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We construct a Sobolev-type space of almost periodic functions, in which we study differential and pseudodifferential operators. We show that the usual theorem on the regularity of solutions of elliptic equations is not true in these spaces, and we present modified variants of this theorem. We also study the question of the defect indices of differential operators in the space of Besicovitch almost periodic functions, and prove a Liouville-type theorem in this space. Bibliography: 16 items.

The notation of Gramian matrix is generalized to operators between function spaces and then used to provide an operator-valued inner product between pairs of Hilbert space-valued Banach function spaces. In this context projection operators, Schwarz's inequality, and orthonormal series are discussed.

Fort ∈ [a, b], letA(t) be the unbounded operator inH 0,p (G) associated with an elliptic-boundary value problem that satisfies Agmon’s conditions on the rays λ=±iτ, τ ≥0. Existence and uniqueness results are obtained for weak and strict solutions of two-point problems of the type (du/dt)−A(t) u(t) =f(t),E 1(α)u (α)=u α,E 2 (β)u (β)=u β. Here [α, β) χ- [a, b],E 1 (α) andE 2 (β) are spectral projections associated withA(α) andA(β) respectively, andA(α)E 1 (α) and =A (β)E 2 (β) are infinitesimal generators of analytic semigroups. WhenA(t) andf(t) are analytic in a convex, complex neighborhoodO of [a, b] we show that for someθ i ,i=1,2, any solution ofdu/dt =A(t)u (t)=f(t) in [a, b] is analytic and satisfies the above equation in the setO∩{t; t ≠ a, t ≠ b, | arg (t −a)

The spectral mapping theorem for joint approximate point spectrum is proved when A is an «-tuple of commuting operators on a Banach space and ƒ is any w-tuple of rational functions for which f (A) is defined. The purpose of the paper is to show how properties of parts of the joint spectrum can be obtained by use of spaces of sequences of vectors. The method (first used for general Banach spaces in [4]) provides very simple proofs of some known results ; but the main result is believed to be new. Throughout the paper, we deal with n-tuples of bounded operators on a Banach space 2£ 9 whose vectors are denoted by x, y, • • •. It will be convenient to use a symbol such as A to denote an «-tuple of operators and a symbol such as A to denote an «-tuple of complex numbers: A = (Al9 • • • , An)9 A=(Al5 • • • , AJ, 0=(0 , • • • , 0). The one-sided spectra of interest in this paper are not defined in terms of existence of Banachalgebra inverses, the context of the papers of Robin Harte ([5], [6]); they are the somewhat different spectra of the following definition. (However, in the special case that 2£ is a Hilbert space, our result is a corollary of Harte's.) DEFINITION. We say 0eov(A) in case there exists a nonzero xe& such that Ax=0 (i.e., AjX=0 for j=l, • • • , « ) . We say 0 e av(A) in case for every £>0 there exists a unit vector xeSC such that \Ax\

1. INTR~DLJcT~~N The connection between the convergence of the sequence {(I - aA*A)nj, where A is a bounded linear operator on a Hilbert space H, and the iterative computation of the pseudoinverse A+ of A has been investigated by various authors [2, 19, 20, 22 and 251. Also, the convergence of the sequence {T”}, where T is a bounded linear operator on a Banach space X, has received much attention over a period of years mostly in connection with the iterativp solution of linear equations [2, 8,9, 14, 15 and 181. The sequence {T”} can be replaced by (u,(T)), h w ere each a,, is an nth degree polynomial whose coefficients are nonnegative and sum to 1 [5, 6, 16 and 171. The convergence of {T”) has also been used as a hypothesis for theorems on series represents tion of Banachspace pseudoinverses [31, 19 and 201. The purpose of this paper is twofold: To study general properties of power convergent operators (bounded linear operators on X for which {Tn) con verges in a suitable operator topology), and to apply these results to the series representation of Banachspace pseudoinverses. In Section 2, we survey some properties of power convergent operators [15, 161, and present some new results and examples on spectral analysis of these operators. Section 3 gives some necessary and sufficient conditions for power convergence of certain classes of mostly Banach-space operators, Section 4 then presents conditions of the Stein type for power convergence of Hilbert-space operators. In Section 5 we give a definition of the pseudoinverse for bounded linear operators on Banach space with arbitrary range. This definition seems to be the most natural generalization of the classical Hilbert-space pseudoinverse for operators with arbitrary ranges [12, 19,20 and 251, and agrees with the definition given in [l] and [31] in the case that the ranges are closed. We then give series representations for the pseudoinverse and partial inverse; these results have numerous applications to the iterative solution of linear operator equa446

The concept of semi-normality and hyponormality are defined for unbounded operators in Hilbert Space and Commutators AB-BA, with A unbounded and seminormal and with B bounded, are studied. Four theorems are proved. Two of them are generalizations of results of C. R. Putman.

