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Andrei D. Polyanin

Publications and source records attributed to Andrei D. Polyanin.

18 recordsLinked to original sources
arXiv2026

Two-dimensional nonlinear Schrödinger equations with potential and dispersion given by arbitrary functions: Reductions and exact solutions

For the first time, a nonlinear Schrödinger equation of the general form is considered, depending on time and two spatial variables, the potential and dispersion of which are specified by two arbitrary functions. This equation naturally generalizes a number of simpler nonlinear partial differential equations encountered in various fields of theoretical physics, including nonlinear optics, superconductivity, and plasma physics. Two- and one-dimensional reductions are described, which reduce the studied nonlinear Schrödinger equation to simpler equations of lower dimension or ordinary differential equations (or systems of ODEs). In addition to the general Schrödinger equation with two arbitrary functions, related nonlinear PDEs are also examined, in which the dispersion function is specified arbitrarily while the potential function is expressed in terms of it. For all considered classes of nonlinear PDEs, using the methods of generalized and functional separation of variables, as well as the semi-inverse approach and the principle of structural analogy of solutions, many new exact solutions have been found, which are expressed in terms of elementary or special functions, or in the form of quadratures. Both Cartesian and polar coordinate systems are employed to analyze the equations under consideration. Special attention is paid to finding solutions with radial symmetry. It is shown that the nonlinear Schrödinger equation, in which the functions defining the potential and dispersion are linearly related (one of these functions can be chosen arbitrarily), can be reduced to a two-dimensional nonlinear PDE that admits exact linearization. The exact solutions obtained in this work can be used as test problems intended for verifying the adequacy and assessing the accuracy of numerical and approximate analytical methods for solving complex nonlinear PDEs of mathematical physics.

nlin.SI↗
arXiv2026

Nonclassical symmetries of polynomial equations and test problems with parameters for computer algebra systems

Nonclassical symmetries and reductions of polynomial equations and systems of polynomial equations are considered. It is shown that specific polynomial equations having "hidden" symmetries can be reduced to classical symmetric systems of polynomial equations by introducing a new additional variable. It has been established that symmetric systems of polynomial equations of mixed type, consisting of symmetric and anti-symmetric polynomials, can be transformed into simpler systems. A method is presented for solving nonclassical symmetric systems of two polynomial equations that change places when the unknowns are permuted. We study polynomial equations containing the second iteration of a given polynomial, which are reduced to nonclassical symmetric systems of equations. New higher-degree polynomial equations containing free parameters that admit solutions in radicals are found. Three such equations of the sixth and ninth degrees are further used as test problems with parameters for analyzing the capabilities of two leading computer algebra systems. It is shown that currently, the Maple and Mathematica systems do not allow us to efficiently find analytical solutions (in radicals) of polynomial equations with free parameters, but they allow us to obtain numerical solutions of equations for fixed numerical values of the parameters. The results of this work and the proposed test problems with parameters can be used to further improve existing computer algebra systems.

math.NA↗
arXiv2025

Exact solutions and reductions of nonlinear Schrödinger equations with delay

For the first time, Schrödinger equations with cubic and more complex nonlinearities containing the unknown function with constant delay are analyzed. The physical considerations that can lead to the appearance of a delay in such nonlinear equations and mathematical models are expressed. One-dimensional non-symmetry reductions are described, which lead the studied partial differential equations with delay to simpler ordinary differential equations and ordinary differential equations with delay. New exact solutions of the nonlinear Schrödinger equation of the general form with delay, which are expressed in quadratures, are found. To construct exact solutions, a combination of methods of generalized separation of variables and the method of functional constraints are used. Special attention is paid to three equations with cubic nonlinearity, which allow simple solutions in elementary functions, as well as more complex exact solutions with generalized separation of variables. Solutions representing a nonlinear superposition of two traveling waves, the amplitude of which varies periodically in time and space, are constructed. Some more complex nonlinear Schrödinger equations of a general form with variable delay are also studied. The results of this work can be useful for the development and improvement of mathematical models described by nonlinear Schrödinger equations with delay and related functional PDEs, and the obtained exact solutions can be used as test problems intended to assess the accuracy of numerical methods for integrating nonlinear equations of mathematical physics with delay.

nlin.SI↗
arXiv2024

Closed-form solutions of the nonlinear Schrödinger equation with arbitrary dispersion and potential

