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  • Mathematics and Statistics  (93)
  • Wiley-Blackwell  (93)
  • 1990-1994  (93)
  • 1992  (93)
  • 1
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    Chichester, West Sussex : Wiley-Blackwell
    Mathematical Methods in the Applied Sciences 15 (1992), S. 315-320 
    ISSN: 0170-4214
    Keywords: Mathematics and Statistics ; Applied Mathematics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics
    Notes: We prove the radial symmetry of the solution of degenerate quasilinear capacity problems, when constant overdetermined Neumann data are assigned.
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  • 2
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    Mathematical Methods in the Applied Sciences 15 (1992), S. 321-330 
    ISSN: 0170-4214
    Keywords: Mathematics and Statistics ; Applied Mathematics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics
    Notes: We describe a mathematical method for computing the attenuation distribution from Positron emission tomography (PET) data. The method renders possible the correction of PET data for attenuation without a transmission scan. It can be applied if the attenuation distribution is sufficiently simple (abdomen and bead). We give the results for computer-generated data and for data on patients.
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  • 3
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    Mathematical Methods in the Applied Sciences 15 (1992), S. 593-597 
    ISSN: 0170-4214
    Keywords: Mathematics and Statistics ; Applied Mathematics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics
    Notes: A useful relationship between solutions is derived for a class of matrix equations arising in the system theory, particularly in the context of the orthogonal approximation of linear dynamic systems. The computational aspects of the result are discussed and illustrated on an example of practical importance.
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  • 4
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    Chichester, West Sussex : Wiley-Blackwell
    Mathematical Methods in the Applied Sciences 15 (1992) 
    ISSN: 0170-4214
    Keywords: Mathematics and Statistics ; Applied Mathematics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics
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  • 5
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    Mathematical Methods in the Applied Sciences 15 (1992), S. 661-675 
    ISSN: 0170-4214
    Keywords: Mathematics and Statistics ; Applied Mathematics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics
    Notes: This paper deals with some mathematical aspects of magnetic resonance imaging (MRI). MRI is used in diagnostic medicine to measure and display the cross section of, for example, a human organ. In this paper we formulate the MRI-reconstruction problem and solve it aftèr restating it as a moment problem in a Hilbert space.
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  • 6
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    Mathematical Methods in the Applied Sciences 15 (1992), S. 223-237 
    ISSN: 0170-4214
    Keywords: Mathematics and Statistics ; Applied Mathematics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics
    Notes: We prove the existence of global solutions for small data to the initial value problem for the non-linear hyperbolic system of partial differential equations describing a thermoelastic medium in a three-dimensional space under the assumption that the coefficients in the non-linear terms are smooth functions of their arguments and behave like 0(∣η∣k0) for k0 ≥ 2 near the origin. The asymptotic behaviour of the solution as t → ∞ is also described.
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  • 7
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    Mathematical Methods in the Applied Sciences 15 (1992) 
    ISSN: 0170-4214
    Keywords: Mathematics and Statistics ; Applied Mathematics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics
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  • 8
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    Mathematical Methods in the Applied Sciences 15 (1992), S. 469-477 
    ISSN: 0170-4214
    Keywords: Mathematics and Statistics ; Applied Mathematics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics
    Notes: We prove the existence of weak global solutions to the degenerate diffusion equation (1) with singular absorption term. Moreover we investigate the regularity up to the quenching time and we show by means of explicit solutions that our regularity results are optimal.
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  • 9
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    Mathematical Methods in the Applied Sciences 15 (1992), S. 599-620 
    ISSN: 0170-4214
    Keywords: Mathematics and Statistics ; Applied Mathematics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics
    Notes: The local existence, uniqueness and regularity of solutions of the initial-value problem for non-stationary Navier-Stokes equations are studied via abstract Besov spaces. The assumptions about Ω the initial data and the external force are improved on by rather simple proofs.
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  • 10
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    Mathematical Methods in the Applied Sciences 15 (1992), S. 647-660 
    ISSN: 0170-4214
    Keywords: Mathematics and Statistics ; Applied Mathematics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics
    Notes: We formulate certain problems in the optimal design of radiating structures such as multicriteria optimization problems. We review the basic background of such problems, prove the existence of Pareto optimal points, and give necessary conditions. We apply the latter to the numerical computation of optimal surface currents for the problem of simultaneously optimizing both the quality factor and the signal-to-noise ratio of a conformal antenna.
