Orthogonal complement

In the mathematical fields of linear algebra and functional analysis, the orthogonal complement of a subspace W of a vector space V equipped with a bilinear form B is the set W^⊥ of all vectors in V that are orthogonal to every vector in W. Informally, it is called the perp, short for perpendicular complement. It is a subspace of V.

General bilinear forms

Let $V$ be a vector space over a field $F$ equipped with a bilinear form $B$ . We define $u$ to be left-orthogonal to $v$ , and $v$ to be right-orthogonal to $u$ , when $B(u,v)=0$ . For a subset $W$ of $V$ we define the left orthogonal complement $W^\bot$ to be

W^\bot=\left\{x\in V : B( x, y ) = 0 \mbox{ for all } y\in W \right\}\,.

There is a corresponding definition of right orthogonal complement. For a reflexive bilinear form, where $B(u,v)=0$ implies $B(v,u)=0$ for all $u$ and $v$ in $V$ , the left and right complements coincide. This will be the case if $B$ is a symmetric or an alternating form.

The definition extends to a bilinear form on a free module over a commutative ring, and to a sesquilinear form extended to include any free module over a commutative ring with conjugation.^[1]

Properties

An orthogonal complement is a subspace of $V$ ;
If $X\subset Y$ then $X^\bot \supset Y^\bot$ ;
The radical $V^\bot$ of $V$ is a subspace of every orthogonal complement;
$W\subset (W^\bot)^\bot$ ;
If $B$ is non-degenerate and $V$ is finite-dimensional, then $\dim(W)+\dim (W^\bot)=\dim V$ .

Example

In special relativity the orthogonal complement is used to determine the simultaneous hyperplane at a point of a world line. The bilinear form η used in Minkowski space determines a pseudo-Euclidean space of events. The origin and all events on the light cone are self-orthogonal. When a time event and a space event evaluate to zero under the bilinear form, then they are hyperbolic-orthogonal. This terminology stems from the use of two conjugate hyperbolas in the pseudo-Euclidean plane: conjugate diameters of these hyperbolas are hyperbolic-orthogonal.

Inner product spaces

This section considers orthogonal complements in inner product spaces.^[2]

Properties

The orthogonal complement is always closed in the metric topology. In finite-dimensional spaces, that is merely an instance of the fact that all subspaces of a vector space are closed. In infinite-dimensional Hilbert spaces, some subspaces are not closed, but all orthogonal complements are closed. In such spaces, the orthogonal complement of the orthogonal complement of $W$ is the closure of $W$ , i.e.,

(W^\bot)^\bot = \overline W

Some other useful properties that always hold are the following. Let $H$ be a Hilbert space and let $X$ and $Y$ be its linear subspaces. Then:

$X^\bot = \overline X^\bot$ ;
if $Y \subset X$ , then $X^\bot \subset Y ^\bot$ ;
$X\cap X^\bot = \{0\}$ ;
$X\subset (X^\bot)^\bot$ ;
if $X$ is a closed linear subspace of $H$ , then $(X^\bot)^\bot = X$ ;
if $X$ is a closed linear subspace of $H$ , then $H = X \oplus X^\bot$ , the (inner) direct sum.

The orthogonal complement generalizes to the annihilator, and gives a Galois connection on subsets of the inner product space, with associated closure operator the topological closure of the span.

Finite dimensions

For a finite-dimensional inner product space of dimension n, the orthogonal complement of a k-dimensional subspace is an (n − k)-dimensional subspace, and the double orthogonal complement is the original subspace:

(W^⊥)^⊥ = W.

If A is an m × n matrix, where Row A, Col A, and Null A refer to the row space, column space, and null space of A (respectively), we have

(Row A)^⊥ = Null A

(Col A)^⊥ = Null A^T.

Banach spaces

There is a natural analog of this notion in general Banach spaces. In this case one defines the orthogonal complement of W to be a subspace of the dual of V defined similarly as the annihilator

W^\bot = \left\{\,x\in V^* : \forall y\in W, x(y) = 0 \, \right\}.\,

It is always a closed subspace of V^∗. There is also an analog of the double complement property. W^⊥⊥ is now a subspace of V^∗∗ (which is not identical to V). However, the reflexive spaces have a natural isomorphism i between V and V^∗∗. In this case we have

i\overline{W} = W^{\bot\,\bot}.

This is a rather straightforward consequence of the Hahn–Banach theorem.

References

↑ Adkins & Weintraub (1992) p.359
↑ Adkins&Weintraub (1992) p.272

Adkins, William A.; Weintraub, Steven H. (1992), Algebra: An Approach via Module Theory, Graduate Texts in Mathematics 136, Springer-Verlag, ISBN 3-540-97839-9, Zbl 0768.00003
Halmos, Paul R. (1974), Finite-dimensional vector spaces, Undergraduate Texts in Mathematics, Berlin, New York: Springer-Verlag, ISBN 978-0-387-90093-3, Zbl 0288.15002
Milnor, J.; Husemoller, D. (1973), Symmetric Bilinear Forms, Ergebnisse der Mathematik und ihrer Grenzgebiete 73, Springer-Verlag, ISBN 3-540-06009-X, Zbl 0292.10016

External links

Instructional video describing orthogonal complements (Khan Academy)

Orthogonal complement

General bilinear forms

Properties

Example

Inner product spaces

Properties

Finite dimensions

Banach spaces

See also

References

External links