# Orthonormality

In linear algebra, two vectors v and w are said to be orthonormal if they are both orthogonal (according to a given inner product) and normalized. A set of vectors which are pairwise orthonormal is called an orthonormal set. A basis which forms an orthonormal set is called an orthonormal basis.

When referring to functions, usually the L²-norm is assumed unless otherwise stated, so that two functions [itex]\phi(x)[itex] and [itex]\psi(x)[itex] are orthonormal over the interval [itex][a,b][itex] if

[itex](2)\quad||\phi(x)||_2 = ||\psi(x)||_2 = \left[\int_a^b|\phi(x)|^2dx\right]^\frac{1}{2} = \left[\int_a^b|\psi(x)|^2dx\right]^\frac{1}{2} = 1.[itex]

An equivalent formulation of the two conditions is done by using the Delta function. A set of vectors (functions, matrices, sequences etc)

[itex] \left\{ u_1 , u_2 , ... , u_n , ... \right\} [itex]

forms an orthonormal set iff

[itex] \forall n,m \ : \quad \left\langle u_n | u_m \right\rangle = \delta_{n,m} [itex]

where < | > is the proper inner product defined over the vector space.

Unfortunately, the word normal is sometimes used synonymously with orthogonal.

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