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Asymptotes in Rational Functions01:30

Asymptotes in Rational Functions

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A rational function is defined as the quotient of two polynomials:  where Q(x)≠0, These functions often exhibit asymptotes, which are the lines that the graph approaches but never touches. These asymptotes are classified based on how the function behaves near specific values of the input.Vertical asymptotes occur where the denominator is zero, and the numerator is not, causing the function to be undefined. These are found by solving Q(x)=0. For example:  has a vertical asymptote at x=3,...
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Routh-Hurwitz Criterion II01:19

Routh-Hurwitz Criterion II

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In the application of the Routh-Hurwitz criterion, two specific scenarios can arise that complicate stability analysis.
The first scenario occurs when a singular zero appears in the first column of the Routh table. This situation creates a division by zero issues. To resolve this, a small positive or negative number, denoted as epsilon (∈), is substituted for the zero. The stability analysis proceeds by assuming a sign for ∈. If ∈ is positive, any sign change in the first...
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Partial Fractions01:28

Partial Fractions

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A partial fraction is a component of a rational expression represented as the sum of simpler fractions. When a rational function is expressed as a ratio of two polynomials, it can often be decomposed into a sum of fractions whose denominators are simpler polynomials, typically linear or irreducible quadratic factors. This process is called partial fraction decomposition, and it is used to simplify complex expressions for integration, solving equations, or analysis.Partial fraction decomposition...
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Pole and System Stability01:24

Pole and System Stability

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The transfer function is a fundamental concept representing the ratio of two polynomials. The numerator and denominator encapsulate the system's dynamics. The zeros and poles of this transfer function are critical in determining the system's behavior and stability.
Simple poles are unique roots of the denominator polynomial. Each simple pole corresponds to a distinct solution to the system's characteristic equation, typically resulting in exponential decay terms in the system's...
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Rational Expressions01:28

Rational Expressions

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Rational expressions are algebraic fractions in which both the numerator and the denominator are polynomials. These expressions follow the arithmetic rules of numerical fractions but require extra care due to the presence of variables. A fundamental part of working with rational expressions is identifying values that make the expression undefined, typically those that result in division by zero or undefined radicals.Determining the DomainThe domain of a rational expression includes all real...
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Divergence and Stokes' Theorems01:06

Divergence and Stokes' Theorems

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The divergence and Stokes' theorems are a variation of Green's theorem in a higher dimension. They are also a generalization of the fundamental theorem of calculus. The divergence theorem and Stokes' theorem are in a way similar to each other; The divergence theorem relates to the dot product of a vector, while Stokes' theorem relates to the curl of a vector. Many applications in physics and engineering make use of the divergence and Stokes' theorems, enabling us to write...
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Updated: Nov 22, 2025

Generation and Coherent Control of Pulsed Quantum Frequency Combs
06:42

Generation and Coherent Control of Pulsed Quantum Frequency Combs

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Exponential node clustering at singularities for rational approximation, quadrature, and PDEs.

Lloyd N Trefethen1, Yuji Nakatsukasa1, J A C Weideman2

  • 1Mathematical Institute, University of Oxford, Oxford, OX2 6GG UK.

Numerische Mathematik
|January 11, 2021
PubMed
Summary
This summary is machine-generated.

Rational approximations with exponentially clustered poles achieve root-exponential convergence. Tapered exponential clustering further enhances convergence rates in various approximation and numerical methods, doubling speed in theoretical models.

Keywords:
41A2065D3265N35

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Area of Science:

  • Numerical Analysis
  • Approximation Theory
  • Scientific Computing

Background:

  • Rational approximations with singularities can exhibit root-exponential convergence when poles are exponentially clustered.
  • This phenomenon is observed in minimax, least-squares, and AAA approximations, conformal mapping, and numerical solutions for Laplace, Helmholtz, and biharmonic equations using the 'lightning' method.

Purpose of the Study:

  • To investigate the impact of non-uniform, linearly tapering exponential pole clustering on approximation convergence rates.
  • To propose a theoretical model for the tapering effect and explore its relationship with quadrature formulas.

Main Methods:

  • Review of existing literature on exponential clustering in various approximation and numerical methods.
  • Conducting extensive numerical experiments to evaluate the performance of tapered clustering.
  • Developing a theoretical model using Hermite contour integral and potential theory.
  • Analyzing the connection between exponential and doubly exponential quadrature formulas via the Gauss-Takahasi-Mori contour integral.

Main Results:

  • Exponential clustering of poles accelerates convergence of rational approximations.
  • Linear tapering of pole density near singularities demonstrably improves convergence rates.
  • Theoretical modeling suggests tapering can double the convergence rate.
  • A mathematical link is established between tapered and non-tapered quadrature formulas.

Conclusions:

  • Tapered exponential clustering is a beneficial strategy for improving the efficiency of rational approximations and numerical methods.
  • The proposed theoretical model provides a framework for understanding and predicting the enhanced convergence.
  • Further research into the application of these findings in quadrature formulas is warranted.