On some singular perturbation problems in the theory of lubrication

Some singular perturbation problems occurring in the theory of hydrodynamic lubrication are studied. We examine in detail the case of the very long or very short journal bearing; the asymptotic problem of small eccentricity ratio is also considered.     

Toward the rank-one singular perturbation theory of self-adjoint operators

The perturbation theory is developed for the case where an arbitrary positive self-adjoint operator is perturbed by the projector on a generalized vector. Similar to the well-known problem –+, we obtain, in the general case, explicit representations for singularly perturbed operators and their resolvents, and we find the point spectrum and an explicit form of the corresponding eigenvectors. Our approach differs from the usual ones and is based on the self-adjoint extension theory of semibounded operators.Translated from Ukrainskii Matematicheskii Zhurnal, Vol. 43, No. 11, pp. 1559–1566, November, 1991.     

Geometric singular perturbation theory for stochastic differential equations

We consider slow-fast systems of differential equations, in which both the slow and fast variables are perturbed by noise. When the deterministic system admits a uniformly asymptotically stable slow manifold, we show that the sample paths of the stochastic system are concentrated in a neighbourhood of the slow manifold, which we construct explicitly. Depending on the dynamics of the reduced system, the results cover time spans which can be exponentially long in the noise intensity squared (that is, up to Kramers’ time). We obtain exponentially small upper and lower bounds on the probability of exceptional paths. If the slow manifold contains bifurcation points, we show similar concentration properties for the fast variables corresponding to non-bifurcating modes. We also give conditions under which the system can be approximated by a lower-dimensional one, in which the fast variables contain only bifurcating modes.     

On the algebraic theory of scattering

Within the context of the algebraic theory for simple quantum mechanical potential scattering, we establish existence and completeness of generalized wave operators for a large class of long-range potential scattering problems. Our approach utilizes a priori knowledge of the spectral properties of the scattering Hamiltonian and an algebraic theorem on the uniqueness of a particular Euclidean group representation.     

On perturbation of roots of homogeneous algebraic systems

A problem concerning the perturbation of roots of a system of homogeneous algebraic equations is investigated. The question of conservation and decomposition of a multiple root into simple roots are discussed. The main theorem on the conservation of the number of roots of a deformed (not necessarily homogeneous) algebraic system is proved by making use of a homotopy connecting initial roots of the given system and roots of a perturbed system. Hereby we give an estimate on the size of perturbation that does not affect the number of roots. Further on we state the existence of a slightly deformed system that has the same number of real zeros as the original system in taking the multiplicities into account. We give also a result about the decomposition of multiple real roots into simple real roots.

     

Application of perturbation theory to the solvability analysis of differential algebraic equations

Linear systems of ordinary differential equations with an identically singular or rectangular matrix multiplying the derivative of the desired vector function are considered. The structure of general solutions is discussed. A special case of perturbed systems is studied by applying the Vishik-Lyusternik method.     

On a class of singular boundary value problems with singular perturbation

In this paper we investigate a class of singular second order differential equations with singular perturbation subject to three-point boundary value conditions, whose solution exhibits a couple of boundary layers at two endpoints. We first establish a lower–upper solutions theorem by using the Schauder fixed point theorem. By the asymptotic expansions and the lower–upper solutions theorem we obtain the existence, asymptotic estimates and uniqueness for the proposed problem. Several examples are given for illustrating our results.     

On the breakdown of perturbation theory

The production of particles in a weakly-coupled theory is believed to be non-perturbatively suppressed. I comment on the prospects of (a) establishing this rigorously, and (b) estimating the effect to exponential accuracy semiclassically, by discussing two closely-related problems: the large-order behavior of few-point Green functions, and induced excitation in quantum mechanics. Induced tunneling in the latter case is exponentially enhanced for frequencies of the order of the barrier height.Published in Teoreticheskaya i Matematicheskaya Fizika, Vol. 95, No. 2, pp. 187–196, May, 1993.     

