Time-dependent propagation speed vs strong damping for degenerate linear hyperbolic equations

We consider a degenerate abstract wave equation with a time-dependent propagation speed. We investigate the influence of a strong dissipation, namely a friction term that depends on a power of the elastic operator.We discover a threshold effect. If the propagation speed is regular enough, then the damping prevails, and therefore the initial value problem is well-posed in Sobolev spaces. Solutions also exhibit a regularizing effect analogous to parabolic problems. As expected, the stronger is the damping, the lower is the required regularity.On the contrary, if the propagation speed is not regular enough, there are examples where the damping is ineffective, and the dissipative equation behaves as the non-dissipative one.     

Unilateral problems for the wave equation with degenerate and localized nonlinear damping: well‐posedness and non‐stability results

Unilateral problems related to the wave model subject to degenerate and localized nonlinear damping on a compact Riemannian manifold are considered. Our results are new and concern two main issues: (a) to prove the global well‐posedness of the variational problem; (b) to establish that the corresponding energy functional is not (uniformly) stable to equilibrium in general, namely, the energy does not converge to zero on the trajectory of every solution, even if a full linear damping is taken in place.     

Generalized differential equation arising in the solution of the inverse scattering problem in a layered medium

We consider a nonclassical ordinary differential equation containing not only an unknown function but also an unknown coefficient depending on the unknown function. We show that if the desired solution is assumed to have bounded variation and be a.e. constant on the interval where the equation is considered, then the problem of finding the solution and the unknown coefficient does not have a unique solution in terms of the classical derivative. We prove that if the derivative is understood as a distribution, than this problem has a unique solution. These results are used to show that the acoustic impedance and the damping factor in the inverse scattering problem in a layered dissipative medium can be determined simultaneously.     

The uniqueness of the solution to the diffusion equation with a damping term

Consider a non-linear diffusion equation with a damping term. If the diffusion coefficient is positive, then the solutions are not unique generally. However, if the diffusion coefficient degenerates, the situation may change. In this paper, not only the existence of the weak solution is established, but also the uniqueness of the weak solutions is proved, even the boundary value condition is not imposed. The conclusions imply that, on the boundary, the degeneracy of diffusion coefficient can eliminate the action from the damping term.     

Global nonexistence for a semilinear Petrovsky equation

In this paper we consider a semilinear Petrovsky equation with damping and source terms. It is proved that the solution blows up in finite time if the positive initial energy satisfies a suitable condition. Moreover for the linear damping case, we show that the solution blows up in finite time even for vanishing initial energy. This is an important breakthrough, since it is only well known that the solution blows up in finite time if the initial energy is negative from all the previous literature.     

Embedding the planar structure of 4-dimensional linear spaces into projective spaces

It is known that a linear spaces of dimensiond has at least as many hyperplanes as points with equality if it is a (possibly degenerate) projective space. If there are only a few more hyperplanes than points, then the linear space can still be embedded in a projective space of the same dimension. But even if the difference between the number of hyperplanes and points is too big to ensure an embedding, it seems likely that the linear space is closely related to a projective space. We shall demonstrate this in the cased=4.     

Differential properties of solutions of variational problems for functionals of linear growth

The differential properties of the vector extremals of convex functionals of linear growth, depending only on the modulus of the gradient of the desired function, are investigated. It is proved that if the integrand is strictly convex and its derivative is concave for large values of the argument, then, under some additional conditions, the generalized solution is regular in an open set of full measure. Another result consists in the fact that if the integrand is linear for all large values of the argument, then, under some additional conditions, there exists an open set in which the gradient of the solution is continuous and its modulus is strictly smaller than the value of the parameter, starting from which the integrand becomes a linear function.Translated from Problemy Matematicheskogo Analiza, No. 11, pp. 51–79, 1990.     

