Geometric properties of some special cones in a Banach space

Let E be a Banach space partially ordered by a cone K. Let B be a closed linear operator in E with domain (B). In this paper certain cones in the Banach space (B) with norm x =x+Bx are singled out for study; a number of their geometric properties are established under the assumption that the cone K in the space E has analogous properties.Translated from Matematicheskie Zametki, Vol. 5, No. 1, pp. 63–70, 1969.The author is indebted to M. A. Krasnosel'skii and V. Ya. Stetsenko for helpful discussions and for their interest in the results of the present paper.     

Problem of instability in the first approximation

Let E be a Banach space, A be a continuous linear operator such that (A) ; Re>0 Ø, and F(t, x) be a continuous function on [0, )×E satisfying the condition F(t, x) q x (q= const). An example of a system dx/dt=Ax + F(t, x) is given which has an exponentially stable zero solution for certain F(t, x) with arbitrarily small q.Translated from Matematicheskie Zametki, Vol. 23, No. 5, pp. 721–723, May, 1978.     

The CMP and Banach Spaces with unique metric lines

This paper is concerned with characterizing the class of Banach Spaces with unique metric lines in the class of complete metric spaces with unique lines.The concept utilized to prove the major theorem is theConsistent Midpoint Property (CMP). We define a binary operation (+) in metric spaces with unique lines and show that, under suitable assumptions, the space is a Banach Space with + as the vector addition and a=d(a, ) for some fixed .     

Formeln für Pole der Resolvente und den Radius eines wesentlichen Spektrums

Let E be a complex Banach space and T a bounded linear operator. If _w(T) is the set of all spectral points of T which are not poles of the resolvent and r_w(T) its radius, we give formulas for the moduli of all poles lying outside the circle with radius r_w(T). We further prove that , where (T)=inf(T–FF(T–F)=(T–F)F=0,FF and F is the Caradus class of operators with rational resolvent. Finally we give an estimation of r_w(S+T) and r_w(S·T).     

Über die Existenz gemeinsamer Elemente bester Approximation bezüglich zweier Normen II

The present paper deals with the possibility of existence of best approximation elements, simultaneously with respect to two norms ·_i,i=1,2, for all the elements of a class of subspaces. In case this class in any of the following: (a) All n-dimensional subspaces, (b) All ·₁-or ·||₂-closed, n-codimensional subspaces, (c) All ·₁-or ·₂-closed subspaces with infinite dimension and codimension, we prove that the two norms differ at most by a constant factor.     

Jordan axioms for C*-algebras

A complex Banach spaceA which is also an associative algebra provided with a conjugate linear vector space involution * satisfying (a ²)^*=(a ^*)², aa ^* a=a³ and ab+ba2ab for alla, b inA is shown to be a C^*-algebra. The assumptions onA can be expressed in terms of the Jordan algebra obtained by symmetrization of the product ofA and are satisfied by any C^*-algebra. Thus we obtain a purely Jordan characterization of C^*-algebras.     

Absorption spectroscopy and excitation of YAlO3∶Er (50%) crystals

Absorption spectra have been measured in highly doped YAlO₃Er (50 %) in the wavelength range from 300 nm up to 2m. The spectra were taken with irradiation to the crystallographic b-axis with polarization a- or c-axis. From the measured spectra the Er energy levels have been determied. Based on the results of absorption spectroscopy the pumplight absorption in a Xenon-flashlamp-pumped YAlO₃Er laser slab has been calculated as a function of wavelength and depth.

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Zusammenfassung In hochdotiertem YAlO₃Er (50%) wurden Absorptionsspektren im Wellenlängenbereich von 300 nm bis 2m gemessen. Die Spektren wurden mit Bestrahlung zu der kristallographischen b-Achse mit Polarisation zur a- oder c-Achse aufgenommen. Aus den gemessenen Spektren wurden die Energieniveaus bestimmt. Mit den Ergebnissen aus der Absorptionsspektroskopie wurde die Pumplichtabsorption in einer mit Xenonblitzlampen gepumpten YAlO₃Er Platte in Abhängigkeit von der Wellenlänge berechnet.

