The Barnett approximation in the theory of hydrodynamic fluctuations

The Langevin dynamics and fluctuational-dissipative relationships for the hydrodynamic fluctuations for systems which are described in the third Barnett order with respect to the gradients of the hydrodynamic variables are generalized on the basis of a kinetic approach.     

Further improvement of weakly nonlinear theory of hydrodynamic stability

The problems of the weakly nonlinear theory of hydrodynamic stability has been re-addressed; namely, (i) why is the radius of convergence of its solution too small? (ii) it is not appropriate to use it to deal with the evolution problem of disturbances; the main reason is that the way of calculating the mean flow distortion is inappropriate, (iii) it cannot be used to deal with the initial value problem. Ways of its improvement were proposed. Its correctness was confirmed by its comparison with the results of numerical simulations.     

Statistical decision theory for quantum systems

Quantum statistical decision theory arises in connection with applied problems of optimal detection and processing of quantum signals. In this paper we give a systematic treatment of this theory, based on operator-valued measures. We study the existence problem for optimal measurements and give sufficient and necessary conditions for optimality. The notion of the maximum likelihood measurement is introduced and investigated. The general theory is then applied to the case of Gaussian (quasifree) states of Bose systems, for which optimal measurements of the mean value are found.     

Toward a theory of nonlinear stochastic systems

The inequalities of Bihari, Langenhop, and Chebyshev are applied to a system of stochastic nonlinear differential equations. As a result, a one-parametric family of determinate estimates from above and below are obtained for the norm of the solution of the system. Each of these estimates is valid with a probability not less than some quantity depending upon the parameter of the family.Translated from Matematicheskii Zametki, Vol. 5, No. 5, pp. 607–614, May, 1969.The author expresses his appreciation to G. Ya. Lyubarski for his interest in the study and his valuable comments.     

Blowup of solutions of nonlinear equations and systems of nonlinear equations in wave theory

We consider the phenomenon of solution blowup for the system of equations describing surface water waves and also for the acoustic wave equation in viscous medium using the test-function method developed by Galactionov, Pokhozhaev, and Mitidieri.     

Statistical learning control of uncertain systems: theory and algorithms

It has recently become clear that many control problems are too difficult to admit analytic solutions. New results have also emerged to show that the computational complexity of some “solved” control problems is prohibitive. Many of these control problems can be reduced to decidability problems or to optimization questions. Even though such questions may be too difficult to answer analytically, or may not be answered exactly given a reasonable amount of computational resources, researchers have shown that we can “approximately” answer these questions “most of the time”, and have “high confidence” in the correctness of the answers.     

Universal theory of dynamical chaos in nonlinear dissipative systems of differential equations

The article presents a new universal theory of dynamical chaos in nonlinear dissipative systems of differential equations, including autonomous and nonautonomous ordinary differential equations (ODE), partial differential equations, and delay differential equations. The theory relies on four remarkable results: Feigenbaum’s period doubling theory for cycles of one-dimensional unimodal maps, Sharkovskii’s theory of birth of cycles of arbitrary period up to cycle of period three in one-dimensional unimodal maps, Magnitskii’s theory of rotor singular point in two-dimensional nonautonomous ODE systems, acting as a bridge between one-dimensional maps and differential equations, and Magnitskii’s theory of homoclinic bifurcation cascade that follows the Sharkovskii cascade. All the theoretical propositions are rigorously proved and illustrated with numerous analytical examples and numerical computations, which are presented for all classical chaotic nonlinear dissipative systems of differential equations.     

Statistical structuring theory in parametrically excitable dynamical systems with a Gaussian pump

Based on the idea of the statistical topography, we analyze the problem of emergence of stochastic structure formation in linear and quasilinear problems described by first-order partial differential equations. The appearance of a parametric excitation on the background of a Gaussian pump is a specific feature of these problems. We obtain equations for the probability density of the solutions of these equations, whence it follows that the stochastic structure formation emerges with probability one, i.e., for almost every realization of the random parameters of the medium.     

Some asymptotic results in hydrodynamic lubrication theory

Exact and asymptotic solutions to Reynolds equation for one-dimensional hydrodynamic lubrication are obtained for the case of rigid bearing surfaces and pressure-dependent viscosity.     

An optimization problem in hydrodynamic lubrication theory

If an arbitrary film profile is given, then we show how to find a film profile which will support more weight than the given profile. We do this for a discrete problem-our methods may be used on a digital computer-and we establish convergence results. The problem of finding a film profile which will support more weight than any other profile is also discussed.