For the first time, the general nonlinear Schrödinger equation is investigated, in which the chromatic dispersion and potential are specified by two arbitrary functions. The equation in question is a natural generalization of a wide class of related nonlinear partial differential equations that are often used in various areas of theoretical physics, including nonlinear optics, superconductivity and plasma physics. To construct exact solutions, a combination of the method of functional constraints and methods of generalized separation of variables is used. Exact closed-form solutions of the general nonlinear Schrödinger equation, which are expressed in quadratures or elementary functions, are found. One-dimensional non-symmetry reductions are described, which lead the considered nonlinear partial differential equation to a simpler ordinary differential equation or a system of such equations. The exact solutions obtained in this work can be used as test problems intended to assess the accuracy of numerical and approximate analytical methods for integrating nonlinear equations of mathematical physics.

nlin.SI↗
arXiv2024

Notes on symmetries and reductions of algebraic equations

Symmetries and reductions of some algebraic equations are considered. Transformations that preserve the form of several algebraic equations, as well as transformations that reduce the degree of these equations, are described. Illustrative examples are provided. The obtained results and solutions can be used as test problems for numerical methods of solving algebraic equations.

math.NA↗
arXiv2024

Principle of structural analogy of solutions and its application to nonlinear PDEs and delay PDEs

Using the principle of structural analogy of solutions, approaches have been developed for constructing exact solutions of complex nonlinear PDEs, including PDEs with delay, based on the use of special solutions to auxiliary simpler related equations. It is shown that to obtain exact solutions of nonlinear non-autonomous PDEs, the coefficients of which depend on time, it is possible to use generalized and functional separable solutions of simpler autonomous PDEs, the coefficients of which do not depend on time. Specific examples of constructing exact solutions to nonlinear PDEs, the coefficients of which depend arbitrarily on time, are considered. It has been discovered that generalized and functional separable solutions of nonlinear PDEs with constant delay can be used to construct exact solutions of more complex nonlinear PDEs with variable delay of general form. A number of nonlinear reaction-diffusion type PDEs with variable delay are described, which allow exact solutions with generalized separation of variables.

nlin.SI↗
arXiv2021

The similarity index of scientific publications with equations and formulas, identification of self-plagiarism, and testing of the iThenticate system

The problems of estimating the similarity index of mathematical and other scientific publications containing equations and formulas are discussed for the first time. It is shown that the presence of equations and formulas (as well as figures, drawings, and tables) is a complicating factor that significantly complicates the study of such texts. It is shown that the method for determining the similarity index of publications, based on taking into account individual mathematical symbols and parts of equations and formulas, is ineffective and can lead to erroneous and even completely absurd conclusions. The possibilities of the most popular software system iThenticate, currently used in scientific journals, are investigated for detecting plagiarism and self-plagiarism. The results of processing by the iThenticate system of specific examples and special test problems containing equations (PDEs and ODEs), exact solutions, and some formulas are presented. It has been established that this software system when analyzing inhomogeneous texts, is often unable to distinguish self-plagiarism from pseudo-self-plagiarism (false self-plagiarism). A model complex situation is considered, in which the identification of self-plagiarism requires the involvement of highly qualified specialists of a narrow profile. Various ways to improve the work of software systems for comparing inhomogeneous texts are proposed. This article will be useful to researchers and university teachers in mathematics, physics, and engineering sciences, programmers dealing with problems in image recognition and research topics of digital image processing, as well as a wide range of readers who are interested in issues of plagiarism and self-plagiarism.

cs.DL↗
arXiv2021

Exact Solutions and Reductions of Nonlinear Diffusion PDEs of Pantograph Type

We study nonlinear pantograph-type reaction-diffusion PDEs, which, in addition to the unknown $u=u(x,t)$, also contain the same functions with dilated or contracted arguments of the form $w=u(px,t)$, $w=u(x,qt)$, and $w=u(px,qt)$, where $p$ and $q$ are the free scaling parameters (for equations with proportional delay we have $0<p<1$, $0<q<1$). A brief review of publications on pantograph-type ODEs and PDEs and their applications is given. Exact solutions and reductions of various types of such nonlinear partial functional differential equations are described for the first time. We present examples of nonlinear pantograph-type PDEs with proportional delay, which admit traveling-wave and self-similar solutions (note that PDEs with constant delay do not have self-similar solutions). Additive, multiplicative and functional separable solutions, as well as some other exact solutions are also obtained. Special attention is paid to nonlinear pantograph-type PDEs of a rather general form, which contain one or two arbitrary functions. In total, more than forty nonlinear pantograph-type reaction-diffusion PDEs with dilated or contracted arguments, admitting exact solutions, have been considered. Multi-pantograph nonlinear PDEs are also discussed. The principle of analogy is formulated, which makes it possible to efficiently construct exact solutions of nonlinear pantograph-type PDEs. A number of exact solutions of more complex nonlinear functional differential equations with varying delay, which arbitrarily depends on time or spatial coordinate, are also described. The presented equations and their exact solutions can be used to formulate test problems designed to evaluate the accuracy of numerical and approximate analytical methods for solving the corresponding nonlinear initial-boundary value problems for PDEs with varying delay.

