A waiting time phenomenon for modulations of pattern in reaction–diffusion systems

In order to describe slow modulations in time and space of stable or slightly unstable spatially periodic stationary solutions of pattern forming reaction–diffusion systems, so-called phase diffusion equations and Cahn–Hilliard equations can be derived via multiple scaling analysis as formal approximation equations. In the case that these equations degenerate, waiting time phenomena are well known to occur. In this paper, we prove that such waiting time phenomena can also occur approximately in the original reaction–diffusion systems by proving estimates between the formal approximations and the exact solutions of the original systems.     

Robust Analysis and Synthesis for the Nonexistence of Periodic Solutions in a Class of Nonlinear Systems

This paper considers the nonexistence of periodic solutions in a class of nonlinear uncertain systems. Based on the generalized Kalman–Yakubovich–Popov (GKYP) lemma, linear matrix inequalities (LMIs) characterizations are derived to guarantee the nonexistence of periodic solutions in a certain frequency range. The new LMI conditions do not involve any product of the Lyapunov matrix and the system matrices. Based on the results, a dynamic output feedback controller is designed to ensure the nonexistence of periodic solutions in such systems. A concrete application to the Chua circuit shows the applicability and validity of the proposed approach. This work was supported by the National Science Foundation of China under Grants 60404007 and 60334030.     

Asymptotic Behavior of Solutions to a Class of Systems of Delay Differential Equations

The authors investigate the asymptotic behavior of solutions to a class of systems of delay differential equations. It is shown that every bounded solution of such a class of systems tends to a constant vector as t→∞. Our results improve and extend some corresponding ones already known.     

Hamiltonian systems of PDEs with selfdual boundary conditions

Selfdual variational calculus is developed further and used to address questions of existence of local and global solutions for various parabolic semi-linear equations, and Hamiltonian systems of PDEs. This allows for the resolution of such equations under general time boundary conditions which include the more traditional ones such as initial value problems, periodic and anti-periodic orbits, but also yield new ones such as “periodic orbits up to an isometry” for evolution equations that may not have periodic solutions. In the process, we introduce a method for perturbing selfdual functionals in order to induce coercivity and compactness, without destroying the selfdual character of the system. N. Ghoussoub was partially supported by a grant from the Natural Sciences and Engineering Research Council of Canada. A. Moameni’s research was supported by a postdoctoral fellowship at the University of British Columbia.     

On Exact Controllability of Networks of Nonlinear Elastic Strings in 3-Dimensional Space

This paper concerns a system of nonlinear wave equations describing the vibrations of a 3-dimensional network of elastic strings.The authors derive the equations and appropriate nodal conditions,determine equilibrium solutions,and,by using the methods of quasilinear hyperbolic systems,prove that for tree networks the natural initial,boundary value problem has classical solutions existing in neighborhoods of the "stretched" equilibrium solutions.Then the local controllability of such networks near such equilibrium configurations in a certain specified time interval is proved.Finally,it is proved that,given two different equilibrium states satisfying certain conditions,it is possible to control the network from states in a small enough neighborhood of one equilibrium to any state in a suitable neighborhood of the second equilibrium over a suffciently large time interval.     

Solutions to the systems of functional equations of multivalued logic

General issues concerning solutions to the systems of functional equations of multivalued logic—the dependence of solutions on functional constants and the possibility of constructing the systems of equations having a given unique solution or a given set of solutions—are considered.     

Hamiltonian systems as selfdual equations

Hamiltonian systems with various time boundary conditions are formulated as absolute minima of newly devised non-negative action functionals obtained by a generalization of Bogomolnyi’s trick of ‘dcompleting squares’. Reminiscent of the selfdual Yang-Mills equations, they are not derived from the fact that they are critical points (i.e., from the corresponding Euler-Lagrange equations) but from being zeroes of the corresponding non-negative Lagrangians. A general method for resolving such variational problems is also described and applied to the construction of periodic solutions for Hamiltonian systems, but also to study certain Lagrangian intersections.      

Noncommutative unitons

By Uhlenbeck’s results, every harmonic map from the Riemann sphere S² to the unitary group U(n) decomposes into a product of so-called unitons: special maps from S² to the Grassmannians Gr _k(ℂⁿ) ⊂ U(n) satisfying certain systems of first-order differential equations. We construct a noncommutative analogue of this factorization, applicable to those solutions of the noncommutative unitary sigma model that are finite-dimensional perturbations of zero-energy solutions. In particular, we prove that the energy of each such solution is an integer multiple of 8π, give examples of solutions that are not equivalent to Grassmannian solutions, and study the realization of non-Grassmannian zero modes of the Hessian of the energy functional by directions tangent to the moduli space of solutions. __________ Translated from Teoreticheskaya i Matematicheskaya Fizika, Vol. 154, No. 2, pp. 220–239, February, 2008.     

On a class of systems of quasilinear conservation laws

We consider hyperbolic conservation laws on matrix algebras. We describe entropies of such systems and study properties of generalized entropy solutions and strong generalized entropy solutions to the Cauchy problem. Bibliography: 15 titles. Translated from Problemy Matematicheskogo Analiza, No. 38, December 2008, pp. 73–91.     

A priori estimates for solutions to systems of nonlinear parabolic equations

A class of nondiagonal systems of nonlinear parabolic equations that can be reduced to a scalar parabolic equation in the phase space of a larger dimension is described. In view of such a reduction, it is possible to state the maximum principle for solutions to systems of nonlinear parabolic equations and derive a priori C^2+α-estimates for a solution to the Cauchy problem. Bibliography: 19 titles. Translated fromProblemy Matematicheskogo Analiza, No. 16. 1997, pp. 41–67.     

