# Bilinear form

In mathematics, a bilinear form on a vector space V over a field F is a mapping V × VF which is linear in both arguments. That is, B : V × VF is bilinear if the maps

[itex]w \mapsto B(v, w)[itex]
[itex]w \mapsto B(w, v)[itex]

are linear for each v in V. This definition applies equally well to modules over a commutative ring with linear maps being module homomorphisms.

Note that a bilinear form is a special case of a bilinear operator.

When F is the field of complex numbers C one is often more interested in sesquilinear forms. These are similiar to bilinear forms but are conjugate linear in one argument instead of linear.

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## Coordinate representation

If V is finite-dimensional with dimension n then any bilinear form B on V can represented in coordinates by a matrix B relative to some ordered basis {ei} for V. The components of the matrix B are given by [itex]B_{ij} = B(e_i,e_j)[itex]. The action of the bilinear form on vectors u and v is then given by matrix multiplication:

[itex]B(u,v) = \mathbf{u}^T \mathbf{Bv} = \sum_{i,j=1}^{n}B_{ij}u^i v^j[itex]

where ui and vj are the components of u and v in this basis.

## 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 [itex]B_1,B_2\colon V \to V^*[itex] by

[itex]B_1(v)(w) = B(v,w)[itex]
[itex]B_2(v)(w) = B(w,v)[itex]

This is often denoted as

[itex]B_1(v) = B(v,{-})[itex]
[itex]B_2(v) = B({-},v)[itex]

where the (–) indicates the slot into which the argument is to be placed.

If V is finite-dimensional then one can identify V with its double dual V**. One can then show that B2 is the transpose of the linear map B1 (if V is infinite-dimensional then B2 is the transpose of B1 restricted to the image of V in V**). Given B one can define the transpose of B to be the bilinear form given by

[itex]B^*(v,w) = B(w,v).[itex]

If V is finite-dimensional then the rank of B1 is equal to the rank of B2. If this number is equal to the dimension of V then B1 and B2 are linear isomorphisms from V to V*. In this case B is said to be nondegenerate.

Given any linear map A : VV* one can bilinear form B on V via

[itex]B(v,w) = A(v)(w)[itex]

This form will be nondegenerate iff A is an isomorphism.

## Symmetry

A bilinear form B : V × VF is said to be:

• symmetric if [itex]B(v,w)=B(w,v)[itex] for all [itex]v,w\in V[itex]
• skew-symmetric if [itex]B(v,w)=-B(w,v)[itex] for all [itex]v,w\in V[itex]
• alternating if [itex]B(v,v)=0[itex] for all [itex]v\in V[itex]

Every alternating form is skew-symmetric; this may be seen by expanding

B(v+w,v+w).

If the characteristic of F is not 2 then the converse is also true (every skew-symmetric form is alternating). If, however, char(F) = 2 then a skew-symmetric form is the same thing as a symmetric form and not all of these are alternating.

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

A bilinear form is symmetric iff the maps [itex]B_1,B_2\colon V \to V^*[itex] are equal, and skew-symmetric iff they are negatives of one another. If char(F) ≠ 2 then one can always decompose a bilinear form into a symmetric and an skew-symmetric part as follows

[itex]B^{\pm} = {1\over 2}(B \pm B^*)[itex]

where B* is the transpose of B (defined above).

## 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 VVF. If B is a bilinear form on V the corresponding linear map is given by

[itex]v\otimes w\mapsto B(v,w).[itex]

The set of all linear maps VVF is the dual space of VV, so bilinear forms may be thought of as elements of

[itex](V\otimes V)^{*} \cong V^{*}\otimes V^{*}.[itex]

Likewise, symmetric bilinear forms may be thought of as elements of S2V* (the second symmetric power of V*), and alternating bilinear forms as elements of Λ2V* (the second exterior power of V*).

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