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### orthogonal complement

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ARTICLE ORIGINS orthogonal complement
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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 WâŠ¥ 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.

## Example

In the case that W is the subspace of V = mathbb{R}^5 (with the usual dot product) spanned by the rows of the next matrix,begin{pmatrix}begin{array}{c c } 1 & 0 0 & 1 end{array}& begin{array}{c c c } 2 & 3 & 5 hline 6 & 9 & 3 hline end{array} end{pmatrix}its orthogonal complement WâŠ¥ is spanned by the three row-vectors of

begin{pmatrix} begin{array}{c|c|} -2& -6 -3&-9 -5&-3 end{array} & begin{array}{c c c} 1 & 0 & 0 0&1&0 0&0&1 end{array} end{pmatrix}
.The fact that every vector on the first list is orthogonal to every vector on the second list can be checked by direct computation. The fact that the spans of these vectors are orthogonal then follows by bilinearity of the dot product. Finally, the fact that these spaces are orthogonal complements follows from the dimension relationships given below.

## 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{xin V : B( x, y ) = 0 mbox{ for all } yin 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.Adkins & Weintraub (1992) p.359

### Properties

• An orthogonal complement is a subspace of V;
• If {{nowrap|Xsubset Y}} then {{nowrap|X^bot supset Y^bot}};
• The radical V^bot of V is a subspace of every orthogonal complement;
• Wsubset (W^bot)^bot;
• If B is non-degenerate and V is finite-dimensional, then dim(W)+dim (W^bot)=dim V.
• If L_1, L_2, ldots, L_r are subspaces of a finite-dimensional space V and L_ = L_1 cap L_2 cap ldots cap L_r, then L_^bot = L_1^bot + L_2^bot + ldots + L_r^bot.

## Inner product spaces

This section considers orthogonal complements in inner product spaces.Adkins&Weintraub (1992) p.272

### 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 {{nowrap| X^bot subset Y ^bot}};
• Xcap X^bot = {0};
• Xsubseteq (X^bot)^bot ;
• if X is a closed linear subspace of H, then {{nowrap|(X^bot)^bot = X}};
• if X is a closed linear subspace of H, then {{nowrap| 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 {{nowrap|(n âˆ’ k)}}-dimensional subspace, and the double orthogonal complement is the original subspace:
(WâŠ¥)âŠ¥ = W.
If A is an {{nowrap|m Ã— n}} matrix, where {{nowrap|Row A}}, {{nowrap|Col A}}, and {{nowrap|Null A}} refer to the row space, column space, and null space of A (respectively), we have
(Row A)âŠ¥ = Null A (Col A)âŠ¥ = Null AT."Orthogonal Complement"

## 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{,xin V^* : forall yin W, x(y) = 0 , right}.,
It is always a closed subspace of Vâˆ—. There is also an analog of the double complement property. WâŠ¥âŠ¥ is now a subspace of Vâˆ—âˆ— (which is not identical to V). However, the reflexive spaces have a natural isomorphism i between V and Vâˆ—âˆ—. In this case we have
ioverline{W} = W^{bot,bot}.
This is a rather straightforward consequence of the Hahnâ€“Banach theorem.

## Applications

In special relativity the orthogonal complement is used to determine the simultaneous hyperplane at a point of a world line. The bilinear form Î· 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.

## References

{{reflist}}
• {{citation | last1=Adkins | first1=William A. | last2=Weintraub | first2=Steven H. | title=Algebra: An Approach via Module Theory | series=Graduate Texts in Mathematics | volume=136 | publisher=Springer-Verlag | year=1992 | isbn=3-540-97839-9 | zbl=0768.00003 }}
• {{Citation | last1=Halmos | first1=Paul R. | author1-link=Paul R. Halmos | title=Finite-dimensional vector spaces | series=Undergraduate Texts in Mathematics | publisher=Springer-Verlag | location=Berlin, New York | isbn=978-0-387-90093-3 | year=1974 | zbl=0288.15002 }}
• {{citation | first1=J. | last1=Milnor | author1-link=John Milnor| first2=D. | last2=Husemoller | title=Symmetric Bilinear Forms | series=Ergebnisse der Mathematik und ihrer Grenzgebiete | volume=73 | publisher=Springer-Verlag | year=1973 | isbn=3-540-06009-X | zbl=0292.10016 }}

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