Nikolai Kudryashov
Publications:
Kudryashov N. A.
Exact Solutions and Integrability of the Duffing–Van der Pol Equation
2018, vol. 23, no. 4, pp. 471479
Abstract
The forcefree Duffing–Van der Pol oscillator is considered. The truncated expansions for finding the solutions are used to look for exact solutions of this nonlinear ordinary differential equation. Conditions on parameter values of the equation are found to have the linearization of the Duffing–Van der Pol equation. The Painlevé test for this equation is used to study the integrability of the model. Exact solutions of this differential equation are found. In the special case the approach is simplified to demonstrate that some wellknown methods can be used for finding exact solutions of nonlinear differential equations. The first integral of the Duffing–Van der Pol equation is found and the general solution of the equation is given in the special case for parameters of the equation. We also demonstrate the efficiency of the method for finding the first integral and the general solution for one of nonlinear secondorder ordinary differential equations.

Garashchuk I. R., Sinelshchikov D. I., Kudryashov N. A.
Nonlinear Dynamics of a Bubble Contrast Agent Oscillating near an Elastic Wall
2018, vol. 23, no. 3, pp. 257272
Abstract
Contrast agent microbubbles, which are encapsulated gas bubbles, are widely used to enhance ultrasound imaging. There are also several new promising applications of the contrast agents such as targeted drug delivery and noninvasive therapy. Here we study three models of the microbubble dynamics: a nonencapsulated bubble oscillating close to an elastic wall, a simple coated bubble and a coated bubble near an elastic wall.We demonstrate that complex dynamics can occur in these models. We are particularly interested in the multistability phenomenon of bubble dynamics. We show that coexisting attractors appear in all of these models, but for higher acoustic pressures for the models of an encapsulated bubble.We demonstrate how several tools can be used to localize the coexisting attractors. We provide some considerations why the multistability can be undesirable for applications.

Kudryashov N. A.
Asymptotic and Exact Solutions of the FitzHugh–Nagumo Model
2018, vol. 23, no. 2, pp. 152160
Abstract
The standard FitzHugh–Nagumo model for description of impulse from one neuron to another is considered. The system of equations is transformed to a nonlinear secondorder ordinary differential equation. It is shown that the differential equation does not pass the Painlevé test in the general case and the general solution of this equation does not exist. The simplest solutions of the system of equations are found. The secondorder differential equation is transformed to another asymptotic equation with the general solution expressed via the Jacobi elliptic function. This transformation allows us to obtain the asymptotic solutions of the FitzHugh–Nagumo model. The perturbed FitzHugh–Nagumo model is studied as well. Taking into account the simplest equation method, the exact solutions of the perturbed system of equations are found. The asymptotic solutions of the perturbed model are presented too. The application of the exact solutions for construction of the neural networks is discussed.

Kudryashov N. A., Gaur I. Y.
Weak Nonlinear Asymptotic Solutions for the Fourth Order Analogue of the Second Painlevé Equation
2017, vol. 22, no. 3, pp. 266271
Abstract
The fourthorder analogue of the second Painlevé equation is considered. The monodromy manifold for a Lax pair associated with the $P_2^2$ equation is constructed. The direct monodromy problem for the Lax pair is solved. Asymptotic solutions expressed via trigonometric functions in the Boutroux variables along the rays $\phi = \frac{2}{5}\pi(2n+1)$ on the complex plane have been found by the isomonodromy deformations technique.

Kudryashov N. A., Sinelshchikov D. I.
On the Integrability Conditions for a Family of Liénardtype Equations
2016, vol. 21, no. 5, pp. 548555
Abstract
We study a family of Liénardtype equations. Such equations are used for the description of various processes in physics, mechanics and biology and also appear as travelingwave reductions of some nonlinear partial differential equations. In this work we find new conditions for the integrability of this family of equations. To this end we use an approach which is based on the application of nonlocal transformations. By studying connections between this family of Liénardtype equations and type III Painlevé–Gambier equations, we obtain four new integrability criteria. We illustrate our results by providing examples of some integrable Liénardtype equations. We also discuss relationships between linearizability via nonlocal transformations of this family of Liénardtype equations and other integrability conditions for this family of equations.

Demina M. V., Kudryashov N. A.
Multiparticle Dynamical Systems and Polynomials
2016, vol. 21, no. 3, pp. 351366
Abstract
Polynomial dynamical systems describing interacting particles in the plane are studied. A method replacing integration of a polynomial multiparticle dynamical system
by finding polynomial solutions of partial differential equations is introduced. The method enables one to integrate a wide class of polynomial multiparticle dynamical systems. The general solutions of certain dynamical systems related to linear secondorder partial differential equations are found. As a byproduct of our results, new families of orthogonal polynomials are derived.

Kudryashov N. A., Sinelshchikov D. I.
On the Connection of the Quadratic Lienard Equation with an Equation for the Elliptic Functions
2015, vol. 20, no. 4, pp. 486496
Abstract
The quadratic Lienard equation is widely used in many applications. A connection between this equation and a linear secondorder differential equation has been discussed. Here we show that the whole family of quadratic Lienard equations can be transformed into an equation for the elliptic functions. We demonstrate that this connection can be useful for finding explicit forms of general solutions of the quadratic Lienard equation. We provide several examples of application of our approach.

Kudryashov N. A.
Analytical Solutions of the Lorenz System
2015, vol. 20, no. 2, pp. 123133
Abstract
The Lorenz system is considered. The Painlevé test for the thirdorder equation corresponding to the Lorenz model at $\sigma \ne 0$ is presented. The integrable cases of the Lorenz system and the first integrals for the Lorenz system are discussed. The main result of the work is the classification of the elliptic solutions expressed via the Weierstrass function. It is shown that most of the elliptic solutions are degenerated and expressed via the trigonometric functions. However, two solutions of the Lorenz system can be expressed via the elliptic functions.

