Evolution Problems involving non-stationary Operators between two Banach Spaces I. Existence and Uniqueness Theorems

Synopsis

(for ‘Evolution Problems involving non-stationary Operators between two Banach Spaces I-II)

In this series of two papers the initial-value problem [B(t)u(t)' = A(t)u(t), Bu(0) = y, with A = A(t) and B = B(t) time-varying operators from one Banach space X to another Banach space Y, and y an arbitrary element of Y, is considered. By making use of the theory of B-evolutions and by integrating certain temporally inhomogeneous equations, a unique solution is obtained for any y in Y. The solution is formulated explicitly in terms of a certain solution operator which involves the B(t)-evolution generated by the closed pair >A(t),B(t)< of operators. Certain properties of the solution operator are also studied. The well-known results, obtained by making use of semigroup theory, for the evolution problem [u(t)]' = A(t)u(t), u(0) = u₀, where A is a closed operator in a Banach space with dense domain, may also be derived from our results.     

Nonlinear and quasilinear evolution equations: Existence,uniqueness, and comparison of solutions; the rate of convergence of the difference method

In the Banach space X one investigates the Cauchy problem where [u](t)=u|_{[o, t]}, f L₁ (0, T; X); for fixed t, w, the nonlinear operator A(t, w)=A is a pseudogenerating operator of the semigroup e^SA (s 0), and for u, v, w(r) Z_r(Z_n is a ball in ZX),; the conditions on the dependence of A(t, w) on w admit the occurrence of w in the leading terms. One proves local and global theorems of existence and uniqueness of the limit-difference solution of the Cauchy problem, one investigates its differentiability and its dependence on u_o and f. Similar results of Crandall-Pazy, Benilan, Crandall-Evans, Evans, Oharu, Pavel, etc. for the equations du(t)/dt=A(t)u(t)+f (t) with- dissipative operators are special cases of ours. In the quasilinear case, our results complement and generalize T. Kato's well-known theorem. In addition, one obtains estimates for the convergence rates of the difference method and estimates for the norm of the difference of the solutions of Cauchy problems with different operators A(t, w); these results are new also for the equations with dissipative operators.Translated from Zapiski Nauchnykh Seminarov Leningradskogo Otdeleniya Matematicheskogo Instituta im. V. A. Steklova AN SSSR, Vol. 127, pp. 181–200, 1983.In conclusion, the author would like to express his sincere gratitude to O. A. Ladyzhenskaya for her interest in this paper.     

Dynamics and interpretation of some integrable systems via multiple orthogonal polynomials

High-order non-symmetric difference operators with complex coefficients are considered. The correspondence between dynamics of the coefficients of the operator defined by a Lax pair and its resolvent function is established. The method of investigation is based on the analysis of the moments for the operator. The solution of a discrete dynamical system is studied. We give explicit expressions for the resolvent function and, under some conditions, the representation of the vector of functionals, associated with the solution for the integrable systems.     

L p-Theory of elliptic differential operators on manifolds of bounded geometry

This paper is devoted to some of the properties of uniformly elliptic differential operators with bounded coefficients on manifolds of bounded geometry in L ^pspaces. We prove the coincidence of minimal and maximal extensions of an operator of a considered type with a positive principal symbol, the existence of holomorphic semigroup, generated by it, and the estimates of L ^p-norms of the operators of this semigroup. Some spectral properties of such operators in L ^pspaces are also studied.     

Homogeneous differential-operator equations in locally convex spaces

We describe a general method that allows us to find solutions to homogeneous differential-operator equations with variable coefficients by means of continuous vector-valued functions. The “homogeneity” is not interpreted as the triviality of the right-hand side of an equation. It is understood in the sense that the left-hand side of an equation is a homogeneous function with respect to operators appearing in that equation. Solutions are represented as functional vector-valued series which are uniformly convergent and generated by solutions to a kth order ordinary differential equation, by the roots of the characteristic polynomial and by elements of a locally convex space. We find sufficient conditions for the continuous dependence of the solution on a generating set. We also solve the Cauchy problem for the considered equations and specify conditions for the existence and the uniqueness of the solution. Moreover, under certain hypotheses we find the general solution to the considered equations. It is a function which yields any particular solution. The investigation is realized by means of characteristics of operators such as the order and the type of an operator, as well as operator characteristics of vectors, namely, the operator order and the operator type of a vector relative to an operator. We also use a convergence of operator series with respect to an equicontinuous bornology.     

