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Orthonormal Basis




These concepts are important both for finite-dimensional and infinite-dimensional spaces. For finite-dimensional spaces the condition of a dense span is the same as 'span', as used in Linear Algebra .

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. In the infinite-dimensional case the distinction matters: the definition given above requires only that the span of an orthonormal basis be ''dense in'' the vector space, not that it equal the entire space.

An orthonormal basis of a vector space ''V'' makes no sense unless ''V'' is given an inner product; Banach Space s do not generally have orthonormal bases.


EXAMPLES


  • The set {(1,0,0),(0,1,0),(0,0,1)} (the standard basis), as well as versions obtained by rotation about an axis through the origin or reflection in a plane through the origin, or a combination, each form an orthonormal basis of R3

  • The set {''f''''n'' : ''n'' ∈ Z} with ''f''''n''(''x'') = Exp (2π''inx'') forms an orthonormal basis of the complex space L2( {Link without Title} ). This is fundamental to the study of Fourier Series .

  • 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'').

  • Eigenfunctions of a Sturm-Liouville Eigenproblem .



BASIC FORMULAE


If ''B'' is an orthogonal basis of ''H'', then every element ''x'' of ''H'' may be written as
  :<math>\x\^2 \sum_{b\in B}\langle x,b angle ^2</math>