Does Huygens' principle hold for small perturbations of the wave equation?

The perturbed wave equation □u + q(x)u = 0 in $\text{R}$³ × $\text{R}$ with C∞ ($\text{R}$³) compactly supported initial data at t = 0 is considered. It is proven that the Huygens' principle does not hold for this equation if the potential is (essentially) non-negative, well-behaved at infinity and small in a suitable sense. The treatment is elementary and based on energy estimates and the positivity of the Riemann function for the wave equation in three space dimensions. The result still holds if the solution u is “small” over some space-time propagation cone. In the ease in which q has compact support, stronger results of this type for the above equation are obtained.     

Green's and Dirichlet spaces associated with fine Markov processes

This is the second paper in a series devoted to Green's and Dirichlet spaces. In the first paper, we have investigated Green's space $\text{K}$ and the Dirichlet space $\text{H}$ associated with a symmetric Markov transition function p_t(x, B). Now we assume that p is a transition function of a fine Markov process X and we prove that: (a) the space $\text{H}$ can be built from functions which are right continuous along almost all paths; (b) the positive cone $\text{K}$⁺ in $\text{K}$ can be identified with a cone M of measures on the state space; (c) the positive cone $\text{H}$⁺ in $\text{H}$ can be interpreted as the cone of Green's potentials of measures μ?M. To every measurable set B in the state space E there correspond a subspace $\text{K}$(B) of $\text{K}$ and a subspace $\text{H}$(B) of $\text{H}$. The orthogonal projections of $\text{K}$ onto $\text{K}$ and of $\text{H}$ onto $\text{H}$(B) can be expressed in terms of the hitting probabilities of B by the Markov process X. As the main tool, we use additive functionals of X corresponding to measures μ?M.     

Simulation of turbulent dispersion using a simple random model of the flow field

A method is devised to simulate the movement and spreading of a patch of contaminant in two-dimensional turbulent flow. The turbulent motion is exponentially divided into components of differing wave number, adjacent components being made to have correlation times differing by a factor of two. The turbulent motion is then reconstructed by replacing each component with a sinusoidal advection field having a randomly directed wave number. Contaminant particles are advected by each of the reconstructed components, the smallest scale components being applied first. A computer simulation was performed, using a Kolmogorov k^{-$\text{5}\text{3}$} turbulent energy spectrum. Batchelor's σ ∝ t^{$\text{3}\text{2}$} law for the spreading of a contaminant patch was reproduced, approximately, as was Richardson's non-Gaussian asymptotic form of the distance-neighbour function.     

The Yang-Mills equations on the universal cosmos

Global existence and regularity of solutions for the Yang-Mills equations on the universal cosmos M?, which has the form R¹ × S³ for each of an 8-parameter continuum of factorizations of M? as time × space, are treated by general methods. The Cauchy problem in the temporal gauge is globally soluble in its abstract evolutionary form with arbitrary data for the field ⊕ potential in L_2,r(S³) ⊕ L_{2,r + 1}(S³), where r is an integer >1 and L_2,r denotes the class of sections whose first r derivatives are square-integrable; if r = 1, the problem is soluble locally in time. When r is 3 or more the solution is identifiable with a classical one; if infinite, the solution is in C^∞(M?). These results extend earlier work and approaches [1–5]. Solutions of the equations on Minkowski space-time M₀ extend canonically (modulo gauge transformations) to solutions on M? provided their Cauchy data are moderately smooth and small near spatial infinity. Precise asymptotic structures for solutions on M₀ follow, and in turn imply various decay estimates. Thus the energy in regions uniformly bounded in direction away from the light cone is $\text{O(¦x}{}_0\text{¦}{}^{\mathrm{?5}}\text{)}$, where x₀ is the Minkowski time coordinate; analysis solely in M₀ [8,9] earlier yielded the estimate $\text{O(¦x}{}_0\text{¦}{}^{\mathrm{?2}}\text{)}$ applicable to the region within the light cone. Similarly it follows that the action integral for a solution of the Yang-Mills equations in M₀ is finite, in fact absolutely convergent.     