On a singular perturbation problem in magnetohydrodynamics

Zusammenfassung Die Strömung einer zähen, inkompressiblen und elektrisch leitenden Flüssigkeit über einen rotationssymmetrischen Körper wird studiert mit Hilfe einer singulären Methode der Störungsrechnung. Eine asymptotische, im ganzen Strömungsfeld gültige Lösung wird gegeben für grosse Hartmann-ZahlenM.Die Resultate ergeben folgendes Strömungsbild: Zwei Totwasser-Bereiche von der LängeO (M) und der BreiteO (1) werden vor und nach dem Körper geformt. Sie sind begrenzt durch eine zylindrische Schubschicht, die vom grössten Durchmesser des Körpers aus parabolisch stromaufwärts und stromabwärts anwächst. In einer Entfernung der GrössenordnungO (M) geht diese Schubschicht in eine Wirbelstrasse über, die sich parabolisch ins Unendliche erstreckt. Die Einzelheiten des Strömungsbildes werden analytisch aufgezeigt. Die Wirbelstrasse wird mit derjenigen der klassischen Navier-Stokes-Theorie verglichen.     

First order spectral perturbation theory of square singular matrix polynomials

We develop first order eigenvalue expansions of one-parametric perturbations of square singular matrix polynomials. Although the eigenvalues of a singular matrix polynomial P(λ) are not continuous functions of the entries of the coefficients of the polynomial, we show that for most perturbations they are indeed continuous. Given an eigenvalue λ₀ of P(λ) we prove that, for generic perturbations M(λ) of degree at most the degree of P(λ), the eigenvalues of P(λ)+?M(λ) admit covergent series expansions near λ₀ and we describe the first order term of these expansions in terms of M(λ₀) and certain particular bases of the left and right null spaces of P(λ₀). In the important case of λ₀ being a semisimple eigenvalue of P(λ) any bases of the left and right null spaces of P(λ₀) can be used, and the first order term of the eigenvalue expansions takes a simple form. In this situation we also obtain the limit vector of the associated eigenvector expansions.     

Nonlinear PI control of chaotic systems using singular perturbation theory

In this paper, we develop the nonlinear PI controllers for a class of chaotic systems based on singular perturbation theory. The original system is decomposed into two reduced order systems, to which the nonlinear uncertain terms belongs. In order to alleviate the deterioration of these nonlinear uncertainties, the nonlinear PI controllers are applied to each subsystem and combined to construct the composite controller for the full order system. The effectiveness and feasibility of the proposed control scheme is demonstrated through numerical simulations on the chaotic Chua’s circuit.     

On singular perturbation of a semilinear hyperbolic equation

We consider a hyperbolic version of Eells-Sampson's equation: . This equation is semilinear with respect to space derivative and time derivative. Letu (x) be the solution with initial data u(0) and (0), and putv (t,x)=u (t,x). We show that when the resistance ,V (t,x) converges to a solution of the original parabolic Eells-Sampson's equation: . Note thatv _t(0)= (0) diverges when . We show that this phenomena occurs in more general situations.This article was processed by the author using the Springer-Verlag Pjourlg macro package.     

A geometric singular perturbation theory approach to constrained differential equations

This paper is concerned with a geometric study of ()‐parameter families of constrained differential systems, where . Our main results say that the dynamics of such a family close to the impasse set is equivalent to the dynamics of a multiple time scale singular perturbation problem (that is a singularly perturbed system containing several small parameters). This enables us to use a geometric theory for multiscale systems in order to describe the behaviour of such a family close to the impasse set. We think that a systematic program towards a combination between geometric singular perturbation theory and constrained systems and problems involving persistence of typical minimal sets are currently emergent. Some illustrations and applications of the main results are provided.     

On singular perturbation in non well posed problems

Summary Let L_εu and L ₀ v be the elliptic and “backward” heat operators defined by(1.1) and(1.2), respectively. The following question is considered for a pair of “non-well posed” initial-boundary value problems for L_ε and L ₀ : if u and v are the respective solutions, under what restrictions on the classes of admissible solutions and in what sense, if any, does u converge to v as ɛ →0? This research was supported in part by the National Science Foundation Grant No. GP 5882 with Cornell University.     

WKB-approximations and spectral asymptotics in one problem of singular perturbation theory

We study features of asymptotic distributions of eigenvalues of non-self-adjoint boundary-value problems for differential equations with small coefficients of the highest derivatives.Translated from Sovremennaya Matematika i Ee Prilozheniya (Contemporary Mathematics and Its Applications), Vol. 8 , Suzdal Conference—2, 2003.This revised version was published online in April 2005 with a corrected cover date.