In
mathematics, given three
vector spaces V,
W and
X over the same base
field F,
a
bilinear operator is a
function B:
Vx
W ->
X such that
for any
w in
W,
v |->
B(
v,
w) is
a
linear operator from
V to
X, and for any
v in
V,
w |->
B(
v,
w)
is a linear operator from
W to
X. In other words, if we hold fixed the first entry to the bilinear operator, while letting the second entry vary, the result is a linear operator, and similarly if we hold fixed the second entry.
The definition works without any changes if instead of vector spaces we use modules over a commutative
ring R. It also can be easily generalized to n-ary functions, where the proper term is multilinear.
For the case of a non-commutative base ring R and
a right module M_{R} and a left
module _{R}N, we can define a bilinear operator
B: MxN -> T, where T is a commutative group, such that
for any n in N, m |-> B(m, n) is
a group homomorphism, and for any m in M, n |-> B(m, n)
is a group homomorphism, and which also satisfies
- B(mr, n) = B(m, rn)
for all m in M, n in N and r in R.
One often thinks of a bilinear operator as a generalized "multiplication" which satisfies the distributive law.
- Matrix multiplication is a bilinear map M(m,n) x M(n,p) -> M(m,p).
- If a vector space V over the real numbers R carries an inner product, then the inner product is a bilinear operator V x V -> R.
- In general, for a vector space V over a field F, a bilinear form on V is the same as a bilinear operator V x V -> F.
- If V is a vector space with dual space V*, then the application operator, b(f, v) = f(v) is a bilinear operator from V^{*}xV 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 operator V x W -> F.
- The cross product in R^{3} is a bilinear operator R^{3} x R^{3} -> R^{3}.
- Let B: VxW->X be a bilinear operator, and L: U->W be a linear operator, then (v, u) -> B(v, Lu) is a bilinear operator on VxU
- The operator B: VxW -> X where B(v, w) = 0 for all v in V and w in W is bilinear
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