Ein operatorwertiger Hahn-Banach Satz

We generalize Arveson's extension theorem for completely positive mappings [1] to a Hahn-Banach principle for matricial sublinear functionals with values in an injective C¹-algebra or an ideal in B($\text{H}$). We characterize injective W¹-algebras by a matricial order condition. We illustrate the matricial Hahn-Banach principle by three applications: (1) Let $\text{A, B, b}$ be unital C¹-algebras, $\text{b}$ a subalgebra of $\text{A}$ and $\text{B, B}$ injective. If ?: $\text{A}$ → $\text{B}$ is a completely bounded self-adjoint $\text{b}$-bihomomorphism, then it can be expressed as the difference of two completely positive $\text{b}$-bihomomorphism. (2) Let $\text{M}$ be a W¹-algebra, containing 1_$\text{H}$, on a Hilbert space $\text{H}$. If $\text{M}$ is finite and hyperfinite, there exists an invariant expectation mapping P of B($\text{H}$) onto $\text{M}$′. P is an extension of the center trace. (3) Combes [7] proved, that a lower semicontinuous scalar weight on a C¹-algebra is the upper envelope of bounded positive functionals. We generalize this result to unbounded completely positive mappings with values in an injective W¹-algebra.     

Roth's theorems and decomposition of modules

It is shown that there is a connection between Roth's theorems on similarity and equivalence of block-triangular matrices and decomposition of modules. The module property is that if $\text{M?}\text{N⊕M}\text{N}$, then N is a summand of M. This holds for any commutative ring if M is finitely presented. New proofs of Roth's theorems are given for commutative rings. Some results are established in the noncommutative case.     

The complex Frobenius Theorem and uniqueness of solutions to the tangential Cauchy-Riemann equations

Suppose M is a C^∞ real k-dimensional CR-submanifold of $\text{C}$ⁿ, n > 1, and suppose that $\text{?}\text{?}\text{t6}{}_{\mathrm{M}}$ is the tangential Cauchy-Riemann operator on M. Let S be a C¹ real (k ? 1)-dimensional submanifold of M which is noncharacteristic for ??t6_M at p?S. Conditions are found so that a C^∞ solution f of $\text{?}\text{?}\text{t6}{}_{\mathrm{M}}\text{f = 0}$ which vanishes on one side of S in M must vanish in a neighborhood of p in M. If M is a real hypersurface, it is known that such unique continuation always exists. If the codimension of M in $\text{C}$ⁿ is greater than 1, and if the excess dimension of the Levi algebra on M is constant, then it is proved that CR-functions on M which vanish on one side of S must vanish in a full neighborhood of p. The assumption on the dimension of the Levi algebra allows us to use the Complex Frobenius Theorem. Other methods to prove such unique continuation results are also developed.     

An index product formula for the study of elliptic resonance problems

Let $\text{(P)}\text{u}\text{?}\text{+ Au = f(u)}$ be a semilinear parabolic equation. If f(0) = 0 and f is of class C¹ in a neighborhood of 0, then there exists a local center manifold M near zero containing all small invariant sets of (P). The purpose of this paper is to prove an index product formula relating the homotopy index h(K) of a small isolated invariant set K relative to (P) to the homotopy index h_M(K) of the same set with respect to the equation induced by (P) on the center manifold M. This formula can be applied to elliptic BVP with resonance at zero. In particular, earlier results of Amann and Zehnder (Ann. Scuola Norm. Sup. Pisa IV7 (1980), 534–603) can be obtained under less restrictive assumptions than those used in that paper. Further-more, the formula permits applications to cases not discussed in Amann and Zehnder's paper. The applications of the index product formula are given in K. P. Rybakowski (Nontrivial solutions of elliptic boundary value problems with resonance at zero, Ann. Mat. Pura Appl., to appear).     

