Hermitian wavelet

Hermitian wavelets are a family of discrete and continuous wavelets used in the continuous and discrete Hermite wavelet transform. The $n^{\textrm {th}}$ Hermitian wavelet is defined as the $n^{\textrm {th}}$ derivative of a Gaussian distribution for each positive $n$:$$\Psi _{n}(x)=(2n)^{-{\frac {n}{2}}}c_{n}\operatorname {He} _{n}\left(x\right)e^{-{\frac {1}{2}}x^{2}},$$where in this case the (probabilist) Hermite polynomial $\operatorname {He} _{n}(x)$ can be considered.

The normalization coefficient $c_{n}$ is given by$$c_{n}=\left(n^{{\frac {1}{2}}-n}\Gamma \left(n+{\frac {1}{2}}\right)\right)^{-{\frac {1}{2}}}=\left(n^{{\frac {1}{2}}-n}{\sqrt {\pi }}2^{-n}(2n-1)!!\right)^{-{\frac {1}{2}}}\quad n\in \mathbb {N} .$$The function $\Psi \in L_{\rho ,\mu }(-\infty ,\infty )$ is said to be an admissible Hermite wavelet if it satisfies the admissibility relation:

$$C_{\Psi }=\sum _{n=0}^{\infty }{\frac {\|{\hat {\Psi }}(n)\|^{2}}{\|n\|}}<\infty$$

where ${\hat {\Psi }}(n)$ is the Hermite transform of $\Psi$.

The perfector $C_{\Psi }$ in the resolution of the identity of the continuous wavelet transform for this wavelet is given by the formula$$C_{\Psi }={\frac {4\pi n}{2n-1}}.$$In computer vision and image processing, Gaussian derivative operators of different orders are frequently used as a basis for expressing various types of visual operations; see scale space and N-jet.

Examples

The first three derivatives of the Gaussian function with $\mu =0,\;\sigma =1$:$$f(t)=\pi ^{-1/4}e^{(-t^{2}/2)},$$are:$${\begin{aligned}f'(t)&=-\pi ^{-1/4}te^{(-t^{2}/2)},\\f''(t)&=\pi ^{-1/4}(t^{2}-1)e^{(-t^{2}/2)},\\f^{(3)}(t)&=\pi ^{-1/4}(3t-t^{3})e^{(-t^{2}/2)},\end{aligned}}$$and their $L^{2}$ norms $\lVert f'\rVert ={\sqrt {2}}/2,\lVert f''\rVert ={\sqrt {3}}/2,\lVert f^{(3)}\rVert ={\sqrt {30}}/4$.

Normalizing the derivatives yields three Hermitian wavelets:$${\begin{aligned}\Psi _{1}(t)&={\sqrt {2}}\pi ^{-1/4}te^{(-t^{2}/2)},\\\Psi _{2}(t)&={\frac {2}{3}}{\sqrt {3}}\pi ^{-1/4}(1-t^{2})e^{(-t^{2}/2)},\\\Psi _{3}(t)&={\frac {2}{15}}{\sqrt {30}}\pi ^{-1/4}(t^{3}-3t)e^{(-t^{2}/2)}.\end{aligned}}$$

See also

• Wavelet
• The Ricker wavelet is the $n=2$ Hermitian wavelet

External links

• Hermitian Clifford–Hermite Wavelets (Department of Mathematical Analysis, Faculty of Engineering, Ghent University)