     

Joint norm of operators and an investigation of nonlinear problems

Nonlinear operator equations of the form x=Fx in a real-valued Hilbert space H are studied. If the operator F is completely continuous and admits the bound Fx< Bx+b, where B is a continuous linear operator then for B<1 the Schauder principle is applicable to the equation x=Fx and this equation possesses at least one solution x H. If the bound Fx<,B₁x+B₂x+b is valid where B₁ and B₂ are bounded linear operators then the simplest conditions for solvability of the equation x=Fx is of the form B₁+B₂<1. This condition could be relaxed. The proposed method is applied to the investigation of a two-point boundary problem (cf., e.g., [1–3]). New conditions for the existence of solutions are obtained.Translated from Ukrainskii Matematicheskii Zhurnal, Vol. 42, No. 12, pp. 1605–1616, December, 1990.     

Sufficient conditions for bang-bang control in Hilbert space

Sufficient conditions for bang-bang and singular optimal control are established in the case of linear operator equations with cost functionals which are the sum of linear and quadratic terms, that is,Ax=u,J(u)=(r,x)+(x,x), >0. For example, ifA is a bounded operator with a bounded inverse from a Hilbert spaceH into itself and the control setU is the unit ball inH, then an optimal control is bang-bang (has norm l) if 0<1/2;A ^–1*r·A ^–1–2, but is singular (an interior point ofU) if >1/2A ^–1*r·A².This work was supported by NRC Grant No. A-4047 and NSF Grant No. GP-7445.     

Affine geometry over free unitary modules

In this paper we consider systems (P,L,,), where P is an arbitrary non-empty set, L a set of subsets of p and resp. a relation on PxP resp. LxL. In successive stages adding geometrical axioms, we characterize the class of those structures (P,L, ,), which coincides — up to isomorphisms — with the class of all Affine Barbilian-Spaces.     

Approximating derivations on ideals ofC *-algebras

For each^*-derivation of a separableC ^*-algebraA and each >0 there is an essential idealI ofA and a self-adjoint multiplierx ofI such that (–ad(ix))|I< and x.     

Blow-up of solutions of nonlinear wave equations in three space dimensions

Let u(x,t) be a solution, uA|u|^p for xIR₃, t0 where is the d'Alembertian, and A, p are constants with A>0, 10–|x–x⁰|, if the initial data u(x,0), u_t(x,0) have their support in the ball |x–x⁰|t₀. In particular global solutions of u=A|u|^p with initial data of compact support vanish identically. On the other hand for A>0, p>1+2 global solutions of u=A|u|^p exist, if the initial data are of compact support and u is sufficiently small in a suitable norm. For p=2 the time at which u becomes infinite is of order u^–2.Dedicated to Hans Lewy and Charles B. Morrey, Jr.The research for this paper was performed at the Courant Institute and supported by the Office of Naval Research under Contract No. N00014-76-C-0301. Reproduction in whole or part is permitted for any purpose of the United States Government.     

On summability of weighted Lagrange interpolation. I

The paper is devoted to the study of summability of weighted Lagrange interpolation on the roots of orthogonal polynomials with respect to a weight function w. Starting from the Lagrange interpolation polynomials we shall construct a wide class of discrete processes (using summations) which are uniformly convergent in a suitable Banach space (C,·) of continuous functions (w denotes a weight). We shall give such conditions with respect to w, , (C,·) and to summation methods for which the uniform convergence holds. Error estimates for the approximation will also be considered.     

Khinchin's inequality and the asymptotic behavior of the sums Σɛnxn in Banach lattices

A series of generalizations of the classical Khinchin inequality to Banach lattices are given. The asymptotic behavior of ₁ ⁿ _ix_i is investigated.Translated from Ukrainskii Matematicheskii Zhurnal, Vol. 42, No. 5, pp. 639–644, May, 1990.     