Integral operators with homogeneous kernels in grand Lebesgue spaces

Sufficient conditions on the kernel and the grandizer that ensure the boundedness of integral operators with homogeneous kernels in grand Lebesgue spaces on ? ⁿ as well as an upper bound for their norms are obtained. For some classes of grandizers, necessary conditions and lower bounds for the norm of these operators are also obtained. In the case of a radial kernel, stronger estimates are established in terms of one-dimensional grand norms of spherical means of the function. A sufficient condition for the boundedness of the operator with homogeneous kernel in classical Lebesgue spaces with arbitrary radial weight is obtained. As an application, boundedness in grand spaces of the one-dimensional operator of fractional Riemann–Liouville integration and of a multidimensional Hilbert-type operator is studied.     

Singular integral operators in a weighted L2-space

Cauchy singular integral operators are characterized as operators in a weighted L²-space. The integral operator arises from a singular integral equation with variable coefficients. An appropriate weight function associated with the singular integral operator is constructed, and the set of polynomials orthogonal with respect to this weight function is defined. The action of the operator on polynomial sets is studied, and the definition of the operator is extended to a weighted L²-space. In this space, the operator is shown to be bounded, and, in some cases, isometric. Formulas are developed for the composition of the singular integral operator and its one sided inverse.     

On a Priori Estimates and Fredholm Property of Differential Operators in Anisotropic Spaces

The paper is devoted to special a priori estimates and Fredholm property of differential operators acting in anisotropic Sobolev spaces in ?ⁿ. Necessary conditions for a priori estimates in terms of the symbol of an operator are obtained. Under appropriate conditions imposed on the coefficients, a priori estimates are obtained in the corresponding weighted spaces.     

Self-adjointness of the dirac operator in a space of vector functions

This paper is devoted to the proof of the self-adjointness of the minimal operator defined on the space L₂(? ∞, ∞; H) (H being a separable Hilbert space) by the expression L=iJ(d/dt)+A+B(t). The coefficients in this expression are self-adjoint operators on H, with A being unbounded, AJ+JA = 0, and the function ∥B(t)∥ H being assumed to lie in L ₂ ^loc (? ∞, ∞). The result obtained is applicable to the Dirac operator.     

Hypoelliptic regularity in kinetic equations

We establish new regularity estimates, in terms of Sobolev spaces, of the solution f to a kinetic equation. The right-hand side can contain partial derivatives in time, space and velocity, as in classical averaging, and f is assumed to have a certain amount of regularity in velocity. The result is that f is also regular in time and space, and this is related to a commutator identity introduced by Hörmander for hypoelliptic operators. In contrast with averaging, the number of derivatives does not depend on the L^p space considered. Three type of proofs are provided: one relies on the Fourier transform, another one uses Hörmander's commutators, and the last uses a characteristics commutator. Regularity of averages in velocity are deduced. We apply our method to the linear Fokker–Planck operator and recover the known optimal regularity, by direct estimates using Hörmander's commutator.     

U-标算子的连续演算

本文给出了U-标算子经连续算子演算后有界的充分条件。另外,对一给定的U-标算子T,证明了当函数μ(t)连续可导时算子μ(T)有界,但存在连续函数使得μ(T)无界。     

A normalization problem for a class of singular integral operators with carleman shift and unbounded coefficients

The solution to a normalization problem for singular integral operators with Carleman shift and degenerate and unbounded coefficients inL _p () is obtained, where is either the unit circle or the real line. The approach followed consists mainly in two steps: the reduction to a singular integral operator with bounded coefficients and the use of the solution to an abstract normalization problem.This research was supported by JNICT under the grant PBIC/C/CEN/1040/92.     

An efficient spectral method for ordinary differential equations with rational function coefficients

We present some relations that allow the efficient approximate inversion of linear differential operators with rational function coefficients. We employ expansions in terms of a large class of orthogonal polynomial families, including all the classical orthogonal polynomials. These families obey a simple 3-term recurrence relation for differentiation, which implies that on an appropriately restricted domain the differentiation operator has a unique banded inverse. The inverse is an integration operator for the family, and it is simply the tridiagonal coefficient matrix for the recurrence. Since in these families convolution operators (i.e., matrix representations of multiplication by a function) are banded for polynomials, we are able to obtain a banded representation for linear differential operators with rational coefficients. This leads to a method of solution of initial or boundary value problems that, besides having an operation count that scales linearly with the order of truncation , is computationally well conditioned. Among the applications considered is the use of rational maps for the resolution of sharp interior layers.