Stability of stratified shear flows

For a steady plane parallel flow of an inviscid, incompressible fluid of variable density under gravity, it is shown that the complex wave velocity for any unstable mode lies in a semiellipse-type region whose major axis coincides with the diameter of Howard's semicircle, while its minor axis depends on the stratification. If kc_i denotes the complex part of wave frequency and J₀ the minimum of the local Richardson number over the flow domain, it is further established that kc_i → 0+ as $\text{J}{}_0\text{→}\text{1}\text{4}\text{?}$. The case of free upper surface and conditional reduction dependent on the curvature of the basic velocity of the unstable region is also studied.     

nth order Stieltjes differential boundary operators and Stieltjes differential boundary systems

This article discusses linear differential boundary systems, which include nth-order differential boundary relations as a special case, in $\text{L}$_n^p[0,1] × $\text{L}$_n^p[0,1], 1 ? p < ∞. The adjoint relation in $\text{L}$_n^q[0,1] × $\text{L}$_n^q[0,1], $\text{1}\text{p}\text{+}\text{1}\text{q}\text{= 1}$, is derived. Green's formula is also found. Self-adjoint relations are found in $\text{L}$_n²[0,1] × $\text{L}$_n²[0,1], and their connection with Coddington's extensions of symmetric operators on subspaces of $\text{L}$_n^p[0,1] × $\text{L}$_n²[0,1] is established.     

Regularizing effects for ut + Aϑ(u) = 0 in L1

Various initial-boundary value problems and Cauchy problems can be written in the form $\text{du}\text{dt}\text{+ A?(u) = 0}$, where ?:R→$\text{R}$ is nondecreasing and A is the linear generator of strongly continuous nonexpansive semigroup e^?tA in an L¹ space. For example, if A = ?Δ (subject, perhaps, to suitable boundary conditions) we obtain equations arising in flow in a porous medium or plasma physics (depending on the choice of ?) while if $\text{A =}\text{?}\text{?x}$ acting in L¹($\text{R}$) we have a scalar conservation law. In this paper we show that if M, m > 0 and m?′² ? ν??′' ? M?′², where ν ? {1,?1}, then (roughly speaking), the norm of $\text{t}\text{du}\text{dt}$ may be estimated in terms of the initial data u₀ in L¹. Such estimates give information about the regularity of solutions, asymptotic behaviour, etc., in applications. Side issues, such as the introduction of sufficiently regular approximate problems on which estimates can be made and the assignment of a precise meaning to the operator A?, are also dealt with. These considerations are of independent interest.     

Almost arcwise connected continua without dense arc components

A continuum M is almost arcwise connected if each pair of nonempty open subsets of M can be joined by an arc in M. An almost arcwise connected plane continuum without a dense arc component can be defined by identifying pairs of endpoints of three copies of the Knaster indecomposable continuum that has two endpoints. In [7] K.R. Kellum gave this example and asked if every almost arcwise connected continuum without a dense arc component has uncountably many arc components. We answer Kellum's question by defining an almost arcwise connected plane continuum with only three arc components none of which are dense. A continuum M is almost Peano if for each finite collection $\text{C}$ of nonempty open subsets of M there is a Peano continuum in M that intersects each element of $\text{C}$. We define a hereditarily unicoherent almost Peano plane continuum that does not have a dense arc component. We prove that every almost arcwise connected planar λ-dendroid has exactly one dense arc component. It follows that every hereditarily unicoherent almost arcwise connected plane continuum without a dense arc component has uncountably many arc components. Using an example of J. Krasinkiewicz and P Minc [8], we define an almost Peano λ-dendroid that do not have a dense arc component. Using a theorem of J.B. Fugate and L. Mohler [3], we prove that every almost arcwise connected λ-dendroid without a dense arc component has uncountably many arc components. In Euclidean 3-space we define an almost Peano continuum with only countably many arc components no one of which is dense. It is not known if the plane contains a continuum with these properties.     

