Equivalence of some coercive estimates for linear partial differential operators

The equivalence of certain coercive estimates for linear partial differential operators on ℝⁿ in a Banach space X is studied. Such estimates play an important role in the theory of differential equations and operators. __________ Translated from Sovremennaya Matematika i Ee Prilozheniya (Contemporary Mathematics and Its Applications), Vol. 38, Suzdal Conference-2004, Part 3, 2006.     

On some partial differential inequalities with gradient terms

Using a modification of the nonlinear capacity method, we obtain necessary conditions for the solvability of some nonlinear partial differential equations and inequalities containing the polyharmonic operator and terms that depend on the norm of the gradient of the solution, both in the entire space and in bounded domains; in the latter case the coefficients of the inequality are allowed to have singularities.     

On spectra of some linear elliptic differential operators

In the study of boundary-value problems, we often need information on the spectra of the corresponding differential operators. In this paper, we discuss the location of spectra of linear elliptic differential operators in ℝⁿ, n ∈ ℕ.Translated from Sovremennaya Matematika i Ee Prilozheniya (Contemporary Mathematics and Its Applications), Vol. 9, Suzdal Conference-3, 2003.     

On trace identities and universal eigenvalue estimates for some partial differential operators

In this article, we prove and exploit a trace identity for the spectra of Schrödinger operators and similar operators. This identity leads to universal bounds on the spectra, which apply to low-lying eigenvalues, eigenvalue asymptotics, and to partition functions (traces of heat operators). In many cases they are sharp in the sense that there are specific examples for which the inequalities are saturated. Special cases corresponding to known inequalities include those of Hile and Protter and of Yang.

     

Differential operators for systems of linear partial differential equations

The aim of this paper is the representation of solutions of systems of formally hyperbolic differential equations of second order. I. N.Vekua gave a representation of the solutions using the Riemann-matrix-function. Here we introduce special differential operators which map holomorphic functions into the set of solutions. An existence theorem for such operators is proved which gives a necessary and sufficient condition on the coefficients of a system. These operators are represented explicitly and several properties of them are investigated. We give different representations of the solutions of such systems and discuss the relation between the integral operator method and the method using differential operators which leads to an explicit representation of the Riemann-matrix-function by means of the differential operators. Two examples of special systems with differential operators are given.     

Opial-type inequalities for differential operators

Some continuous and discrete versions of Opial-type inequalities which are readily applicable to differential and difference operators are established. These generalize earlier results of Anastassiou and Pe?ari?, and of Koliha and Pe?ari?.     

Factoring linear partial differential operators and the Darboux method for integrating nonlinear partial differential equations

Using a new definition of the generalized factorization of linear partial differential operators, we discuss possible generalizations of the Darboux integrability of nonlinear partial differential equations. Translated from Teoreticheskaya i Matematicheskaya Fizika, Vol. 122, No. 1, pp. 144–160, January, 1999.     

A hierarchy of generalized invariants for linear partial differential operators

We study invariants of linear partial differential operators in two variables under gauge transformations. Using the Beals-Kartashova factorization, we construct a hierarchy of generalized invariants for operators of an arbitrary order. We study the properties of these invariants and give some examples. We also show that the classic Laplace invariants correspond to some particular cases of generalized invariants. __________ Translated from Teoreticheskaya i Matematicheskaya Fizika, Vol. 147, No. 3, pp. 470–478, June, 2006.     

Global solvability for first order real linear partial differential operators

F. Treves, in [17], using a notion of convexity of sets with respect to operators due to B. Malgrange and a theorem of C. Harvey, characterized globally solvable linear partial differential operators on C^∞(X), for an open subset X of Rn.Let P=L+c be a linear partial differential operator with real coefficients on a C^∞ manifold X, where L is a vector field and c is a function. If L has no critical points, J. Duistermaat and L. Hörmander, in [2], proved five equivalent conditions for global solvability of P on C^∞(X).Based on Harvey-Treves's result we prove sufficient conditions for the global solvability of P on C^∞(X), in the spirit of geometrical Duistermaat-Hörmander's characterizations, when L is zero at precisely one point. For this case, additional non-resonance type conditions on the value of c at the equilibrium point are necessary.     

