Exact solutions of the generalized Bretherton equation

The generalized Bretherton equation is studied. The Bäcklund transformations between traveling wave solutions of the generalized Bretherton equation and solutions of polynomial ordinary differential equation are constructed. The classification problem for meromorphic solutions of the latter equation is discussed. Several new families of exact solutions for the generalized Brethenton equation are given.     

Petrov classification of vacuum axisymmetric space-times

Using the null bivector approach, Petrov classification is studied for axisymmetric vacuum space-times with orthogonally transitive Killing vectors. It is shown that the equation on which the classification is based is biquadratic. This excludes that any such space-time can be type III. The only type-N manifolds are the radiative solutions considered by Hoffman. Van Stockum solutions are the most general type-II solutions. Degenerate Weyl solutions and Kinnersley solutions cover typeD. Stationary solutions with functional dependence of the potential are then examined. It is found that, except for special cases, Papapetrou and Lewis solutions are algebraically general.     

First- and second-order finite volume methods for the one-dimensional nonconservative Euler system

Gas flow in porous media with a nonconstant porosity function provides a nonconservative Euler system. We propose a new class of schemes for such a system for the one-dimensional situations based on nonconservative fluxes preserving the steady-state solutions. We derive a second-order scheme using a splitting of the porosity function into a discontinuous and a regular part where the regular part is treated as a source term while the discontinuous part is treated with the nonconservative fluxes. We then present a classification of all the configurations for the Riemann problem solutions. In particularly, we carefully study the resonant situations when two eigenvalues are superposed. Based on the classification, we deal with the inverse Riemann problem and present algorithms to compute the exact solutions. We finally propose new Sod problems to test the schemes for the resonant situations where numerical simulations are performed to compare with the theoretical solutions. The schemes accuracy (first- and second-order) is evaluated comparing with a nontrivial steady-state solution with the numerical approximation and convergence curves are established.     

Group analysis of the nonlinear electrodynamics equations

The symmetry of the Maxwell equations is investiated in this paper in a nonlinear isotropic medium without currents and charges. A group-theoretic classification is given of the material equations and partially invariant solutions. Exact solutions of the Maxwell equations are found.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 27–31, July, 1990.     

New classes of exact solutions and some transformations of the Navier-Stokes equations

New classes of exact solutions of three-dimensional nonstationary Navier-Stokes equations are described. These solutions contain arbitrary functions. Many periodic solutions (both with respect to the spatial coordinate and with respect to time) and aperiodic solutions are obtained, which can be expressed in terms of elementary functions. A Crocco-type transformation is presented, which reduces the order of the equation for the longitudinal component of the velocity. Problems concerning the nonlinear stability/instability of the solutions thus obtained are investigated. It turns out that a specific feature of many solutions of the Navier-Stokes equations is their instability. It is shown that instability can take place not only for rather large Reynolds numbers but also for arbitrarily small ones (and can be independent of the velocity profile of the fluid). A general physical interpretation and classification of solutions is given.     

From Laurent series to exact meromorphic solutions: The Kawahara equation

Meromorphic traveling wave solutions of the Kawahara equation and the modified Kawahara equations are studied. An algorithm for constructing meromorphic solutions in explicit form is described. The classification problem for meromorphic solutions of autonomous nonlinear ordinary differential equations is discussed.     

On the fixed points and invariants of Dykhne transformations and the stability of the solutions of the problem of the effective conductivity of randomly inhomogeneous two-phase media

The fixed points and invariants of Dykhne transformations are determined. It is established that they correspond to the exact solutions and duality relations for the effective characteristics of an inhomogeneous medium. The stability of the exact solutions for the effective conductivity, which are fixed points of the Dykhne transformations, is studied by bifurcation-theory methods, and a classification of these fixed points by stability type is given. It is shown that the effective conductivity tensor of a two-phase medium in magnetic and ac electric fields for certain parameters of the medium can be an unstable point of the “saddle” type.     

带有扰动非线性源的多孔介质方程的近似泛函分离变量

讨论了带有扰动非线性源的多孔介质方程的近似泛函分离变量问题. 给出了该类方程容许近似泛函分离变量解的完全分类. 举例阐述了近似泛函分离变量解的求解步骤,获得了分类方程的近似泛函分离变量解.     

The classification of the single travelling wave solutions to the variant Boussinesq equations

The discrimination system for the polynomial method is applied to variant Boussinesq equations to classify single travelling wave solutions. In particular, we construct corresponding solutions to the concrete parameters to show that each solution in the classification can be realized.     

The complexiton solutions to the nonisospectral Korteweg-de Vries equation

Starting with the solution classification for a linear differential equations, the complexiton solutions to nonisospectral Korteweg-de Vries equation are presented. The basic technique adopted is the Wronskian technique for Hirota's bilinear equation.     