Geometric conditions for the existence of positive eigenvalues of matrices

Finite-dimensional theorems of Perron-Frobenius type are proved. For A∈$\text{C}$ⁿⁿ and a nonnegative integer k, we let w_k (A) be the cone generated by A^k, A^k+1,…in $\text{C}$ⁿⁿ. We show that A satisfies the Perron-Schaefer condition if and only if the closure $\text{W}$_k(A) of w_k(A) is a pointed cone. This theorem is closely related to several known results. If k?v₀(A), the index of the eigenvalue 0 in spec A, we prove that A has a positive eigenvalue if and only if w_k(A) is a pointed nonzero cone or, equivalently $\text{W}$_k(A) is not a real subspace of $\text{C}$ⁿⁿ. Our proofs are elementary and based on a method of Birkhoff's. We discuss the relation of this method to Pringsheim's theorem.     

Great intersecting families of edges in hereditary hypergraphs

Chvátal stated in 1972 the following conjecture: If $\text{H}$ is a hereditary hypergraph on S and $\text{M}$ ? $\text{H}$ is a family of maximum cardinality of pairwise intersecting members of $\text{H}$, then there exists an x ∈ S such that d_$\text{H}$(x) = |{H ∈ $\text{H}$: x ∈ H}| = |$\text{M}$|. Berge and Schönheim proved that |$\text{M}$|?$\text{1}\text{2}$|$\text{H}$| for every $\text{H}$ and $\text{M}$. Now we prove that if there exists an $\text{M}$ ? $\text{H}$, |$\text{M}$| = $\text{1}\text{2}$|$\text{H}$| then Chvátal's conjecture is true for this $\text{H}$.     

3-part splittings for singular and rectangular linear systems

We explore iterative schemes for obtaining a solution to the linear system $\text{(}{}^1\text{) Ax = b, A ? C}{}^{\mathrm{m\; \times \; n}}$, if the system is solvable, or for obtaining an approximate solution to (¹) if the system is not solvable. Our iterative schemes are obtained via a 3-part splitting of A into A = M ? Q₁ ? Q₂. The 3-part splitting of A is, in turn, a refinement of a (2-part) subproper splitting of A into A = M ? Q. We indicate the possible usefulness of such refinements (of a 2-part splitting of A) to systems (¹) which arise from a discrete analog to the Neumann problem, where the conventional iterative schemes (i.e., iterative schemes induced by a 2-part splitting of A) are not necessarily convergent.     

Left mean-ergodicity,fixed points,and invariant means

Recently Lau [15] generalized a result of Yeadon [25]. In the present paper we generalize Yeadon's result in another direction recasting it as a theorem of ergodic type. We call the notion of ergodicity required left mean-ergodicity and show how it relates to the mean-ergodicity of Nagel [21]. Connections with the existence of invariant means on spaces of continuous functions on semitopological semigroups S are made, connections concerning, among other things, a fixed point theorem of Mitchell [20] and Schwartz's property P of W¹-algebras [22]. For example, if $\text{M}$(S) is a certain subspace of C(S) (which was considered by Mitchell and is of almost periodic type, i.e., the right translates of a member of $\text{M}$(S) satisfy a compactness condition), then the assumption that $\text{M}$(S) has a left invariant mean is equivalent to the assumption that every representation of S of a certain kind by operators on a linear topological space X is left mean-ergodic. An analog involving the existence of a (left and right) invariant mean on $\text{M}$(S) is given, and we show our methods restrict in the Banach space setting to give short direct proofs of some results in [4], results involving the existence of an invariant mean on the weakly almost periodic functions on S or on the almost periodic functions on S. An ergodic theorem of Lloyd [16] is generalized, and a number of examples are presented.     

Grothendieck's theorem for noncommutative C1-algebras,with an Appendix on Grothendieck's constants

We study a conjecture of Grothendieck on bilinear forms on a C¹-algebra $\text{Ol}$. We prove that every “approximable” operator from $\text{Ol}$ into $\text{Ol}$¹ factors through a Hilbert space, and we describe the factorization. In the commutative case, this is known as Grothendieck's theorem. These results enable us to prove a conjecture of Ringrose on operators on a C¹-algebra. In the Appendix, we present a new proof of Grothendieck's inequality which gives an improved upper bound for the so-called Grothendieck constant k_G.