Representations of bounded linear operators on Banach function spaces of vector-valued functions to Banach spaces are given in terms of operator-valued measures. Then spaces whose duals are Banach function spaces are characterized. With this last information, reflexivity of this type of space is discussed. Finally, the structure of compact operators on these spaces is studied, and an observation is made on the approximation problem in this context.

Introduction. The question of ergodicity of a semigroup of bounded linear operators on a Banach space has been reduced, by Alaoglu and Birkhoff [1],' Day [2, 3], and Eberlein [4], to the study firstly of the ergodicity of the semigroup itself and secondly, of the ergodicity of each element of the Banach space with respect to this ergodic semigroup. In the case of a bounded and commutative semigroup operating on a reflexive Banach space, the ergodicity has been established [2, Corollary 6], [4, Corollary 5.1, Theorem 5.5]. For noncommutative semigroups, Alaoglu and Birkhoff have proved ergodicity by restricting the Banach space [1, Theorem 6]. A general discussion of the noncommutative case is given by Day [3 ]. The following is an attempt to prove ergodicity, in the strong topology, for a certain noncommutative case, where we use a weakened form of the commutative law. Our method uses a fixed point theorem of Kakutani [5], and a theorem about the group of cluster points of a directed subsemigroup, which we have developed elsewhere [6]. It should be observed, however, that the theorem which is proved here, though being similar to, is not a generalization of the results mentioned above. In fact it is apparently independent of them, because we postulate here a compactness in the strong topology of operators; whereas in the analogous theorems of Day and Eberlein, a kind of weak compactness is used, by considering bounded semigroups acting on reflexive spaces, a natural legacy from the study of Hilbert space. However our theoremn is a generalization of a theorem of Yosida [7, Theorem 2 ].

Some fixed point theorems for a sum of two operators are proved, generalizing to locally convex spaces a fixed point theorem of M. A. Krasnoselskii, for a sum of a completely continuous and a contraction mapping, as well as some of its recent variants. A notion of stability of solutions of nonlinear operator equations in linear topological spaces is formulated in terms of specific topologies on the set of nonlinear operators, and a theorem on the stability of fixed points of a sum of two operators is given. As a byproduct, sufficient conditions for a mapping to be open or to be onto are obtained.

A representation of functions as integrals of a kernel ψ ( t ; x ) \psi (t;x) , which was introduced by Studden, with respect to functions of bounded variation in [ 0 , ∞ ) [0,\infty ) is obtained whenever the functions satisfy some conditions involving the differential operators ( d / d t ) { f ( t ) / w i ( t ) } , i = 0 , 1 , 2 , … (d/dt)\{ f(t)/{w_i}(t)\} ,i = 0,1,2, \ldots . The results are related to the concepts of generalized completely monotonic functions and generalized absolutely monotonic functions in ( 0 , ∞ ) (0,\infty ) . Some approximation operators for the approximation of continuous functions in [ 0 , ∞ ) [0,\infty ) arise naturally and are introduced; some sequence-to-function summability methods are also introduced.

The purpose of this paper is to extend an interpolation theorem for sublinear operators on Lp spaces, obtained by Calderon and Zygmund, to similar operators acting on Hp spaces, p >0 [I]. We begin by making a few definitions. Let T be a mapping defined on a class of functions that are analytic in the interior of the unit circle I w| 0, if the domain of T includes Ha and there exists a constant M, independent of F in Ha, such that jjTFjjb

The following Theorem 1 and Theorem 2 are established. The proof utilizes the elementary properties of symmetric operators. It is shown that the symmetry condition is necessary for these theorems to hold.

