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Orthogonal basis

## Summary

In mathematics, particularly linear algebra, an orthogonal basis for an inner product space ${\displaystyle V}$ is a basis for ${\displaystyle V}$ whose vectors are mutually orthogonal. If the vectors of an orthogonal basis are normalized, the resulting basis is an orthonormal basis.

## As coordinates

Any orthogonal basis can be used to define a system of orthogonal coordinates ${\displaystyle V.}$  Orthogonal (not necessarily orthonormal) bases are important due to their appearance from curvilinear orthogonal coordinates in Euclidean spaces, as well as in Riemannian and pseudo-Riemannian manifolds.

## In functional analysis

In functional analysis, an orthogonal basis is any basis obtained from an orthonormal basis (or Hilbert basis) using multiplication by nonzero scalars.

## Extensions

### Symmetric bilinear form

The concept of an orthogonal basis is applicable to a vector space ${\displaystyle V}$  (over any field) equipped with a symmetric bilinear form ${\displaystyle \langle \cdot ,\cdot \rangle ,}$  where orthogonality of two vectors ${\displaystyle v}$  and ${\displaystyle w}$  means ${\displaystyle \langle v,w\rangle =0.}$  For an orthogonal basis ${\displaystyle \left\{e_{k}\right\}:}$

${\displaystyle \langle e_{j},e_{k}\rangle ={\begin{cases}q(e_{k})&j=k\\0&j\neq k,\end{cases}}}$

where ${\displaystyle q}$  is a quadratic form associated with ${\displaystyle \langle \cdot ,\cdot \rangle :}$  ${\displaystyle q(v)=\langle v,v\rangle }$  (in an inner product space, ${\displaystyle q(v)=\|v\|^{2}.}$ ).

Hence for an orthogonal basis ${\displaystyle \left\{e_{k}\right\},}$

${\displaystyle \langle v,w\rangle =\sum _{k}q(e_{k})v^{k}w^{k},}$

where ${\displaystyle v_{k}}$  and ${\displaystyle w_{k}}$  are components of ${\displaystyle v}$  and ${\displaystyle w}$  in the basis.

The concept of orthogonality may be extended to a vector space (over any field) equipped with a quadratic form ${\displaystyle q(v)}$ . Starting from the observation that, when the characteristic of the underlying field is not 2, the associated symmetric bilinear form ${\displaystyle \langle v,w\rangle ={\tfrac {1}{2}}(q(v+w)-q(v)-q(w))}$  allows vectors ${\displaystyle v}$  and ${\displaystyle w}$  to be defined as being orthogonal with respect to ${\displaystyle q}$  when ${\displaystyle q(v+w)-q(v)-q(w)=0.}$