Simultaneous approximation of algebraic irrationalities

This paper proves three theorems concerning the simultaneous approximation of numbers from a totally real algebraic number field. It is shown that for two given numbers ₁ and ₂ from a totally real algebraic number field, the constant ₁₂ can be explicitly calculated, this being the upper limit of the numbers c₁₂ such that the inequality max (q₁, q₂)(qc₁₂)^–1/2 holds for infinitely many natural numbers q; likewise for the constant a₁₂ such that the inequality q₁·q₂< a₁₂(q^logq) holds for infinitely many natural numbers q. It is shown that there exist n –1 numbers ₁, ..., _n–1 in an algebraic number field of degree n and discriminant d such that the inequality holds only for finitely many natural numbers q if 2^{ - \left[ {\tfrac{{n - 1}}{2}} \right]} \sqrt d $$ " align="middle" border="0"> . is fixed.Translated from Zapiski Nauchnykh Seminarov Leningradskogo Otdeleniya Matematicheskogo Instituta im. V. A. Steklova AN SSSR, Vol. 116, pp. 142–154, 1982.     

Isolation theorem for forms corresponding to purely real algebraic fields

Let M be the complete module of a purely real algebraic field of degree n 3, let be a lattice in this module, and let F(X) be its form. We use to denote any lattice for which we have = , where is a nondiagonal matrix for which – I . With each lattice we can associate a factorizable formF (X) in a natural manner. We denote the complete set of forms corresponding to the set {} by {F (X)}. It is proved that for any > 0 there exists an > 0 such that for eachF (X) {F } we have |F (X₀₎| for some integer vector X₀ 0.Translated from Zapiski Nauchnykh Seminarov Leningradskogo Otdeleniya Matematicheskogo Instituta im. V. A. Steklova Akademii Nauk SSSR, Vol. 185, pp. 5–12, 1990.In conclusion, the author would like to express his deep gratitude to B. F. Skubenko for stating the problem and for his constant attention.     

Nonnegative Hall Polynomials

The number of subgroups of type and cotype in a finite abelian p-group of type is a polynomialg with integral coefficients. We prove g has nonnegative coefficients for all partitions and if and only if no two parts of differ by more than one. Necessity follows from a few simple facts about Hall-Littlewood symmetric functions; sufficiency relies on properties of certain order-preserving surjections that associate to each subgroup a vector dominated componentwise by . The nonzero components of (H) are the parts of , the type of H; if no two parts of differ by more than one, the nonzero components of – (H) are the parts of , the cotype of H. In fact, we provide an order-theoretic characterization of those isomorphism types of finite abelian p-groups all of whose Hall polynomials have nonnegative coefficients.     

L p -estimates for the solutions of second order elliptic equations

Summary We investigate the homogeneous Dirichlet problem in H^2,p for a second order elliptic partial differential equation in nondivergence form Lu=f in the case in which the leading coefficients of L belong to H^1,n(), Rⁿ. We prove that if p belongs to a suitable neighbourhood of 2, then the above problem, has a unique solution u satisfying D²u_p Cf_p; furthermore, if f H^k,p, k=1,2, ..., and the coefficients of L satisfy some natural conditions, then the solution satisfies .Lavoro eseguito nell'ambito del gruppi 40% e 60% del M.P.I.     

On the type of spectral operators and the non-spectrality of several differential operators on Lp

A spectral operator, not necessarily bounded, which is the infinitesimal generator of a strongly continuous group of operators {U(t)|t} where U(t) = (|t|^K), is of type k N_o, i.e. T = S + N, where S is spectral of scalar type, N is bounded, N^k+1 = O and S and N are commuting. This result yields a simple proof of the non-spectrality of certain differential operators on L^p, p 2, which are known to be selfadjoint for p = 2.     

Best approximations of continuous functions by spline functions

An investigation of the approximation on [0, 1] of functionsf (x) by spline functions s(f,; x) of degree 2r-1 and of deficiency r (r>1) depending on the vector function = ₁ (x),..., _r-1(x) and interpolatingf (x) at fixed points. For the optimal choice of the vector ₀, exact estimates are obtained of the norms f(x)-s (f, ₀; x)_C[0,1] and f (x)-s (f, ₀; x)_{L[0, 1]} on the function classes H Translated from Matematicheskie Zametki, Vol. 8, No. 1, pp. 41–46, July, 1970.In conclusion we would like to thank N. P. Korneichuk for suggesting this problem and for his valuable advice.