nlin.SI↗
arXiv2021

Construction of complex solutions to nonlinear partial differential equations using simpler solutions

The paper describes a number of simple but quite effective methods for constructing exact solutions of PDEs, that involve a relatively small amount of intermediate calculations. The methods employ two main ideas: (i) simple exact solutions can serve to construct more complex solutions of the equations under consideration and (ii) exact solutions of some equations can serve to construct solutions of other, more complex equations. In particular, we propose a method for constructing complex solutions from simple solutions using translation and scaling. We show that in some cases, rather complex solutions can be obtained by adding one or more terms to simpler solutions. There are situations where nonlinear superposition allows us to construct a complex composite solution using similar simple solutions. We also propose a few methods for constructing complex exact solutions to linear and nonlinear PDEs by introducing complex-valued parameters into simpler solutions. The effectiveness of the methods is illustrated by a large number of specific examples (over 30 in total). These include nonlinear heat/diffusion equations, wave type equations, Klein--Gordon type equations, hydrodynamic boundary layer equations, Navier--Stokes equations, and some other PDEs. Apart from exact solutions to `ordinary' PDEs, we also describe some exact solutions to more complex nonlinear delay PDEs. Along with the unknown function at the current time, $u=u(x,t)$, these equations contain the same function at a past time, $w=u(x,t-τ)$, where $τ>0$ is the delay time. Furthermore, we look at nonlinear partial functional-differential equations of the pantograph type, which in addition to the unknown $u=u(x,t)$, also contain the same functions with dilated or contracted arguments, $w=u(px,qt)$, where $p$ and $q$ are scaling parameters.

nlin.SI↗
arXiv2020

Functional separation of variables in nonlinear PDEs: General approach, new solutions of diffusion-type equations

The study gives a brief overview of existing modifications of the method of functional separation of variables for nonlinear PDEs. It proposes a more general approach to the construction of exact solutions to nonlinear equations of applied mathematics and mathematical physics, based on a special transformation with an integral term and the generalized splitting principle. The effectiveness of this approach is illustrated by nonlinear diffusion-type equations that contain reaction and convective terms with variable coefficients. The focus is on equations of a fairly general form that depend on one, two or three arbitrary functions (such nonlinear PDEs are most difficult to analyze and find exact solutions). A number of new functional separable solutions and generalized traveling wave solutions are described (more than 30 exact solutions have been presented in total). It is shown that the method of functional separation of variables can, in certain cases, be more effective than (i) the nonclassical method of symmetry reductions based on an invariant surface condition, and (ii) the method of differential constraints based on a single differential constraint. The exact solutions obtained can be used to test various numerical and approximate analytical methods of mathematical physics and mechanics.

math-ph↗
arXiv2019

The direct method of functional separation of variables can provide more exact solutions than the compatibility analysis of PDEs based on a single differential constraint

This note shows that in looking for exact solutions to nonlinear PDEs, the direct method of functional separation of variables can, in certain cases, be more effective than the method of differential constraints based on the compatibility analysis of PDEs with a single constraint (invariant surface condition). This fact is illustrated by examples of nonlinear reaction-diffusion and convection-diffusion equations with variable coefficients, nonlinear Klein--Gordon type equations, and hydrodynamic boundary layer equations. A few new exact solutions are given.

nlin.SI↗
arXiv2018

Hypersingular nonlinear boundary-value problems with a small parameter

For the first time, some hypersingular nonlinear boundary-value problems with a small parameter~$\varepsilon$ at the highest derivative are described. These problems essentially (qualitatively and quantitatively) differ from the usual linear and quasilinear singularly perturbed boundary-value problems and have the following unusual properties: (i) in hypersingular boundary-value problems, super thin boundary layers arise, and the derivative at the boundary layer can have very large values of the order of $e^{1/\varepsilon}$ and more (in standard problems with boundary layers, the derivative at the boundary has the order of $\varepsilon^{-1}$ or less); (ii) in hypersingular boundary-value problems, the position of the boundary layer is determined by the values of the unknown function at the boundaries (in standard problems with boundary layers, the position of the boundary layer is determined by coefficients of the given equation, and the values of the unknown function at the boundaries do not play a role here); (iii) hypersingular boundary-value problems do not admit a direct application of the method of matched asymptotic expansions (without a preliminary nonlinear point transformation of the equation under consideration). Examples of hypersingular nonlinear boundary-value problems with ODEs and PDEs are given and their exact solutions are obtained. It is important to note that the exact solutions presented in this paper can be used to compare the effectiveness of various methods of numerical integration of singularly perturbed problems with boundary layers, and also to develop new numerical and approximate analytical methods.