Let A and B be closed operators on Banach spaces X and Y. Assume that A and B have nonempty resolvent sets and that the spectra of A and B are unbounded. Let α be a uniform cross norm on X ⊗ Y. Using the Gelfand theory and resolvent algebra techniques, a spectral mapping theorem is proven for a certain class of rational functions of A and B. The class of admissable rational functions (including polynomials) depends on the spectra of A and B. The theory is applied to the cases A ⊗ I + I ⊗ B and A ⊗ B where A and B are the generators of bounded holomorphic semigroups.

The trace of a positive bounded operator in a Hilbert space is defined and shown to be independent of the basis used in the definition. The trace of the product WA is defined for bounded A and positive bounded W with unit trace and is also shown to be basis‐invariant. An integral representation is derived and used to define Tr(WA) for unbounded A. Linearity, isotony, and continuity with respect to uniform covergence are proved for Tr(WA) as a function of (bounded) A. Several formulas used in statistical mechanics are derived, and it is proved that if A and B are commuting positive operators, then Tr(W(A + B)) = Tr(WA) + Tr(WB). A counter example is given which shows that the commonly used definition of Tr(WA) in terms of an orthonormal basis is not invariant under permutation of the basis vectors even in the case of a very simple unbounded operator A.An expectation‐value function M which associates the expectation value M(A) to the operator A is assumed given and subject to certain restrictions. For bound...

Introduction. In a recent paper [17] we obtained a spectral representation for the bounded normal operator solutions of a certain functional equation whose special cases include the semigroup equation (U,,, = U,LUV) as well as many others. In this note we continue our study of the abstract functional equation expressed in terms of algebras of measures. We shall consider therefore a mapping z -* UZ of a locally compact space Z into the set of unbounded normal operators on a Hilbert space. Since the operators are not necessarily bounded, the conditions imposed on the mapping z -* Uz vary somewhat from the conditions in the bounded case. The main result (Theorem 1) which we obtain here is a spectral representation analogous to the result valid in the bounded case [17; 18, Theorem 2, p. 8]. The methods which we use are suggested by the papers [12] and [13] of C. Ionescu Tulcea. For the general theory and terminology of unbounded operators we refer the reader to the works of E. Hille and R. Phillips [10], C. Ionescu Tulcea [11], F. Riesz and B. Sz.-Nagy [21]. Spectral representations for semigroups of unbounded operators are also discussed in A. Devinatz [3], A. Devinatz and A. E. Nussbaum [4; 5], R. Getoor [7], C. Ionescu Tulcea and A. Simon [14] and A. E. Nussbaum [19]. Theorem 1 of this paper contains the main part of Theorem 1 in [14] (and henceforth certain of the results given in [3; 4; 5; 7; 19]). The setting of the problem in algebras of measures is given in ??1, 2, and 5. Spectral families of Radon measures and direct sums thereof are summarized in ??3 and 4. The main result is stated and proved in ?6. In ?8 we apply the main theorem to derive a corollary which in the bounded case extends a result of S. Kurepa [16].

We introduce a class of linear operators, containing the positive operators, in which P. P. Korovkin's theorem on the conditions and order of convergence of positive operators hold. We consider also the class of linear operators, convergent to the derivative, in which similar theorems hold.

Recall that the spectrum, σ(T), of a linear operator T in a complex Banach space is the set of complex numbers λ such that T—λI does not have a densely defined bounded inverse. It is known [7, § 5.1] that σ(T) is a closed subset of the complex plane C. If T is not bounded, σ(T) may be empty or the whole of C. If σ(T) ≠ C and T is closed the spectral mapping theorem, is valid for complex polynomials p(z) [7, §5.7]. Also, if T is closed and λ ∉ σ(T), (T–λI)−1 is everywhere defined.

We prove an existence and uniqueness theorem for solutions of equations of Hammerstein type (1)in Banach spaces. The main difference between this study and previous ones is to be found in the assumptions that is a closed operator from one Banach space into another, and that bounds on are imposed only on certain subsets of die space in question. The proof of the basic results requires an extension of the nonlinear mappings; we do not assume continuity of these mappings. The concept of a generalized solution is introduced, and sufficient conditions are found for it to be unique, and to coincide with an exact solution.Bibliography: 11 items.