Approximate Derivative-Dependent Functional Variable Separation for the Generalized Diffusion Equations with Perturbation

As an extension to the derivative-dependent functional variable separation approach, the approximate derivative-dependent functional variable separation approach is proposed, and it is applied to study the generalized diffusion equations with perturbation. Complete classification of these perturbed equations which admit approximate derivative-dependent functional separable solutions is obtained. As a result, the corresponding approximate derivative-dependent functional separable solutions to some resulting perturbed equations are derived by way of examples.     

Approximate Derivative-Dependent Functional Variable Separation for the Generalized Diffusion Equations with Perturbation

As an extension to the derivative-dependent functional variable separation approach, the approximate derivative-dependent functional variable separation approach is proposed, and it is applied to study the generalized diffusion equations with perturbation. Complete classification of these perturbed equations which admit approximate derivative-dependent functional separable solutions is obtained. As a result, the corresponding approximate derivative-dependent functional separable solutions to some resulting perturbed equations are derived by way of examples.     

Classification of degenerate solutions of linear differential equations

An algorithm of obtaining partial solutions of linear differential equations in partial derivatives that admit certain nontrivial symmetry algebra but are nonintegrable by the standard methods is described. The notion of degenerate solution is introduced. Natural classification of solutions is suggested.     

Symmetries,Integrability and Exact Solutions to the (2+1)-Dimensional Benney Types of Equations

This paper is concerned with the (2+1)-dimensional Benney types of equations. By the complete Lie group classification method, all of the point symmetries of the Benney types of equations are obtained, and the integrable condition of the equation is given. Then, the symmetry reductions and exact solutions to the (2+1)-dimensional nonlinear wave equations are presented. Especially, the shock wave solutions of the Benney equations are investigated by the symmetry reduction and trial function method.     

Classification and Functional Separable Solutions to Extended Nonlinear Wave Equations

The generalized conditional symmetry approach is applied to study the variable separation of the extended wave equations. Complete classification of those equations admitting functional separable solutions is obtained and exact separable solutions to some of the resulting equations are constructed.     

Classification of Static, Spherically Symmetric Solutions of the Einstein-Yang-Mills Theory with Positive Cosmological Constant

We give a complete classification of all static, spherically symmetric solutions of the SU(2) Einstein-Yang-Mills theory with a positive cosmological constant. Our classification proceeds in two steps. We first extend solutions of the radial field equations to their maximal interval of existence. In a second step we determine the Carter-Penrose diagrams of all 4-dimensional space-times constructible from such radial pieces. Based on numerical studies we sketch a complete phase space picture of all solutions with a regular origin.     

Classification and Functional Separable Solutions to Extended Nonlinear Wave Equations

The generalized conditional symmetry approach is applied to study the variable separation of the extended wave equations. Complete classification of those equations admitting functional separable solutions is obtained and exact separable solutions to some of the resulting equations are constructed.     

Stekkel space of electrovacuum with isotropic complete sets of the type (1.1)

A complete classification of the Stekkel spaces of the electrovacuum, admitting one isotropic Killing vector field and two Killing tensor fields, is presented. All solutions of the Einstein-Maxwell equations found refer to the Petrov type N.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 79–83, October, 1988.In conclusion we note that all solutions presented above are of the Petrov type N.     

Symmetries,Symmetry Reductions and Exact Solutions to the Generalized Nonlinear Fractional Wave Equations

In this paper, the Lie group classification method is performed on the fractional partial differential equation (FPDE), all of the point symmetries of the FPDEs are obtained. Then, the symmetry reductions and exact solutions to the fractional equations are presented, the compatibility of the symmetry analysis for the fractional and integer-order cases is verified. Especially, we reduce the FPDEs to the fractional ordinary differential equations (FODEs) in terms of the Erdélyi-Kober (E-K) fractional operator method, and extend the power series method for investigating exact solutions to the FPDEs.     

Real-World Data Difficulty Estimation with the Use of Entropy

In the era of the Internet of Things and big data, we are faced with the management of a flood of information. The complexity and amount of data presented to the decision-maker are enormous, and existing methods often fail to derive nonredundant information quickly. Thus, the selection of the most satisfactory set of solutions is often a struggle. This article investigates the possibilities of using the entropy measure as an indicator of data difficulty. To do so, we focus on real-world data covering various fields related to markets (the real estate market and financial markets), sports data, fake news data, and more. The problem is twofold: First, since we deal with unprocessed, inconsistent data, it is necessary to perform additional preprocessing. Therefore, the second step of our research is using the entropy-based measure to capture the nonredundant, noncorrelated core information from the data. Research is conducted using well-known algorithms from the classification domain to investigate the quality of solutions derived based on initial preprocessing and the information indicated by the entropy measure. Eventually, the best 25% (in the sense of entropy measure) attributes are selected to perform the whole classification procedure once again, and the results are compared.