Kudryashov N. A., Sinelshchikov D. I.
Special Solutions of a Highorder Equation for Waves in a Liquid with Gas Bubbles
2014, vol. 19, no. 5, pp. 576585
Abstract
A fifthorder nonlinear partial differential equation for the description of nonlinear waves in a liquid with gas bubbles is considered. Special solutions of this equation are studied. Some elliptic and simple periodic traveling wave solutions are constructed. Connection of selfsimilar solutions with Painlevé transcendents and their highorder analogs is discussed.

Borisov A. V., Kudryashov N. A.
Paul Painlevé and His Contribution to Science
2014, vol. 19, no. 1, pp. 119
Abstract
The life and career of the great French mathematician and politician Paul Painlevé is described. His contribution to the analytical theory of nonlinear differential equations was significant. The paper outlines the achievements of Paul Painlevé and his students in the investigation of an interesting class of nonlinear secondorder equations and new equations defining a completely new class of special functions, now called the Painlevé transcendents. The contribution of Paul Painlevé to the study of algebraic nonintegrability of the $N$body problem, his remarkable observations in mechanics, in particular, paradoxes arising in the dynamics of systems with friction, his attempt to create the axiomatics of mechanics and his contribution to gravitation theory are discussed.

Kudryashov N. A.
Higher Painlevé Transcendents as Special Solutions of Some Nonlinear Integrable Hierarchies
2014, vol. 19, no. 1, pp. 4863
Abstract
It is well known that the selfsimilar solutions of the Korteweg–de Vries equation and the modified Korteweg–de Vries equation are expressed via the solutions of the first and second Painlevé equations. In this paper we solve this problem for all equations from the Korteveg–de Vries, modified Korteweg–de Vries, Kaup–Kupershmidt, Caudrey–Dodd–Gibbon and Fordy–Gibbons hierarchies. We show that the selfsimilar solutions of equations corresponding to hierarchies mentioned above can be found by means of the general solutions of higherorder Painlevé hierarchies introduced more than ten years ago.

Demina M. V., Kudryashov N. A.
Relative Equilibrium Configurations of Point Vortices on a Sphere
2013, vol. 18, no. 4, pp. 344355
Abstract
The problem of constructing and classifying equilibrium and relative equilibrium configurations of point vortices on a sphere is studied. A method which enables one to find any such configuration is presented. Configurations formed by the vortices placed at the vertices of Platonic solids are considered without making the assumption that the vortices possess equal in absolute value circulations. Several new configurations are obtained.

Demina M. V., Kudryashov N. A.
Point Vortices and Classical Orthogonal Polynomials
2012, vol. 17, no. 5, pp. 371384
Abstract
Stationary equilibria of point vortices in the plane and on the cylinder in the presence of a background flow are studied. Vortex systems with an arbitrary choice of circulations are considered. Differential equations satisfied by generating polynomials of vortex configurations are derived. It is shown that these equations can be reduced to a single one. It is found that polynomials that are Wronskians of classical orthogonal polynomials solve the latter equation. As a consequence vortex equilibria at a certain choice of background flows can be described with the help of Wronskians of classical orthogonal polynomials.

Demina M. V., Kudryashov N. A.
Point Vortices and Polynomials of the Sawada–Kotera and Kaup–Kupershmidt Equations
2011, vol. 16, no. 6, pp. 562576
Abstract
Rational solutions and special polynomials associated with the generalized $K_2$ hierarchy are studied. This hierarchy is related to the Sawada–Kotera and Kaup–Kupershmidt equations and some other integrable partial differential equations including the Fordy–Gibbons equation. Differential–difference relations and differential equations satisfied by the polynomials are derived. The relationship between these special polynomials and stationary configurations of point vortices with circulations $\Gamma$ and $−2\Gamma$ is established. Properties of the polynomials are studied. Differential–difference relations enabling one to construct these polynomials explicitly are derived. Algebraic relations satisfied by the roots of the polynomials are found.

Kudryashov N. A., Soukharev M. B.
Popular Ansatz Methods and Solitary Wave Solutions of the Kuramoto–Sivashinsky Equation
2009, vol. 14, no. 3, pp. 407419
Abstract
Some methods to look for exact solutions of nonlinear differential equations are discussed. It is shown that many popular methods are equivalent to each other. Several recent publications with "new" solitary wave solutions for the Kuramoto–Sivashinsky equation are analyzed. We demonstrate that all these solutions coincide with the known ones.

Kudryashov N. A.
Solitary and Periodic Solutions of the Generalized Kuramoto–Sivashinsky Equation
2008, vol. 13, no. 3, pp. 234238
Abstract
The generalized Kuramoto–Sivashinsky equation in the case of the power nonlinearity with arbitrary degree is considered. New exact solutions of this equation are presented.

Efimova O. Y., Kudryashov N. A.
Power Expansions for the SelfSimilar Solutions of the Modified Sawada–Kotera Equation
2007, vol. 12, no. 2, pp. 198218
Abstract
The fourthorder ordinary differential equation that denes the selfsimilar solutions of the Kaup–Kupershmidt and Sawada–Kotera equations is studied. This equation belongs to the class of fourthorder analogues of the Painlevé equations. All the power and nonpower asymptotic forms and expansions near points $z = 0$, $z = \infty$ and near an arbitrary point $z = z_0$ are found by means of power geometry methods. The exponential additions to the solutions of the studied equation are also determined.