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  • 11
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    Mathematical Methods in the Applied Sciences 15 (1992), S. 159-166 
    ISSN: 0170-4214
    Keywords: Mathematics and Statistics ; Applied Mathematics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics
    Notes: The Radon transform R(p, θ), θ∊Sn-1, p∊∝1, of a compactly supported function f(x) with support in a ball Ba of radius a centred at the origin is given for all \documentclass{article}\pagestyle{empty}\begin{document}$ \theta \in \mathop {S^{n - 1} }\limits^\tilde $\end{document}, where \documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {S^{n - 1} }\limits^\tilde $\end{document} is an open set on Sn-1, and all p∊(- ∞, ∞), n≥2. An approximate formula is given to calculate f(x) from the given data.
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  • 12
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    Mathematical Methods in the Applied Sciences 15 (1992) 
    ISSN: 0170-4214
    Keywords: Mathematics and Statistics ; Applied Mathematics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics
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  • 13
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    Mathematical Methods in the Applied Sciences 15 (1992), S. 239-264 
    ISSN: 0170-4214
    Keywords: Mathematics and Statistics ; Applied Mathematics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics
    Notes: This paper introduces some methods (including an approximation method) for investigating pseudodifferential equations and related problems (Cauchy problems, boundary value problems,…) based on the technique of pseudodifferential operators with real analytic symbols.
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  • 14
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    Mathematical Methods in the Applied Sciences 15 (1992), S. 299-314 
    ISSN: 0170-4214
    Keywords: Mathematics and Statistics ; Applied Mathematics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics
    Notes: The initial-boundary value problem in a semi-infinite strip (0, ∞)×(0, T) for a degenerate parabolic equation of the form u, t= ϕ(u)xx + b(x)ϕ(u)x is considered. The properties of solutions in the case where the initial function is compactly supported and for constant initial and boundary data are investigated.
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  • 15
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    Mathematical Methods in the Applied Sciences 15 (1992) 
    ISSN: 0170-4214
    Keywords: Mathematics and Statistics ; Applied Mathematics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics
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  • 16
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    Mathematical Methods in the Applied Sciences 15 (1992), S. 511-523 
    ISSN: 0170-4214
    Keywords: Mathematics and Statistics ; Applied Mathematics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics
    Notes: We present a mathematical model of a communication system perturbed by statistical sampling errors (timing jitter). The aim is to find an ‘optimal’ impulse response for the system, the optimization problem actually being a minimax problem. that is we put the model into a game-theoretical framework. The basic game turns out to be a statistical game similar to those arising from estimation problems in statistics.Earlier results concerning least-favourable sampling error distributions published by Krabs and Vogel are supplemented by estimations of the number of support points of the least-favourable distribution.Furthermore, we state the existence of the saddle points of our game, which formerly has only been proved for some special cases. In the first part we treat the general situation, where one strategy set for the game - the set of all feasible impulse responses - forms a vector space with infinite dimension. In the second part we discuss the problem in the case that the impulse responses are restricted to a finite-dimensional subspace of the whole infinite-dimensional subspace.
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  • 17
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    Mathematical Methods in the Applied Sciences 15 (1992), S. 167-186 
    ISSN: 0170-4214
    Keywords: Mathematics and Statistics ; Applied Mathematics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics
    Notes: A linear integrodifferential equation describing the heat flow in a material with memory is considered. This equation contains a pair of time-dependent convolution kernels that are unknown. Such kernels are recovered by using an overdetermined set of data. A local existence theorem for small times and the continuous dependence of the solutions on the data are shown.
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  • 18
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    Mathematical Methods in the Applied Sciences 15 (1992), S. 187-204 
    ISSN: 0170-4214
    Keywords: Mathematics and Statistics ; Applied Mathematics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics
    Notes: In this paper a non-linear system of equations in a one-dimensional domain is considered, where both the viscosity, v, and R are positive. The existence of solutions in the case of Neumann boundary conditions for θ and a polynomial form of p(θ, ∊) is proved. When θ satisfies the Dirichlet boundary condition, the existence of solutions is obtained under additional assumptions limiting the growth of p(θ, ∊) with respect to θ. In both cases the uniqueness of the solutions is also established.
