On instability in the presence of several resonances : PMM vol. 43, no. 6, 1979, pp. 970–974

The equilibrium position stability of an autonomous system of ordinary differential equations is considered in the case of n pairs of pure imaginary roots with the simultaneous presence of several resonances. It is shown using Chetaev's theorem [1] that when among the solutions of the model system there are increasing solutions of the invariant ray type, the complete system is Liapunov unstable.     

Vibration op two circular stamps in a layered medium : PMM vol. 40, n 6, 1976, pp. 1104–1112

The problem of the harmonic oscillations on the surface of an elastic layered medium by two circular stamps of radii a₁ and a₂ is considered. The spacing between the centers of the stamps is b > a₁ + a₂. It is assumed that there is no friction in the area of contact.By using a method developed in [1], the initial system is reduced to a system of Fredholm equations of the second kind, for whose solution approximate methods are proposed. On the basis of the results obtained, an applied theory for the vibrations of two stamps can be constructed which also takes account of the dispersion properties of the medium, in contrast to all other known applied theories.It is simple to investigate the case of vibrations of a system of n stamps by the method elucidated in this paper, however, we limit ourselves to the case of two stamps for the sake of brevity.     

On the investigation of oscillations of quasilinear systems with almost-periodic coefficients : PMM vol.43, no. 6, 1979, pp. 980–991

A method is developed for investigating the oscillations of systems with almost-periodic coefficients, based on Kamenkov's ideas [1] on the construction of stationary solutions of systems with periodic coefficients and on the separation of motions. In contrast to [1] it is assumed that under the vanishing of a small parameter μ the system's characteristic equation has, besides n pairs of pure imaginary roots, m zero roots and h roots with negative real parts. Non-resonance and resonance cases are considered. Conditions are obtained for the existence of stationary solutions with respect to terms of first order in the small parameter. An example is presented.     

Asymptotic stability and smooth equivalence op ordinary equations : PMM vol.40, n 6, 1976, pp. 1048–1057

Under certain specified conditions the asymptotic stability is a coarse property [1],(i.e. addition of fairly smooth functions to the right-hand sides of equations, does not disturb the asymptotic stability). It is shown below that in this cage the unperturbed system is coarse in a more general sense, namely, any smooth system acted upon by fairly small smooth perturbations, can be returned to its unperturbed state by a smooth reversible transformation. The value and order of the perturbations and the domain of existence of the transformation are all estimated explicitly. The condition required for the above assertion to hold, is that of the existence of a Liapunov function admitting, together with its derivative, specified estimates. This requirement holds, in particular, in the case when the right-hand sides of the unperturbed system are homogeneous functions, the position of equilibrium is asymptotically stable, and its neighborhood contains no solutions bounded when −∞ <t < ∞ (see [1]). If the system is analytic, the requirement will hold in at least all critical cases investigated in which the asymptotic stability with t → ∞ or t → −∞ is fixed, since in these cases the Liapunov function will be analytic, or simply polynomial. It follows therefore from the theorem which we prove, that in all the cases in question, the system is reduced by a smooth transformation, to the polynomial form. If the unperturbed system is linear, then from the theorem proved follows a theorem on linearization appearing in [2]; if the system is nonlinear but of second order, a theorem from [3] ensues. The results obtained in this paper for the nonlinear autonomous systems are extended to the case when the perturbations are continuous and bounded functions of time. This makes possible the investigation of the dynamics of the process in the neighborhood of asymptotically stable equilibria and of periodic modes, ignoring a wide range of external perturbations.     

On the determination op forces op constraint reaction : PMM vol. 39, n 6, 1975, pp. 1129–1134

The equations of motion of mechanical systems with multipliers are reduced to the form enabling the separation of these equations into two groups, the first group describing the motions of the system, and the second group defining the multipliers. Each multiplier is determined independently of the remaining multipliers, and this makes it easy to assess the dynamic effect of each constraint on the system. On the basis of this approach, we study the following problems: determination of the constraint reactions [1], study of the motion of controlled systems with prescribed constraints [2, 3] and utilization of the method of nonholonomic mechanical systems in the case when the first integrals exist [4].     

On a System of Differential Equations Connected with the Gravitational Instability of a Multicomponent Medium in Newtonian Cosmology

The gravitational clustering of a multicomponent medium in an expanding universe has been considered by many authors in recent years. The system of differential equations associated with such a multicomponent medium is generalized in this article by incorporating arbitrary parameters. Explicit analytic solutions of this generalized system are given for various situations. Physical interpretations of the solutions are not considered, but the various solutions of physical problems obtained by various authors can be seen to be special cases of the general solutions given in this article.     

On a Set of Models of Magnets and Chiral Fields: Integrability, the Darboux Transformation, and Exact Solutions

Two matrix linear systems that are polynomial pencils with a complex parameter are considered. Consistency conditions for these systems are found and examples of integrable equations of magnets and chiral fields are given. For some equations, exact solutions are constructed. Bibliography: 16 titles.     