On analytic microlocal hypoellipticity of linear partial differential operators of principal type

It is shown that if P is a linear partial differential uperator with analytic coefficients defined near a point xo in Rⁿ and if P in Rⁿ - 0 is such that: the principal symbol p_m,(x, ξ) vanishes at (x₀. ξ⁰). the differential of p_m, with respect to ξ is different from zero at (x₀, ξ⁰). the Poisson bracket {P_m, P_m} is zero at (x₀. ξ⁰) and the Poisson bracket {p_m, {p_m.p_m }} is different from zero at (x₀, ξ⁰), then P is analytic hypoelliptic at (x₀, ξ⁰). It is also proved that P is analytic hypoelliptic under the assumption that the first non-vanishing repeated Poisson bracket of p_m, and p_m, is of odd length and under some additional hypothesis on the commutators of the Hamilton fields of Re p_m, and Im p_m,     

On some fundamental partial integral inequalities

This paper presents several new partial integral inequalities in two independent variables which can be used in the analysis of various problems in the theory of partial differential and integral equations as ready and powerful tools. An elementary technique of reducing the integral inequality to second order partial differential inequality and then integrating it by Riemann's method is used to establish our results.     

Global solvability for first order real linear partial differential operators II

Let L be a real $C^{\infty }$ vector field on a smooth manifold X, vanishing at exactly one point $x_0$. From the pioneering work of B. Malgrange (1955–1956) [6], we know that solvability of $P=L+c$ on $C^{\infty }(X)$, for $c\in C^{\infty }(X,\mathbb{C})$, implies that: (a) X is L-convex. Also, it follows: (b) a non-resonance condition for the jet-solvability at $x_0$.In a previous paper, in addition to (a) and (b), the authors showed that P is globally solvable on $C^{\infty }$ if we assume: (c) a non-resonance condition in order to linearize L near $x_0$; that (d) the only relatively compact orbit of L is $\{x_0\}$; and that (e) c is real.Here we obtain the same conclusion without (c) and (e).     

On the critical index of Gevrey solvability for some linear partial differential equations

We propose precise estimates of the critical index of Gevrey solvability of some classes of linear partial differential equations with multiple characteristics.

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Sunto Si propongono stime precise dell’indice critico della risolubilità di Gevrey di alcune classi di equazioni lineari differenziali con caratteristiche multiple.

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To the memory of L. Cattabriga

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Research partially supported by a Coordinated Research Project of the University of Cagliari and by INDAM-GNAFA, Italy.     

A continuum of unusual self-adjoint linear partial differential operators

In an earlier publication a linear operator _THar$T_{\mathrm{Har}}$ was defined as an unusual self-adjoint extension generated by each linear elliptic partial differential expression, satisfying suitable conditions on a bounded region Ω$\Omega$ of some Euclidean space. In this present work the authors define an extensive class of _THar$T_{\mathrm{Har}}$-like self-adjoint operators on the Hilbert function space _L2(Ω);$L_2(\Omega );$ but here for brevity we restrict the development to the classical Laplacian differential expression, with Ω$\Omega$ now the planar unit disk. It is demonstrated that there exists a non-denumerable set of such _THar$T_{\mathrm{Har}}$-like operators (each a self-adjoint extension generated by the Laplacian), each of which has a domain in _L2(Ω)$L_2(\Omega )$ that does not lie within the usual Sobolev Hilbert function space W²(Ω)$W^2(\Omega )$. These _THar$T_{\mathrm{Har}}$-like operators cannot be specified by conventional differential boundary conditions on the boundary of ∂Ω$\mathrm{\partial }\Omega$, and may have non-empty essential spectra.