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  • 19
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    Mathematical Methods in the Applied Sciences 15 (1992) 
    ISSN: 0170-4214
    Keywords: Mathematics and Statistics ; Applied Mathematics
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    Topics: Mathematics
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  • 20
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    Mathematical Methods in the Applied Sciences 15 (1992) 
    ISSN: 0170-4214
    Keywords: Mathematics and Statistics ; Applied Mathematics
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    Topics: Mathematics
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  • 21
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    Mathematical Methods in the Applied Sciences 15 (1992), S. 547-558 
    ISSN: 0170-4214
    Keywords: Mathematics and Statistics ; Applied Mathematics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics
    Notes: In this paper a necessary and sufficient condition for a pair of vector functions to be radiation patterns is presented. More precisely, it is proved that two vector functions, the first in the radial direction and the second in the tangential one, are radiation patterns if and only if there are two entire harmonic vector functions whose radial and tangential projections, respectively, are identical with the previous functions on the unit sphere and whose L2-norm over a sphere of radius R is a function of exponential type in the variable R.
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  • 22
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    Mathematical Methods in the Applied Sciences 15 (1992), S. 11-21 
    ISSN: 0170-4214
    Keywords: Mathematics and Statistics ; Applied Mathematics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics
    Notes: Sufficient conditions are obtained for the linear stability of the positive equilibrium of the neutral system in terms of the parameters of the system. The case n=2 is considered in detail and the general case is discussed briefly.
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  • 23
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    Mathematical Methods in the Applied Sciences 15 (1992), S. 39-55 
    ISSN: 0170-4214
    Keywords: Mathematics and Statistics ; Applied Mathematics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics
    Notes: We consider first and second-order implicit time stepping procedures for the non-stationary Stokes equations in bounded domains of ∝3. Using energy estimates we prove the optimal convergence properties in the Sobolev spaces Hm(G)(m = 0, 1, 2) uniformly in time, provided that the Stokes solution has a certain degree of regularity. Here in the case of the second-order scheme (method of Crank-Nicholson) the Stokes solution has to satisfy a non-local compatibility condition at the initial time t = O, which can be satisfied by a special initial construction.
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  • 24
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    Mathematical Methods in the Applied Sciences 15 (1992), S. 57-71 
    ISSN: 0170-4214
    Keywords: Mathematics and Statistics ; Applied Mathematics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics
    Notes: Given self-adjoint operators Hj, on Hilbert spaces Hj, j = 0,l, and J ∊ B (H0, H1) (where B (H0 H1) denotes the set of bounded linear operators from H0to H1), define the wave operators where P0 is the projection onto the subspace for absolute continuity for H0. We use (i) to study the scattering problem associated with a pair of equations each of the form where L is a positive, self-adjoint operator on a Hilbert space X, m is a positive integer and the αj are distinct positive constants. Methods patterned after those of Kato are used to study two equations (that is for L = L0 and L = Ll) each of the form (ii). We show that they are equivalent to equations of the form where each Ĥk is a self-adjoint operator on an associated Hilbert space Hk. Now suppose∼he-wave operators W±,(L1 L0) exist and are complete. Then we can find a J ∊ B(H1 H0) such that W+(Ĥl, Ĥ0,J) exists. In the case where Lo and L1 have the same domain, H1 and H0 are equal as vector spaces, and under certain conditions (on Li, i = 0, 1) H0 and H1 have equivalent norms. Assuming these conditions, let J'∊ B(H1' H0) be the identity map. We show that (with an additional assumption on L0 and L1) W+(Ĥ1Ĥ0,J) exists andisequal to W+(Ĥl,Ĥ0, J).
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    Mathematical Methods in the Applied Sciences 15 (1992), S. 109-121 
    ISSN: 0170-4214
    Keywords: Mathematics and Statistics ; Applied Mathematics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics
    Notes: An estimate of the rate of convergence is given for the domain decomposition method for the second-order parabolic transmission problem. A brief discussion of the method and some of its applications are presented.
