Orthonormal basis
In
mathematics, an
orthonormal basis of an
inner product space (i.e., a
vector space with an inner product), or in particular of a
Hilbert space, is a set of elements whose span is dense in the space, in which the elements are mutually
orthogonal and normal, that is of length 1. Note that an orthonormal basis is not generally a "basis", i.e., it is not generally possible to write every member of the space as a linear combination of
finitely many members of an orthonormal basis. This is why the definition given above required only that the span of an orthonormal basis be
dense in the vector space, not that it equal the entire space. Nor is it possible to speak of an orthormal basis of any vector space unless it first has an inner product; Banach spaces do not generally have orthonormal bases.
Examples of orthonormal bases include:
 the set {(1,0,0),(0,1,0),(0,0,1)} forms an orthonormal basis of R^{3}
 the set {f_{n} : n ∈ Z} with f_{n}(x) = exp(2πinx) forms an orthonormal basis of the complex space L^{2}([0,1])
 the set {e_{b} : b ∈ B} with e_{b}(c) = 1 if b=c and 0 otherwise forms an orthonormal basis of l^{2}(B).
Note that in the infinitedimensional case, an orthonormal basis will not be a basis in the sense of
linear algebra; to distinguish the two, the latter bases are also called
Hamel bases.
Using Zorn's lemma, one can show that every Hilbert space admits an orthonormal basis; furthermore, any two orthonormal bases of the same space have the same cardinality. A Hilbert space is separable if and only if it admits a countable orthonormal basis.
If B is an orthonormal basis of H, then every element x of H may be written as

and the
norm of
x can be given by
 .
Even if
B is uncountable, only countably many terms in this sum will be nonzero, and the expression is therefore welldefined. This sum is also called the
Fourier expansion of
x.
If B is an orthonormal basis of H, then H is isomorphic to l^{2}(B) in the following sense: there exists a bijective linear map Φ : H > l^{2}(B) such that

for all
x and
y in
H.
See also Hamel basis. Hamel bases are of little if any interest in inner product spaces, but realizing that a Hamel basis is something that an orthonormal basis is not, may shed some light on what an orthonormal basis is.