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**Havriliak–Negami relaxation** is an empirical modification of the Debye relaxation model, accounting for the asymmetry and broadness of the dielectric dispersion curve. The model was first used to describe the dielectric relaxation of some polymers,^{ [1] } by adding two exponential parameters to the Debye equation:

**Asymmetry** is the absence of, or a violation of, symmetry. Symmetry is an important property of both physical and abstract systems and it may be displayed in precise terms or in more aesthetic terms. The absence of or violation of symmetry that are either expected or desired can have important consequences for a system.

A **polymer** is a large molecule, or macromolecule, composed of many repeated subunits. Due to their broad range of properties, both synthetic and natural polymers play essential and ubiquitous roles in everyday life. Polymers range from familiar synthetic plastics such as polystyrene to natural biopolymers such as DNA and proteins that are fundamental to biological structure and function. Polymers, both natural and synthetic, are created via polymerization of many small molecules, known as monomers. Their consequently large molecular mass relative to small molecule compounds produces unique physical properties, including toughness, viscoelasticity, and a tendency to form glasses and semicrystalline structures rather than crystals. The terms polymer and resin are often synonymous with plastic.

In mathematics, an **exponential function** is a function of the form

- Mathematical properties
- Real and imaginary parts
- Loss peak
- Superposition of Lorentzians
- Logarithmic moments
- Inverse Fourier transform
- References
- See also

where is the permittivity at the high frequency limit, where is the static, low frequency permittivity, and is the characteristic relaxation time of the medium. The exponents and describe the asymmetry and broadness of the corresponding spectra.

In electromagnetism, **absolute permittivity**, often simply called **permittivity**, usually denoted by the Greek letter ε (epsilon), is the measure of capacitance that is encountered when forming an electric field in a particular medium. More specifically, permittivity describes the amount of charge needed to generate one unit of electric flux in a particular medium. Accordingly, a charge will yield more electric flux in a medium with low permittivity than in a medium with high permittivity. Permittivity is the measure of a material's ability to store an electric field in the polarization of the medium.

Depending on application, the Fourier transform of the stretched exponential function can be a viable alternative that has one parameter less.

The **stretched exponential function**

For the Havriliak–Negami equation reduces to the Cole–Cole equation, for to the Cole–Davidson equation.

The **Cole–Cole equation** is a relaxation model that is often used to describe dielectric relaxation in polymers.

The storage part and the loss part of the permittivity (here: ) can be calculated as

and

with

The maximum of the loss part lies at

The Havriliak–Negami relaxation can be expressed as a superposition of individual Debye relaxations

with the distribution function

where

if the argument of the arctangent is positive, else^{ [2] }

The first logarithmic moment of this distribution, the average logarithmic relaxation time is

where is the digamma function and the Euler constant.^{ [3] }

In mathematics, the **digamma function** is defined as the logarithmic derivative of the gamma function:

The inverse Fourier transform of the Havriliak-Negami function (the corresponding time-domain relaxation function) can be numerically calculated.^{ [4] } It can be shown that the series expansions involved are special cases of the Fox-Wright function.^{ [5] } In particular, in the time-domain the corresponding of can be represented as

where is the Dirac delta function and

is a special instance of the Fox-Wright function and, precisely, it is the three parameters Mittag-Leffler function ^{ [6] } also known as the Prabhakar function. The function can be numerically evaluated, for instance, by means of a Matlab code .^{ [7] }

**Bremsstrahlung**, from *bremsen* "to brake" and *Strahlung* "radiation"; i.e., "braking radiation" or "deceleration radiation", is electromagnetic radiation produced by the deceleration of a charged particle when deflected by another charged particle, typically an electron by an atomic nucleus. The moving particle loses kinetic energy, which is converted into radiation, thus satisfying the law of conservation of energy. The term is also used to refer to the process of producing the radiation. Bremsstrahlung has a continuous spectrum, which becomes more intense and whose peak intensity shifts toward higher frequencies as the change of the energy of the decelerated particles increases.

**Synchrotron radiation** is the electromagnetic radiation emitted when charged particles are accelerated radially, i.e., when they are subject to an acceleration perpendicular to their velocity. It is produced, for example, in synchrotrons using bending magnets, undulators and/or wigglers. If the particle is non-relativistic, then the emission is called cyclotron emission. If, on the other hand, the particles are relativistic, sometimes referred to as ultrarelativistic, the emission is called synchrotron emission. Synchrotron radiation may be achieved artificially in synchrotrons or storage rings, or naturally by fast electrons moving through magnetic fields. The radiation produced in this way has a characteristic polarization and the frequencies generated can range over the entire electromagnetic spectrum which is also called continuum radiation.

**Chebyshev filters** are analog or digital filters having a steeper roll-off and more passband ripple or stopband ripple than Butterworth filters. Chebyshev filters have the property that they minimize the error between the idealized and the actual filter characteristic over the range of the filter, but with ripples in the passband. This type of filter is named after Pafnuty Chebyshev because its mathematical characteristics are derived from Chebyshev polynomials. The type I Chebyshev filters are called usually as just "Chebyshev filters", the type II ones are usually called as "inverse Chebyshev filters".

The **step response** of a system in a given initial state consists of the time evolution of its outputs when its control inputs are Heaviside step functions. In electronic engineering and control theory, step response is the time behaviour of the outputs of a general system when its inputs change from zero to one in a very short time. The concept can be extended to the abstract mathematical notion of a dynamical system using an evolution parameter.

