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.

A bilinear map can also be defined for modules. For that, see the article pairing.

Definition

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Vector spaces

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Let   and   be three vector spaces over the same base field  . A bilinear map is a function   such that for all  , the map     is a linear map from   to   and for all  , the map     is a linear map from   to   In other words, when we hold the first entry of the bilinear map fixed while letting the second entry vary, the result is a linear operator, and similarly for when we hold the second entry fixed.

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 (for example: scalar product, inner product, and quadratic form).

Modules

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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 × NT with T an (R, S)-bimodule, and for which any n in N, mB(m, n) is an R-module homomorphism, and for any m in M, nB(m, n) is an S-module homomorphism. This satisfies

B(rm, n) = rB(m, n)
B(m, ns) = 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

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An immediate consequence of the definition is that B(v, w) = 0X whenever v = 0V or w = 0W. This may be seen by writing the zero vector 0V as 0 ⋅ 0V (and similarly for 0W) 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(ei, fj), 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

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  • 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 × VF.
  • If V is a vector space with dual space V, then the canonical evaluation map, 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 × WF.
  • 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

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

  1. for all   the map   given by   is continuous;
  2. 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

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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 map 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

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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:

  1. give all three the topology of bounded convergence;
  2. give all three the topology of compact convergence;
  3. 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

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  • Tensor product – Mathematical operation on vector spaces
  • Sesquilinear form – Generalization of a bilinear form
  • Bilinear filtering – Method of interpolating functions on a 2D grid
  • Multilinear map – Vector-valued function of multiple vectors, linear in each argument

References

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  1. ^ a b c d e Trèves 2006, pp. 424–426.
  2. ^ Schaefer & Wolff 1999, p. 118.

Bibliography

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  • 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.
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