Perturbation analysis of a modified second grade fluid over a porous plate

A modified second grade non-Newtonian fluid model is considered. The model is a combination of power-law and second grade fluids in which the fluid may exhibit normal stresses, shear thinning or shear thickening behaviors. The flow of this fluid is considered over a porous plate. Equations of motion in dimensionless form are derived. When the power-law effects are small compared to second grade effects, a regular perturbation problem arises which is solved. The validity criterion for the solution is derived. When second grade effects are small compared to power-law effects, or when both effects are small, the problem becomes a boundary layer problem for which the solutions are also obtained. Perturbation solutions are contrasted with the numerical solutions. For the regular perturbation problem of small power-law effects, an excellent match is observed between the solutions if the validity criterion is met. For the boundary layer solution of vanishing second grade effects however, the agreement with the numerical data is not good. When both effects are considered small, the boundary layer solution leads to the same solution given in the case of a regular perturbation problem.     

Properties of solutions of a mixed problem for a nonlinear ultraparabolic equation

Mixed problems for a nonlinear ultraparabolic equation are considered in domains bounded and unbounded with respect to the space variables. Conditions for the existence and uniqueness of solutions of these problems are established and some estimates for these solutions are obtained.     

Small-parameter method for constructing approximate strategies in a class of differential games : PMM vol. 39, n 6, 1975, pp. 1006–1016

We examine a class of problems in which the pay-off is some function of the terminal state of a conflict-controlled system. When the opportunities of one of the players are small in relation with the opportunities of the other, we propose methods for constructing approximate optimal strategies of the players, based on solving the Bellman equation containing a small parameter. We have shown that the players' approximate optimal strategies can be constructed if the solutions of the corresponding optimal control problems are known. The error bounds for the methods are proved and examples are considered. The arguments used rely on the results in [1–6] on the theory of differential games and on [7–11] devoted to optimal control synthesis methods for systems subject to random perturbations of small intensity.     

Travelling wave solutions of a generalized Camassa-Holm-Degasperis-Procesi equation

The travelling wave solutions of a generalized Camassa-Holm-Degasperis-Procesi equation ut-uxxt + (1 + b)umux = buxuxx + uuxxx are considered where b > 1 and m are positive integers. The qualitative analysis methods of planar autonomous systems yield its phase portraits. Its soliton wave solutions, kink or antikink wave solutions, peakon wave solutions, compacton wave solutions, periodic wave solutions and periodic cusp wave solutions are obtained. Some numerical simulations of these solutions are also give...     

Logarithmic corrections in a quantization rule. The polaron spectrum

A nonlinear integrodifferential equation that arises in polaron theory is considered. The integral nonlinearity is given by a convolution with the Coulomb potential. Radially symmetric solutions are sought. In the semiclassical limit, an equation for the self-consistent potential is found and studied. The potential has a logarithmic singularity at the origin, and also a turning point at 1. The phase shifts at these points are determined. The quantization rule that takes into account the logarithmic corrections gives a simple asymptotic formula for the polaron spectrum. Global semiclassical solutions of the original nonlinear equation are constructed.Moscow Institute of Electronic Engineering; Moscow Power Institute. Translated from Teoreticheskaya i Matematicheskaya Fizika, Vol. 97, No. 1, pp. 78–93, October, 1993.     

Asymptotic expansions of solutions to inverse problems for a hyperbolic equation with a small parameter multiplying the highest derivative

Two inverse problems for a hyperbolic equation with a small parameter multiplying the highest derivative are considered. The existence and uniqueness of their solutions are proved. As the small parameter tends to zero, the solutions of the inverse problems are proved to converge to solutions of inverse problems for a parabolic equation.     

Results for a turbulent system with unbounded viscosities: Weak formulations,existence of solutions,boundedness and smoothness

We consider a circulation system arising in turbulence modelling in fluid dynamics with unbounded eddy viscosities. Various notions of weak solution are considered and compared. We establish existence and regularity results. In particular we study the boundedness of weak solutions. We also establish an existence result for a classical solution.     

Global solvability of the equations of one-dimensional nonisothermic motion of a two-phase mixture. I. Statement of the problem and auxiliary assertions

The parabolic regularization is considered of the problem of one-dimensional motion of two interpenetrative viscous incompressible fluids. Some estimates of solutions are obtained that are uniform with respect to the regularization parameter on every finite time interval.     

On a certain class of a nonlinear evolution partial differential equations

An existence result and a priori bound for the solution of a second-order nonlinear parabolic equation are established. Also a generalized tanh-function method is used for constructing exact travelling wave solutions for the nonlinear diffusion equation of Fisher type originated from the considered partial differential equation. And new multiple soliton solutions are obtained.     

Baryon string model. II. Special solutions of classical three-string equations of motion

Conclusions We have considered the simplest solutions of the three-string equations of motion; these are solutions with a finite number of excited degrees of freedom. It is of interest to construct the quantum theory of such motions of the relativistic three-string. Quantum theory of a meson string with finite number of degrees of freedom was constructed in [4]. Quantization of finite-mode solutions of the baryon string model will be considered in the third paper of the present work.Institute of High Energy Physics, Serpukhov. Translated from Teoreticheskaya i Matematicheskaya Fizika. Vol. 64, No. 2, pp. 245–258, August, 1985.     

On Bounded Solutions of the Emden-Fowler Equation in a Semi-cylinder

Bounded solutions of the Emden-Fowler equation in a semi-cylinder are considered. For small solutions the asymptotic representations at infinity are derived. It is shown that there are large solutions whose behavior at infinity is different. These solutions are constructed when some inequalities between the dimension of the cylinder and the homogeneity of the nonlinear term are fulfilled. If these inequalities are not satisfied then it is proved, for the Dirichlet problem, that all bounded solutions tend to zero and have the same asymptotics as small solutions.