     

Neumann problem with the integro-differential operator in the boundary condition

The Neumann problem for a second-order parabolic equation with integro-differential operator in the boundary condition is considered. A well-posedness theorem is proved, in particular, the integral representation of the solution is obtained, estimates for the derivatives of the solution are established, and the kernel of the inverse operator of the problem is explicitly expressed.     

An existence principle for a periodic solution of a differential equation in Banach space

The equation d²x/dt²=Ax +f(t, x) is considered in a Banach space E, where A is a fixed unbounded linear operator, andf(t, x) is a nonlinear operator which is periodic in t and satisfies a Lipschitz condition with respect to x E. Existence conditions have been obtained for a well defined generalized periodic solution of this equation, and also when this solution coincides with the true solution. Similar results have been obtained for the first order equation.Translated from Matematicheskie Zametki, Vol. 4, No. 1, pp. 105–112, July, 1968.     

On a Class of Fully Nonlinear Parabolic Equations

We consider a class of intial boudnary value problems for parabolic equaitons of the form u$sub:t$esub:=f(t,x,u,Du,au) in a bounded domain Ω where A is an elliptic operator with continous coefficients. Scuh problems can be modeled by nonlinear evolation equaitons in Banach spaces, and we use abstract parabolic equairtions technique to show existence, uniqueness, regularity of a local solution, and to give sufficient conditions for existence in the large. In particular, we don't need growth assumptions on f with respect to Au to get existence in the large. In the case where Ω is a ball, A=D and f=f(t,|x||Du|², Du) we show that the solution is radially symmetric if the initial value is     

On the asymptotic behavior of solutions of quasilinear elliptic equations

The asymptotic behavior of solutions of second-order quasilinear elliptic and nonhyperbolic partial differential equations defined on unbounded domains inR ⁿ contained in\(\{ x_1 ,...,x_n :\left| {x_n } \right|< \lambda \sqrt {x_1^2 + ...x_{n - 1}^2 } \) for certain sublinear functions λ is investigated when such solutions satisfy Dirichlet boundary conditions and the Dirichlet boundary data has appropriate asymptotic behavior at infinity. We prove Phragmèn-Lindelöf theorems for large classes of nonhyperbolic operators, without «lower order terms”, including uniformly elliptic operators and operators with well-definedgenre, using special barrier functions which are constructed by considering an operator associated to our original operator. We also estimate the rate at which a solution converges to its limiting function at infinity in terms of properties of the top order coefficienta _nn of the operator and the rate at which the boundary values converge to their limiting function; these results are proven using appropriate barrier functions which depend on the behavior of the coefficients of the operator and the rate of convergence of boundary values.     

Description of the algebraic structure of second-order differential operators that can be represented in divergence form

In the paper one proves that the second-order differential operators ?(x,u,u _x ,u _xx ), representable in divergence form, can be written as ?=c^AΔ_A, where Δ_A is the corresponding minor of order m of the Hessian \(\det (u_{xx} ) = \Delta _{\left( {\begin{array}{*{20}c} {1...n} \\ {1...n} \\ \end{array} } \right)} \) whose representation coefficients c^A=c^A(x,u,u _x ) satisfy certain algebraic relations. One introduces the concept of a second-order D-elliptic differential operator and one establishes that the totally elliptic operators of divergence form are linear with respect to the second derivatives of the function u.     

Spectral analysis of finite convolution operators with matrix kernels

Let N be a finite dimensional complex Hilbert space. A finite convolution operator on the vector function space L _N ² (0,1) is an operator T of the form (Tf) (x) = ₀ ^x k (x–t)f(t)dt, where k(t) is a norm integrableB(N)-valued function on [0,1]. A symbol for T is any function A(z) of the form A(z) = ₀ ¹ k(t)e^itzdt + e^izG(z), where G(z) is aB(N)-valued function which is bounded and analytic in a half plane y > . It is shown that under suitable restrictions two finite convolution operators are similar if their symbols are asymptotically close as z in a half plane y > .This paper was written at the University of Virginia and is based on the author's doctoral dissertation [6], which was supervised by James Rovnyak.     

Estimates for imaginary powers of Laplace operator in variable Lebesgue spaces and applications

In this paper we study some estimates of norms in variable exponent Lebesgue spaces for singular integral operators that are imaginary powers of the Laplace operator in ? ⁿ . Using the Mellin transform argument, fromthese estimates we obtain the boundedness for a family of maximal operators in variable exponent Lebesgue spaces, which are closely related to the (weak) solution of the wave equation.