ON THE BREAKDOWNS OF THE GALERKIN AND LEAST-SQUARES METHODS

1　ＩｎｔｒｏｄｕｃｔｉｏｎＷｅｃｏｎｓｉｄｅｒｌｉｎｅａｒｓｙｓｔｅｍｓｏｆｔｈｅｆｏｒｍＡｘ=ｂ,(1 )ｗｈｅｒｅＡ∈ＣＮ×Ｎｉｓｎｏｎｓｉｎｇｕｌａｒａｎｄｐｏｓｓｉｂｌｙｎｏｎ Ｈｅｒｍｉｔｉａｎ .Ａｍａｊｏｒｃｌａｓｓｏｆｍｅｔｈｏｄｓｆｏｒｓｏｌｖｉｎｇ (1 )ｉｓｔｈｅｃｌａｓｓｏｆＫｒｙｌｏｖｓｕｂｓｐａｃｅｍｅｔｈｏｄｓ (ｓｅｅ[6] ,[1 3]ｆｏｒｏｖｅｒｖｉｅｗｓｏｆｓｕｃｈｍｅｔｈｏｄｓ) ,ｄｅｆｉｎｅｄｂｙｔｈｅｐｒｏｐｅｒｔｉｅｓｘｍ ∈ｘ0 +Ｋｍ(ｒ0 ,Ａ) ;(2 )ｒｍ ⊥Ｌｍ, (3)ｗｈｅ…     

Equilibria of Runge-Kutta methods

Summary It is known that certain Runge-Kutta methods share the property that, in a constant-step implementation, if a solution trajectory converges to a bounded limit then it must be a fixed point of the underlying differential system. Such methods are calledregular. In the present paper we provide a recursive test to check whether given method is regular. Moreover, by examining solution trajectories of linear equations, we prove that the order of ans-stage regular method may not exceed 2[(s+2)/2] and that the maximal order of regular Runge-Kutta method with an irreducible stability function is 4.     

Simultaneous diagonalization of three real symmetric matrices

We formulate and prove necessary and sufficient conditions of simultaneous diagonalization of three real symmetric matrices of regular pencil. The conditions are algebraic and consist, in particular, of two spectral requirements and one matrix equality. For the degenerate matrix pencil we suggest an approach that allows reducing of the analysis to a regular pencil. With the use of obtained theorems we investigate a decomposition of linear gyroscopic system into subsystems of an order not higher than two and the stability of trivial solution to a system.     

Polynomial-time algorithms for regular set-covering and threshold synthesis

A set-covering problem is called regular if a cover always remains a cover when any column in it is replaced by an earlier column. From the input of the problem - the coefficient matrix of the set-covering inequalities - it is possible to check in polynomial time whether the problem is regular or can be made regular by permuting the columns. If it is, then all the minimal covers are generated in polynomial time, and one of them is an optimal solution. The algorithm also yields an explicit bound for the number of minimal covers. These results can be used to check in polynomial time whether a given set-covering problem is equivalent to some knapsack problem without additional variables, or equivalently to recognize positive threshold functions in polynomial time. However, the problem of recognizing when an arbitrary Boolean function is threshold is NP-complete. It is also shown that the list of maximal non-covers is essentially the most compact input possible, even if it is known in advance that the problem is regular.     

Global attractor for the one dimensional wave equation with displacement dependent damping

We study the long-time behavior of solutions of the one dimensional wave equation with nonlinear damping coefficient. We prove that if the damping coefficient function is strictly positive near the origin then this equation possesses a global attractor.     

Well-posedness for the fractional Landau–Lifshitz equation without Gilbert damping

The main purpose is to consider the well-posedness of the fractional Landau– Lifshitz equation without Gilbert damping. The local existence of classical solutions is obtained by combining Kato’s method and vanishing viscosity method, by carefully choosing the working space. Since this equation is strongly degenerate and nonlocal and no regularizing effect is available, it is a challenging problem to extend this smooth solution to global. Instead, we give some regularity criteria to show that the solution is global if some additional regularity is assumed, which seems minimal in the sense of dimensional analysis. Finally, we introduce the commutator and show the global existence of weak solutions by vanishing viscosity method.     

