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In mathematics, a wavelet series is a representation of a square-integrable (real- or complex-valued) function by a certain orthonormal series generated by a wavelet. This article provides a formal, mathematical definition of an orthonormal wavelet and of the integral wavelet transform.

Contents

Formal definition

A function \psi\in L^2(\mathbb{R}) is called an orthonormal wavelet if it can be used to define a Hilbert basis, that is a complete orthonormal system, for the Hilbert space L^2(\mathbb{R}) of square integrable functions. The Hilbert basis is constructed as the family of functions \{\psi_{jk}:j,k\in\Z\} by means of dyadic translations and dilations of \psi\,,

\psi_{jk}(x) = 2^{j/2} \psi(2^jx-k)\,

for integers j,k\in \mathbb{Z}. This family is an orthonormal system if it is orthonormal under the inner product

\langle\psi_{jk},\psi_{lm}\rangle = \delta_{jl}\delta_{km}

where \delta_{jl}\, is the Kronecker delta and \langle f,g\rangle is the standard inner product \langle f,g\rangle = \int_{-\infty}^\infty \overline{f(x)}g(x)dx on L^2(\mathbb{R}). The requirement of completeness is that every function f\in L^2(\mathbb{R}) may be expanded in the basis as

f(x)=\sum_{j,k=-\infty}^\infty c_{jk} \psi_{jk}(x)

with convergence of the series understood to be convergence in the norm. Such a representation of a function f is known as a wavelet series. This implies that an orthonormal wavelet is self-dual.

Wavelet transform

The integral wavelet transform is the integral transform defined as

\left[W_\psi f\right](a,b) = \frac{1}{\sqrt{|a|}}
\int_{-\infty}^\infty \overline{\psi\left(\frac{x-b}{a}\right)}f(x)dx\,

The wavelet coefficients cjk are then given by

c_{jk}= \left[W_\psi f\right](2^{-j}, k2^{-j})

Here, a = 2 j is called the binary dilation or dyadic dilation, and b = k2 j is the binary or dyadic position.

General remarks

Unlike the Fourier transform, which is an integral transform in both directions, the wavelet series is an integral transform in one direction, and a series in the other, much like the Fourier series.

The canonical example of an orthonormal wavelet, that is, a wavelet that provides a complete set of basis elements for L^2(\mathbb{R}), is the Haar wavelet.

See also

References

  • Charles K. Chui, An Introduction to Wavelets, (1992), Academic Press, San Diego, ISBN 0121745848

External links

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