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  • 26
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    Mathematical Methods in the Applied Sciences 15 (1992), S. 421-432 
    ISSN: 0170-4214
    Keywords: Mathematics and Statistics ; Applied Mathematics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics
    Notes: We investigate an unsteady viscous flow problem where ‘good’ boundary conditions are available on part of the boundary only. This problem appears when the flow phenomena one is interested in are concentrated on part of the flow region and, for reasons of computational economy, are numerically computed in this subregion only. Assuming that outside of the subregion the flow is not subjected to any acceleration forces, we develop an (abstract) combined finite-element/boundary element scheme to compute the flow approximately. This scheme leads to a proof of the existence of a weak solution of the corresponding Navier-Stokes problem as well.
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  • 27
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    Mathematical Methods in the Applied Sciences 15 (1992), S. 433-451 
    ISSN: 0170-4214
    Keywords: Mathematics and Statistics ; Applied Mathematics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics
    Notes: Recent monographs and research articles have demonstrated that the highly complicated bifurcation structure, which usually arises in the presence of high symmetries, can often be systematically reduced with group theoretic concepts. Equivariant branching theorems and bifurcation subgroups have been the principle tools in this direction of investigation. The generalizations that we give allow the discussions of new classes of problems and the weakening of the usual hypotheses of absolute irreducibility of the group representation or of the appropriate subspaces. These extensions are illustrated in the case of a class of planar semilinear elliptic partial differential equations, which are usually treated as model problems in fluid mechanics and chemical reactions.
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    Mathematical Methods in the Applied Sciences 15 (1992) 
    ISSN: 0170-4214
    Keywords: Mathematics and Statistics ; Applied Mathematics
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    Topics: Mathematics
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    Mathematical Methods in the Applied Sciences 15 (1992), S. 23-37 
    ISSN: 0170-4214
    Keywords: Mathematics and Statistics ; Applied Mathematics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics
    Notes: We examine the numerical approximation of the integral equation (λ - K)u =f, where K is the double layer (harmonic) potential operator on a closed polyhedral surface in ∝3 and λ, ∣λ∣≥1, is a complex constant. The solution is approximated by Galerkin's method, which is based on piecewise polynomials of arbitrary degree on graded triangulations. By utilizing spline spaces which are modified in that the trial functions vanish on some of the triangles closest to the vertices and edges, we investigate the stability of this method in L2. Furthermore, the use of suitably graded meshes leads to the same quasioptimal error estimates as in the case of a smooth surface.
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    Mathematical Methods in the Applied Sciences 15 (1992), S. 79-88 
    ISSN: 0170-4214
    Keywords: Mathematics and Statistics ; Applied Mathematics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics
    Notes: This note is a case study of a general SU(2) gauge vortex model where the two Higgs multiplets are in the adjoint representation of the gauge group and the Higgs potential energy density contains cross interaction terms The vacuum decay of finite-energy solutions is proved and the existence of a non-trivial solution is established. Moreover, for a special choice of the parameter region, the occurrence of a non-linear desingularization phenomenon is observed.
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    Mathematical Methods in the Applied Sciences 15 (1992), S. 89-108 
    ISSN: 0170-4214
    Keywords: Mathematics and Statistics ; Applied Mathematics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics
    Notes: In the present paper we use a time delay ∊ 〉 0 for an energy conserving approximation of the non-linear term of the non-stationary Navier-Stokes equations. We prove that the corresponding initial-value problem (N∊) in smoothly bounded domains G ⊆ ∝3 is well-posed. We study a semidiscretized difference scheme for (N∊) and prove convergence to optimal order in the Sobolev space H2(G). Passing to the limit ∊→0 we show that the sequence of stabilized solutions has an accumulation point such that it solves the Navier-Stokes problem (No) in a weak sense (Hopf).
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  • 32
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    Mathematical Methods in the Applied Sciences 15 (1992), S. 145-157 
    ISSN: 0170-4214
    Keywords: Mathematics and Statistics ; Applied Mathematics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics
    Notes: It is shown that the moving model, which is a variant of the Preisach model for hysteresis, possesses the wiping out property and is continuous in C[0, T] under natural assumptions.