In mathematics, there are several integrals known as the **Dirichlet integral**, after the German mathematician Peter Gustav Lejeune Dirichlet.

**Linear phase** is a property of a filter, where the phase response of the filter is a linear function of frequency. The result is that all frequency components of the input signal are shifted in time by the same constant amount, which is referred to as the group delay. And consequently, there is no phase distortion due to the time delay of frequencies relative to one another.

In calculus, **Leibniz's rule** for differentiation under the integral sign, named after Gottfried Leibniz, states that for an integral of the form

The **Duffing equation**, named after Georg Duffing (1861–1944), is a non-linear second-order differential equation used to model certain damped and driven oscillators. The equation is given by

**Instantaneous phase** and **instantaneous frequency** are important concepts in signal processing that occur in the context of the representation and analysis of time-varying functions. The instantaneous phase of a *complex-valued* function *s*(*t*), is the real-valued function:

In mathematics, in particular in algebraic geometry and differential geometry, **Dolbeault cohomology** is an analog of de Rham cohomology for complex manifolds. Let *M* be a complex manifold. Then the Dolbeault cohomology groups depend on a pair of integers *p* and *q* and are realized as a subquotient of the space of complex differential forms of degree (*p*,*q*).

A **theoretical motivation for general relativity**, including the motivation for the geodesic equation and the Einstein field equation, can be obtained from special relativity by examining the dynamics of particles in circular orbits about the earth. A key advantage in examining circular orbits is that it is possible to know the solution of the Einstein Field Equation *a priori*. This provides a means to inform and verify the formalism.

The **Newman–Penrose** (**NP**) **formalism** is a set of notation developed by Ezra T. Newman and Roger Penrose for general relativity (GR). Their notation is an effort to treat general relativity in terms of spinor notation, which introduces complex forms of the usual variables used in GR. The NP formalism is itself a special case of the tetrad formalism, where the tensors of the theory are projected onto a complete vector basis at each point in spacetime. Usually this vector basis is chosen to reflect some symmetry of the space-time, leading to simplified expressions for physical observables. In the case of the NP formalism, the vector basis chosen is a null tetrad: a set of four null vectors—two real, and a complex-conjugate pair. The two real members asymptotically point radially inward and radially outward, and the formalism is well adapted to treatment of the propagation of radiation in curved spacetime. The most often-used variables in the formalism are the Weyl scalars, derived from the Weyl tensor. In particular, it can be shown that one of these scalars-- in the appropriate frame—encodes the outgoing gravitational radiation of an asymptotically flat system.

In many-body theory, the term **Green's function** is sometimes used interchangeably with correlation function, but refers specifically to correlators of field operators or creation and annihilation operators.

A **biarc** is a smooth curve formed from two circular arcs. In order to make the biarc smooth, the two arcs should have the same tangent at the connecting point where they meet.

A **Sommerfeld expansion** is an approximation method developed by Arnold Sommerfeld for a certain class of integrals which are common in condensed matter and statistical physics. Physically, the integrals represent statistical averages using the Fermi–Dirac distribution.

The **narrow escape problem** is a ubiquitous problem in biology, biophysics and cellular biology.

The **table of chords**, created by the astronomer, geometer, and geographer Ptolemy in Egypt during the 2nd century AD, is a trigonometric table in Book I, chapter 11 of Ptolemy's *Almagest*, a treatise on mathematical astronomy. It is essentially equivalent to a table of values of the sine function. It was the earliest trigonometric table extensive enough for many practical purposes, including those of astronomy. Centuries passed before more extensive trigonometric tables were created. One such table is the *Canon Sinuum* created at the end of the 16th century.

In thermal quantum field theory, the **Matsubara frequency** summation is the summation over discrete imaginary frequencies. It takes the following form

- ↑ Havriliak, S.; Negami, S. (1967). "A complex plane representation of dielectric and mechanical relaxation processes in some polymers".
*Polymer*.**8**: 161–210. doi:10.1016/0032-3861(67)90021-3. - ↑ Zorn, R. (1999). "Applicability of Distribution Functions for the Havriliak–Negami Spectral Function".
*Journal of Polymer Science Part B*.**37**(10): 1043–1044. Bibcode:1999JPoSB..37.1043Z. doi:10.1002/(SICI)1099-0488(19990515)37:10<1043::AID-POLB9>3.3.CO;2-8. - ↑ Zorn, R. (2002). "Logarithmic moments of relaxation time distributions" (PDF).
*Journal of Chemical Physics*.**116**(8): 3204–3209. Bibcode:2002JChPh.116.3204Z. doi:10.1063/1.1446035. - ↑ Schönhals, A. (1991). "Fast calculation of the time dependent dielectric permittivity for the Havriliak-Negami function".
*Acta Polymerica*.**42**: 149–151. - ↑ Hilfer, J. (2002). "
*H*-function representations for stretched exponential relaxation and non-Debye susceptibilities in glassy systems".*Physical Review E*.**65**: 061510. Bibcode:2002PhRvE..65f1510H. doi:10.1103/physreve.65.061510. - ↑ Gorenflo, Rudolf; Kilbas, Anatoly A.; Mainardi, Francesco; Rogosin, Sergei V. (2014). Springer, ed.
*Mittag-Leffler Functions, Related Topics and Applications*. ISBN 978-3-662-43929-6. - ↑ Garrappa, Roberto. "The Mittag-Leffler function" . Retrieved 3 November 2014.

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