math.AP↗
arXiv2017

Numerical integration of blow-up problems on the basis of non-local transformations and differential constraints

Several new methods of numerical integration of Cauchy problems with blow-up solutions for nonlinear ordinary differential equations of the first- and second-order are described. Solutions of such problems have singularities whose positions are unknown a priori (the standard numerical methods for solving problems with blow-up solutions can lead to significant errors). The first proposed method is based on the transition to an equivalent system of equations by introducing a new independent variable chosen as the first derivative. The second method is based on introducing a new auxiliary non-local variable with the subsequent transformation to the Cauchy problem for the corresponding system of ODEs. The third method is based on adding to the original equation of a differential constraint, which is an auxiliary ODE connecting the given variables and a new variable. The proposed methods lead to problems whose solutions are represented in parametric form and do not have blowing-up singular points; therefore the transformed problems admit the application of standard fixed-step numerical methods. The efficiency of these methods is illustrated by solving a number of test problems that admit an exact analytical solution. It is shown that: (i) the methods based on non-local transformations of a special kind are more efficient than several other methods, (ii) among the proposed methods, the most general method is the method based on the differential constraints. Some examples of nonclassical blow-up problems are considered. Simple theoretical estimates are derived for the critical value of an independent variable. It is shown that the method based on a non-local transformation of the general form as well as the method based on the differential constraints admit generalizations to the $n$th-order ODEs and systems of coupled ODEs.

math.NA↗
arXiv2017

New numerical methods for blow-up problems

Two new methods of numerical integration of Cauchy problems for ODEs with blow-up solutions are described. The first method is based on applying a differential transformation, where the first derivative (given in the original equation) is chosen as a new independent variable. The second method is based on introducing a new non-local variable that reduces ODE to a system of coupled ODEs. Both methods lead to problems whose solutions do not have blowing-up singular points, therefore the standard numerical methods can be applied. The efficiency of the proposed methods is illustrated with several test problems.

math.NA↗
arXiv2013

A new method for constructing exact solutions to nonlinear delay partial differential equations

We propose a new method for constructing exact solutions to nonlinear delay reaction--diffusion equations of the form $$ u_t=ku_{xx}+F(u,w), $$ where $u=u(x,t)$, $w=u(x,t-τ)$, and $τ$ is the delay time. The method is based on searching for solutions in the form $u=\sum^N_{n=1}ξ_n(x)η_n(t)$, where the functions $ξ_n(x)$ and $η_n(t)$ are determined from additional functional constraints (which are difference or functional equations) and the original delay partial differential equation. All of the equations considered contain one or two arbitrary functions of a single argument. We describe a considerable number of new exact generalized separable solutions and a few more complex solutions representing a nonlinear superposition of generalized separable and traveling wave solutions. All solutions involve free parameters (in some cases, infinitely many parameters) and so can be suitable for solving certain problems and testing approximate analytical and numerical methods for nonlinear delay PDEs. The results are extended to a wide class of nonlinear partial differential-difference equations involving arbitrary linear differential operators of any order with respect to the independent variables $x$ and $t$ (in particular, this class includes the nonlinear delay Klein--Gordon equation) as well as to some partial functional differential equations with time-varying delay.

nlin.SI↗
arXiv2009

The Crocco transformation: order reduction and construction of Backlund transformations and new integrable equations

Wide classes of nonlinear mathematical physics equations are described that admit order reduction through the use of the Crocco transformation, with a first-order partial derivative taken as a new independent variable and a second-order partial derivative taken as the new dependent variable. Associated Backlund transformations are constructed for evolution equations of general form (special cases of which are Burgers, Korteweg-de Vries, and many other nonlinear equations of mathematical physics). The results obtained are used for order reduction and constructing exact solutions of hydrodynamics equations (Navier-Stokes, Euler, and boundary layer). A number of new integrable nonlinear equations, inclusive of the generalized Calogero equation, are considered.

nlin.SI↗
arXiv2009

The von Mises transformation: order reduction and construction of Backlund transformations and new integrable equations

Wide classes of nonlinear mathematical physics equations are described that admit order reduction through the use of the von Mises transformation, with the unknown function taken as the new independent variable and an appropriate partial derivative taken as the new dependent variable. RF-pairs and associated Bäcklund transformations are constructed for evolution equations of general form (special cases of which are Burgers, Korteweg--de Vries, and Harry Dym type equations as well as many other nonlinear equations of mathematical physics). The results obtained are used for order reduction and constructing exact solutions of hydrodynamics equations. A generalized Calogero equation and a number of other new integrable nonlinear equations are considered.

math-ph↗