In mathematics, a **bilinear map** is a function combining elements of two vector spaces to yield an element of a third vector space, and is linear in each of its arguments. Matrix multiplication is an example.

## Definition

### Vector spaces

Let and be three vector spaces over the same base field . A bilinear map is a function

Such a map satisfies the following properties.

- For any ,
- The map is additive in both components: if and then and

If and we have *B*(*v*, *w*) = *B*(*w*, *v*) for all then we say that *B* is *symmetric*. If *X* is the base field *F*, then the map is called a **bilinear form**, which are well-studied (see for example *Scalar product*, *Inner product* and *Quadratic form*).

### Modules

The definition works without any changes if instead of vector spaces over a field *F*, we use modules over a commutative ring *R*. It generalizes to *n*-ary functions, where the proper term is *multilinear*.

For non-commutative rings *R* and *S*, a left *R*-module *M* and a right *S*-module *N*, a bilinear map is a map *B* : *M* × *N* → *T* with *T* an (*R*, *S*)-bimodule, and for which any *n* in *N*, *m* ↦ *B*(*m*, *n*) is an *R*-module homomorphism, and for any *m* in *M*, *n* ↦ *B*(*m*, *n*) is an *S*-module homomorphism. This satisfies

*B*(*r*⋅*m*,*n*) =*r*⋅*B*(*m*,*n*)*B*(*m*,*n*⋅*s*) =*B*(*m*,*n*) ⋅*s*

for all *m* in *M*, *n* in *N*, *r* in *R* and *s* in *S*, as well as *B* being additive in each argument.

## Properties

An immediate consequence of the definition is that *B*(*v*, *w*) = 0_{X} whenever *v* = 0_{V} or *w* = 0_{W}. This may be seen by writing the zero vector 0_{V} as 0 ⋅ 0_{V} (and similarly for 0_{W}) and moving the scalar 0 "outside", in front of *B*, by linearity.

The set *L*(*V*, *W*; *X*) of all bilinear maps is a linear subspace of the space (viz. vector space, module) of all maps from *V* × *W* into *X*.

If *V*, *W*, *X* are finite-dimensional, then so is *L*(*V*, *W*; *X*). For that is, bilinear forms, the dimension of this space is dim *V* × dim *W* (while the space *L*(*V* × *W*; *F*) of *linear* forms is of dimension dim *V* + dim *W*). To see this, choose a basis for *V* and *W*; then each bilinear map can be uniquely represented by the matrix *B*(*e*_{i}, *f*_{j}), and vice versa.
Now, if *X* is a space of higher dimension, we obviously have dim *L*(*V*, *W*; *X*) = dim *V* × dim *W* × dim *X*.

## Examples

- Matrix multiplication is a bilinear map M(
*m*,*n*) × M(*n*,*p*) → M(*m*,*p*). - If a vector space
*V*over the real numbers carries an inner product, then the inner product is a bilinear map - In general, for a vector space
*V*over a field*F*, a bilinear form on*V*is the same as a bilinear map*V*×*V*→*F*. - If
*V*is a vector space with dual space*V*^{∗}, then the application operator,*b*(*f*,*v*) =*f*(*v*) is a bilinear map from*V*^{∗}×*V*to the base field. - Let
*V*and*W*be vector spaces over the same base field*F*. If*f*is a member of*V*^{∗}and*g*a member of*W*^{∗}, then*b*(*v*,*w*) =*f*(*v*)*g*(*w*) defines a bilinear map*V*×*W*→*F*. - The cross product in is a bilinear map
- Let be a bilinear map, and be a linear map, then (
*v*,*u*) ↦*B*(*v*,*Lu*) is a bilinear map on*V*×*U*.

## Continuity and separate continuity

Suppose are topological vector spaces and let be a bilinear map.
Then *b* is said to be **separately continuous** if the following two conditions hold:

- for all the map given by is continuous;
- for all the map given by is continuous.

Many separately continuous bilinear that are not continuous satisfy an additional property: hypocontinuity.^{[1]}
All continuous bilinear maps are hypocontinuous.

### Sufficient conditions for continuity

Many bilinear maps that occur in practice are separately continuous but not all are continuous. We list here sufficient conditions for a separately continuous bilinear to be continuous.

- If
*X*is a Baire space and*Y*is metrizable then every separately continuous bilinear map is continuous.^{[1]} - If are the strong duals of Fréchet spaces then every separately continuous bilinear map is continuous.
^{[1]} - If a bilinear map is continuous at (0, 0) then it is continuous everywhere.
^{[2]}

### Composition map

Let be locally convex Hausdorff spaces and let be the composition map defined by In general, the bilinear map is not continuous (no matter what topologies the spaces of linear maps are given). We do, however, have the following results:

Give all three spaces of linear maps one of the following topologies:

- give all three the topology of bounded convergence;
- give all three the topology of compact convergence;
- give all three the topology of pointwise convergence.

- If is an equicontinuous subset of then the restriction is continuous for all three topologies.
^{[1]} - If is a barreled space then for every sequence converging to in and every sequence converging to in the sequence converges to in
^{[1]}

## See also

- Tensor product – Operation on vector spaces, generalized throughout mathematics
- Sesquilinear form – Scalar-valued function of two complex variables that is linear in one variable and conjugate-linear in the other
- Bilinear filtering
- Multilinear map – Vector-valued function of multiple vectors, linear in each argument

## References

- ^
^{a}^{b}^{c}^{d}^{e}Trèves 2006, pp. 424–426. **^**Schaefer & Wolff 1999, p. 118.

## Bibliography

- Schaefer, Helmut H.; Wolff, Manfred P. (1999).
*Topological Vector Spaces*. GTM. Vol. 8 (Second ed.). New York, NY: Springer New York Imprint Springer. ISBN 978-1-4612-7155-0. OCLC 840278135. - Trèves, François (2006) [1967].
*Topological Vector Spaces, Distributions and Kernels*. Mineola, N.Y.: Dover Publications. ISBN 978-0-486-45352-1. OCLC 853623322.

## External links

- "Bilinear mapping",
*Encyclopedia of Mathematics*, EMS Press, 2001 [1994]