On the Asymptotic Representation of a Regularization Approach to Nonlinear Semiexplicit Higher-Index Differential-Algebraic Equations

Differential-algebraic equations with a higher index can beapproximated by regularization algorithms. One such possibilitywas introduced by März for linear time-varying index 2systems and extended by the author to index 3 systems. In thepresent paper, März’s approach is generalized tononlinear semiexplicit index 2 and 3 equations. The structureof the regularized solutions and their convergence propertiesare characterized in terms of asymptotic expansions.     

Globally optimal solutions of max–min systems

A variety of problems in computer science, operations research, control theory, etc., can be modeled as non-linear and non-differentiable max–min systems. This paper introduces the global optimization into such systems. The criteria for the existence and uniqueness of the globally optimal solutions are established using the high matrix, optimal max-only projection set and k ^s -control vector of max–min functions. It is also shown that the global optimization can be accomplished through the partial max-only projection representation with algebraic and combinatorial features. The methods are constructive and lead to an algorithm of finding all globally optimal solutions.     

Minimizing Sequences for a Family of Functional Optimal Estimation Problems

Rates of convergence are derived for approximate solutions to optimization problems associated with the design of state estimators for nonlinear dynamic systems. Such problems consist in minimizing the functional given by the worst-case ratio between the ℒ _p -norm of the estimation error and the sum of the ℒ _p -norms of the disturbances acting on the dynamic system. The state estimator depends on an innovation function, which is searched for as a minimizer of the functional over a subset of a suitably-defined functional space. In general, no closed-form solutions are available for these optimization problems. Following the approach proposed in (Optim. Theory Appl. 134:445–466, 2007), suboptimal solutions are searched for over linear combinations of basis functions containing some parameters to be optimized. The accuracies of such suboptimal solutions are estimated in terms of the number of basis functions. The estimates hold for families of approximators used in applications, such as splines of suitable orders.     

Four-Point Boundary-Value Problems for Differential–Algebraic Systems

We apply the monotone iterative method to nonlinear four-point boundary conditions for differential–algebraic systems with causal operators. Sufficient conditions under which such problems have solutions (extremal or unique) are given. An example illustrates the theoretical results.     

On linear homogeneous almost periodic systems that satisfy the Favard condition

We prove the existence of a linear homogeneous almost periodic system of differential equations that has nontrivial bounded solutions and is such that all systems from a certain neighborhood of it have no nontrivial almost periodic solutions. Institute of Mathematics, Ukrainian Academy of Sciences, Kiev. Translated from Ukrainskii Matematicheskii Zhurnal, Vol. 50, No. 3. pp. 409–413, March, 1998.     

Gibbs ensembles,equidistribution of the energy of sympathetic oscillators and statistical models of thermostat

The paper develops an approach to the proof of the “zeroth” law of thermodynamics. The approach is based on the analysis of weak limits of solutions to the Liouville equation as time grows infinitely. A class of linear oscillating systems is indicated for which the average energy becomes eventually uniformly distributed among the degrees of freedom for any initial probability density functions. An example of such systems are sympathetic pendulums. Conditions are found for nonlinear Hamiltonian systems with finite number of degrees of freedom to converge in a weak sense to the state where the mean energies of the interacting subsystems are the same. Some issues related to statistical models of the thermostat are discussed.      

Spreading Speeds and Traveling Waves for Non-cooperative Reaction�CDiffusion Systems

Much has been studied on the spreading speed and traveling wave solutions for cooperative reaction–diffusion systems. In this paper, we shall establish the spreading speed for a large class of non-cooperative reaction–diffusion systems and characterize the spreading speed as the slowest speed of a family of non-constant traveling wave solutions. Our results are applied to a partially cooperative system describing interactions between ungulates and grass.     

Stability conditions for singularly perturbed delay differential equations

Singular perturbation problems containing a small positive parameter ε occur in many areas, including biochemical kinetics, genetics, plasma physics, and mechanical and electrical systems. A uniformly valid, reliable interpretable approximation of such problems is required. This paper provides sufficient conditions to ensure the exponential stability of the analytical and numerical solutions of the singularly perturbed delay differential equations with a bounded time-lag for suf.ciently small ε > 0. The Halanay inequality is used to prove the main results of the paper. A numerical example is provided to illustrate the methodology and clarify the need for a stiff solver for numerical solutions of these problems.     

Diophanting geometry over groups II: Completions,closures and formal solutions

This paper is the second in a series on the structure of sets of solutions to systems of equations in a free group, projections of such sets, and the structure of elementary sets defined over a free group. In the second paper we generalize Merzlyakov’s theorem on the existence of a formal solution associated with a positive sentence [Me]. We first construct a formal solution to a generalAE sentence which is known to be true over some variety, and then develop tools that enable us to analyze the collection of all such formal solutions. Partially supported by an Israel Academy of Sciences Fellowship.     

Solution of equations in Boolean algebra

The explicit form of solutions of Boolean equations with one unknown is obtained. The effectiveness of the method is demonstrated for a number of equations whose solution previously has been found only in “tabular” form. The proposed approach leads to a method for solving systems of equations in Boolean set algebra. We use it to analyze the famous paradoxes of set theory, such as the barber paradox and the liar paradox, as well as Russell's and Cantor's paradoxes. Translated from Nelineinaya Dinamika i Upravlenie, pp. 119–132, 1999.