Nontrivial solutions to boundary value problems for semilinear strongly degenerate elliptic differential equations

In this paper we study boundary value problems for semilinear equations involving strongly degenerate elliptic differential operators. Via a Pohozaev??s type identity we show that if the nonlinear term grows faster than some power function then the boundary value problem has no nontrivial solution. Otherwise when the nonlinear term grows slower than the same power function, by establishing embedding theorems for weighted Sobolev spaces associated with the strongly degenerate elliptic equations, then applying the theory of critical values in Banach spaces, we prove that the problem has a nontrivial solution, or even infinite number of solutions provided that the nonlinear term is an odd function.     

A conservation law with spatially localized sublinear damping

We consider a general conservation law on the circle, in the presence of a sublinear damping. If the damping acts on the whole circle, then the solution becomes identically zero in finite time, following the same mechanism as the corresponding ordinary differential equation. When the damping acts only locally in space, we show a dichotomy: if the flux function is not zero at the origin, then the transport mechanism causes the extinction of the solution in finite time, as in the first case. On the other hand, if zero is a non-degenerate critical point of the flux function, then the solution becomes extinct in finite time only inside the damping zone, decays algebraically uniformly in space, and we exhibit a boundary layer, shrinking with time, around the damping zone. Numerical illustrations show how similar phenomena may be expected for other equations.     

Global classical solution to partially dissipative quasilinear hyperbolic systems

For 1-D quasilinear hyperbolic systems, the strict dissipation or the weak linear degeneracy can prevent the formation of singularity. More precisely, if all the inhomogeneous sources are strictly dissipative, or all the characteristics are weakly linearly degenerate and the system is homogeneous, then the Cauchy problem with small and decaying initial data admits a unique global classical solution. In this paper, under some suitable hypotheses on the interaction, new kinds of weighted formulas of wave decomposition are developed to show the same result for a general class of combined systems, in which a part of equations possesses the strict dissipation and the others are weakly linearly degenerate.     

分数阶一般退化微分系统的通解

本文研究了系数矩阵不是方阵情形的分数阶一般退化微分系统的解.通过定义可解阵对,获得分数阶一般退化微分系统的通解表达式.该结果推广了整数阶退化微分系统和分数阶常微分系统解的相应结论.     

Stability of a Hamiltonian system in a limiting case

We give a fairly simple geometric proof that an equilibrium point of a Hamiltonian system of two degrees of freedom is Liapunov stable in a degenerate case. That is the 1: −1 resonance case where the linearized system has double pure imaginary eigenvalues ±iω, ω ≠ 0 and the Hamiltonian is indefinite. The linear system is weakly unstable, but if a particular coefficient in the normalized Hamiltonian is of the correct sign then Moser’s invariant curve theorem can be applied to show that the equilibrium point is encased in invariant tori and thus it is stable.     

Primary resonance of Duffing oscillator with fractional-order derivative

In this paper the primary resonance of Duffing oscillator with fractional-order derivative is researched by the averaging method. At first the approximately analytical solution and the amplitude-frequency equation are obtained. Additionally, the effect of the fractional-order derivative on the system dynamics is analyzed, and it is found that the fractional-order derivative could affect not only the viscous damping, but also the linear stiffness, which is characterized by the equivalent damping coefficient and the equivalent stiffness coefficient. This conclusion is remarkably different from the existing research results about nonlinear system with fractional-order derivative. Moreover, the comparisons of the amplitude-frequency curves by the approximately analytical solution and the numerical integration are fulfilled, and the results certify the correctness and satisfactory precision of the approximately analytical solution. At last, the effects of the two parameters of the fractional-order derivative, i.e. the fractional coefficient and the fractional order, on the amplitude-frequency curves are investigated, which are different from the traditional integer-order Duffing oscillator.     

Optimal damping ratio for linear second-order systems

In this paper, it is shown that the optimal damping ratio for linear second-order systems that results in minimum-time no-overshoot response to step inputs is of bang-bang type. The optimal damping ratio is zero at the outset and is switched to some maximum value at an appropriate instant of time. The switching time is shown to be a function of the maximum damping ratio and the system natural frequency. Furthermore, it is shown that the larger the maximum damping ratio is, the shorter it takes for the system to reach the desired set point. Finally, it is shown that, if the optimal damping ratio is switched as a function of the system state, then the minimum-time no-overshoot criterion is satisfied, irrespective of the magnitude of the uncertainty in the value of the system natural frequency.