**) for all**v**,**w**in*V*;**alternating**if*B*(**v**,**v**) = 0 for all**v**in*V*;**skew-symmetric**if*B*(**v**,**w**) = −*B*(**w**,**v**) for all**v**,**w**in*V*;

Proposition:Every alternating form is skew-symmetric.

Proof:This can be seen by expandingB(v+w,v+w).

If the characteristic of *K* is not 2 then the converse is also true: every skew-symmetric form is alternating. If, however, char(*K*) = 2 then a skew-symmetric form is the same as a symmetric form and there exist symmetric/skew-symmetric forms which are not alternating.

A bilinear form is symmetric (resp. skew-symmetric) if and only if its coordinate matrix (relative to any basis) is symmetric (resp. skew-symmetric). A bilinear form is alternating if and only if its coordinate matrix is skew-symmetric and the diagonal entries are all zero (which follows from skew-symmetry when char(*K*) ≠ 2).

A bilinear form is symmetric if and only if the maps *B*_{1}, *B*_{2}: *V* → *V*^{∗} are equal, and skew-symmetric if and only if they are negatives of one another. If char(*K*) ≠ 2 then one can decompose a bilinear form into a symmetric and a skew-symmetric part as follows

where *B*^{∗} is the transpose of *B* (defined above).

## Derived quadratic form

For any bilinear form *B* : *V* × *V* → *K*, there exists an associated quadratic form *Q* : *V* → *K* defined by *Q* : *V* → *K* : **v** ↦ *B*(**v**,**v**).

When char(*K*) ≠ 2, the quadratic form *Q* is determined by the symmetric part of the bilinear form *B* and is independent of the antisymmetric part. In this case there is a one-to-one correspondence between the symmetric part of the bilinear form and the quadratic form, and it makes sense to speak of the symmetric bilinear form associated with a quadratic form.

When char(*K*) = 2 and dim *V* > 1, this correspondence between quadratic forms and symmetric bilinear forms breaks down.

## Reflexivity and orthogonality

Definition:A bilinear formB:V×V→Kis calledreflexiveifB(v,w) = 0 impliesB(w,v) = 0 for allv,winV.

Definition:LetB:V×V→Kbe a reflexive bilinear form.v,winVareorthogonal with respect toif and only ifBB(v,w) = 0.

A form *B* is reflexive if and only if it is either symmetric or alternating.^{[3]} In the absence of reflexivity we have to distinguish left and right orthogonality. In a reflexive space the left and right radicals agree and are termed the *kernel* or the *radical* of the bilinear form: the subspace of all vectors orthogonal with every other vector. A vector **v**, with matrix representation *x*, is in the radical of a bilinear form with matrix representation *A*, if and only if *Ax* = 0 ⇔ *x*^{T}*A* = 0. The radical is always a subspace of *V*. It is trivial if and only if the matrix *A* is nonsingular, and thus if and only if the bilinear form is nondegenerate.

Suppose *W* is a subspace. Define the *orthogonal complement*^{[4]}

For a non-degenerate form on a finite dimensional space, the map *W* ↦ *W*^{⊥} is bijective, and the dimension of *W*^{⊥} is dim(*V*) − dim(*W*).

## Different spaces

Much of the theory is available for a bilinear mapping to the base field

*B*:*V*×*W*→*K*.

In this situation we still have induced linear mappings from *V* to *W*^{∗}, and from *W* to *V*^{∗}. It may happen that these mappings are isomorphisms; assuming finite dimensions, if one is an isomorphism, the other must be. When this occurs, *B* is said to be a **perfect pairing**.

In finite dimensions, this is equivalent to the pairing being nondegenerate (the spaces necessarily having the same dimensions). For modules (instead of vector spaces), just as how a nondegenerate form is weaker than a unimodular form, a nondegenerate pairing is a weaker notion than a perfect pairing. A pairing can be nondegenerate without being a perfect pairing, for instance **Z** × **Z** → **Z** via (*x*,*y*) ↦ 2*xy* is nondegenerate, but induces multiplication by 2 on the map **Z** → **Z**^{∗}.

Terminology varies in coverage of bilinear forms. For example, F. Reese Harvey discusses "eight types of inner product".^{[5]} To define them he uses diagonal matrices *A _{ij}* having only +1 or −1 for non-zero elements. Some of the "inner products" are symplectic forms and some are sesquilinear forms or Hermitian forms. Rather than a general field

*K*, the instances with real numbers

**R**, complex numbers

**C**, and quaternions

**H**are spelled out. The bilinear form

is called the **real symmetric case** and labeled R(*p*, *q*), where *p* + *q* = *n*. Then he articulates the connection to traditional terminology:^{[6]}

- Some of the real symmetric cases are very important. The positive definite case
**R**(*n*, 0) is called*Euclidean space*, while the case of a single minus,**R**(*n*−1, 1) is called*Lorentzian space*. If*n*= 4, then Lorentzian space is also called*Minkowski space*or*Minkowski spacetime*. The special case**R**(*p*,*p*) will be referred to as the*split-case*.

## Relation to tensor products

By the universal property of the tensor product, bilinear forms on *V* are in 1-to-1 correspondence with linear maps *V* ⊗ *V* → *K*. If *B* is a bilinear form on *V* the corresponding linear map is given by

**v**⊗**w**↦*B*(**v**,**w**)

The set of all linear maps *V* ⊗ *V* → *K* is the dual space of *V* ⊗ *V*, so bilinear forms may be thought of as elements of

- (
*V*⊗*V*)^{∗}≅*V*^{∗}⊗*V*^{∗}

Likewise, symmetric bilinear forms may be thought of as elements of Sym^{2}(*V*^{∗}) (the second symmetric power of *V*^{∗}), and alternating bilinear forms as elements of Λ^{2}*V*^{∗} (the second exterior power of *V*^{∗}).

## On normed vector spaces

Definition:A bilinear form on a normed vector space (V, ‖·‖ ) isbounded, if there is a constantCsuch that for allu,v∈V,

Definition:A bilinear form on a normed vector space (V, ‖·‖ ) iselliptic, or coercive, if there is a constantc> 0 such that for allu∈V,

## See also

- Bilinear map
- Bilinear operator
- Inner product space
- Linear form
- Multilinear form
- Quadratic form
- Positive semi definite
- Sesquilinear form

## Notes

- ↑ Template:Harvnb p.346
- ↑ {{#invoke:citation/CS1|citation |CitationClass=book }}
- ↑ Template:Harvnb
- ↑ Adkins & Weintraub (1992) p.359
- ↑ Harvey p. 22
- ↑ Harvey p 23

## References

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- Harvey, F. Reese (1990)
*Spinors and calibrations*, Ch 2:The Eight Types of Inner Product Spaces, pp 19–40, Academic Press, ISBN 0-12-329650-1 . - M. Hazewinkel ed. (1988) Encyclopedia of Mathematics, v.1, p. 390, Kluwer Academic Publishers
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## External links

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*This article incorporates material from Unimodular on PlanetMath, which is licensed under the Creative Commons Attribution/Share-Alike License.*