Kato [3] has introduced a class of operators called strictly singular operators. These operators have many properties in common with compact operators. In fact the concept of a strictly singular operator is an extension of the concept of a compact operator. Kato has proved that if X and X′ are Banach Spaces, then the singular operators of X into X′ forms a closed subspace of the space of bounded linear operators of X into X′ and if X = X′, then these operators forms a two-sided ideal in the ring of bounded linear operators on X. He has also shown that the Riers-Schauder theorem holds for the spectrum of a strictly singular operator. Gohberg, Feldman and Markus [23] have treated the same class of operators with an equivalent definition.

In this paper it is shown that if D D and E E are continuous linear operators on a Banach space X X , then the following are equivalent: (i) B B is a right factor of A A , (ii) B B majorizes A A and (iii) the range of B ∗ {B^\ast } contains the range of A ∗ {A^\ast } .

Recently there has been a great amount of research both qualitative and quantitative on the problem of approximating functions with certain growth conditions by means of linear positive operators. This paper discusses a quantitative problem for unbounded functions, i.e., the order of approximation to unbounded functions by means of linear positive operators.The theorems presented are, in several instances, extensions of known results from the case of bounded functions to the unbounded case. Applications of these theorems are given to spline functions, which are presently the subject of intensive investigation, and to the well-known Szasz-Mirakyan-Hille operators.

One of the most useful tools for dealing with the theory of nonlinear operators in concrete analytical problems is the Leray-Schauder fixed point theorem [5]. A disadvantage is that it is only available for the completely continuous (that is, compact and continuous) operators, and moreover the proof of the theorem uses the theory of the topological degree of a mapping. However, in a recent publication, Browder [1] has given an elementary proof without recourse to degree theory, tt was observed by the authors that this method of proof is applicable to a wider class of operators, namely those we shall call strongly P-compact. In this note we shall show that for a restricted class of Banach spaces (including, however, all separable Hilbert spaces) the class of strongly P-compact operators is quite extensive, and shall prove a fixed point theorem like that of Leray-Schauder for this class. Throughout this paper X will denote a real Banach space which has the following property: (re)l: There exists a sequence {F.} of finite dimensional subspaces of X, with F.CF,+I for each n, and U F, dense in X, and a corresponding sequence

The purpose of this paper is to study unbounded determinable subsets of real Banach spaces. The main results obtained are the following: (i) Characterization of unbounded, convex and circled subsets of a real Banach space B that are determinable. (ii) Characterizations of unbounded, convex and circled subsets of $L( B )$, the real Banach space of all continuous linear operators on B, that are determinable in principle and determinable in practice. Disproofs of a theorem and a conjecture of Varaiya are also given.

We prove that a normal operator on a separable Hilbert space can be written as a diagonal operator plus a compact operator. If, in addition, the spectrum lies in a rectifiable curve we show that the compact operator can be made Hilbert-Schmidt.

In Part I of this paper we shall be concerned with the representation as convolutions of continuous linear operators which act on various function- spaces linked with a locally compact group and which commute with left — or right — translations; cf. the results in [12]. For completeness some known results are included whenever they follow from the general procedure. We have tried to follow simple general approaches as much as possible.

This paper comprises a number of applications of the results of Part I. We use essentially the same notation as in Part I with a few additions necessary for the problems at hand.

In this paper we prove some theorems about the n‐representability problem for reduced density operators. The first theorem (Theorem 6) sharpens a theorem proved by Garrod and Percus. Let Tnp be the set of all n‐representable p‐density operators. Then a density operator Dp belongs to Tnp¯ (the bar indicates the closure with respect to a certain topology) if and only if Tr (DpBp) ≥ 0 for all bounded self‐adjoint p‐particle operators Bp, such that their n‐expansion (pn)ΓpnBp≡ ∑ i1

Let A be the closed unbounded operator inLp(G) that is associated with an elliptic boundary value problem for a bounded domainG. We prove the existence of a spectral projectionE determined by the set Γ = {λ;θ1≦argλ≦θ2} and show thatAE is the infinitesimal generator of an analytic semigroup provided that the following conditions hold: 1

In this article we present the basic facts of a general theory of linear operators in Hilbert spaces equipped with a Hermitian-bilinear metric. In particular, we investigate the spectra of operators of certain classes, most frequently encountered in applications.