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    Mathematical Methods in the Applied Sciences 15 (1992), S. 205-221 
    ISSN: 0170-4214
    Keywords: Mathematics and Statistics ; Applied Mathematics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics
    Notes: In this paper we consider a phase transition in binary systems describing the solidification of a dilute alloy in one space dimension, in which the equations of heat and mass transfer are coupled through the conditions at the free boundary. A kinetic condition is introduced to suggest a regularization of the model, which admits a unique solution, and to obtain the generalized solution of the dilute-alloy problem under an additional assumption.
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    Mathematical Methods in the Applied Sciences 15 (1992), S. 275-286 
    ISSN: 0170-4214
    Keywords: Mathematics and Statistics ; Applied Mathematics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics
    Notes: In this paper we continue our studying of the iterative maximum-likelihood reconstruction method. We consider only the continuous case and show some convergence properties of the algorithm. In the discrete case convergence has already been proved. An example demonstrating divergence of the iterative method shows how to handle the continuous case.
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    Mathematical Methods in the Applied Sciences 15 (1992), S. 265-274 
    ISSN: 0170-4214
    Keywords: Mathematics and Statistics ; Applied Mathematics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics
    Notes: We study boundary velocity feedbacks to damp the transverse deflections of heat-conducting thin rods with on-linear stress-strain relations that may be both non-monotone and temperature dependent. It is shown at for suitable feedback laws global solutions exist.
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    Mathematical Methods in the Applied Sciences 15 (1992), S. 453-468 
    ISSN: 0170-4214
    Keywords: Mathematics and Statistics ; Applied Mathematics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics
    Notes: We study the initial-boundary value problem for ∂t2u(t,x)+A(t)u(t,x)+B(t)∂tu(t,x)=f(t,x) on [0,T]×Ω(Ω⊂∝n) with a homogeneous Dirichlet boundary condition; here A(t) denotes a family of uniformly strongly elliptic operators of order 2m, B(t) denotes a family of spatial differential operators of order less than or equal to m, and u is a scalar function. We prove the existence of a unique strong solution u. Furthermore, an energy estimate for u is given.
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    Mathematical Methods in the Applied Sciences 15 (1992), S. 495-509 
    ISSN: 0170-4214
    Keywords: Mathematics and Statistics ; Applied Mathematics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics
    Notes: We examine the existence and regularity results for a scalar conservation law with a convexity condition and solve its weak solution with shocks by using a special method of characterization combined with a representation formula for the weak solution.
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    Mathematical Methods in the Applied Sciences 15 (1992), S. 365-374 
    ISSN: 0170-4214
    Keywords: Mathematics and Statistics ; Applied Mathematics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics
    Notes: We consider a collisionless plasma, which consists of electrons and positively charged ions and is confined to a bounded domain in ∝3. The distribution functions of the particles are assumed to satisfy specular reflections on the boundary of the domain and the boundary is assumed to be perfectly conducting. We establish the existence of stationary plasmas in the non-relativistic, electrostatic case described by the Vlasov-Poisson system as well as in the relativistic, electrodynamic case described by the relativistic Vlasov-Maxwell system.
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    Mathematical Methods in the Applied Sciences 15 (1992), S. 395-409 
    ISSN: 0170-4214
    Keywords: Mathematics and Statistics ; Applied Mathematics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics
    Notes: We prove the existence of guided waves propagating with a velocity strictly larger than the S (shear) wave velocity at infinity in the case of unbounded elastic media invariant under translation in one space direction and asymptotically homogeneous at infinity.These waves correspond to the existence of eigenvalues embedded in the essential spectrum of the self-adjoint elastic propagator.
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    Mathematical Methods in the Applied Sciences 15 (1992), S. 375-393 
    ISSN: 0170-4214
    Keywords: Mathematics and Statistics ; Applied Mathematics
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    Topics: Mathematics
    Notes: The paper deals with the stationary Boltzmann equation in a bounded convex domain Ω. The boundary ∂Ω is assumed to be a piecewise algebraic variety of the C2-class that fulfils Liapunov's conditions. On the boundary we impose the so-called Maxwell boundary conditions, that is a convex combination of specular and diffusive reflections. The non-linear Boltzmann equation is considered with additional volume and boundary source terms and it has been proved that for sufficiently small sources the problem possesses a unique solution in a properly chosen subspace of C(Ω × ∝3). The proof is a refined version of the proof delivered by Guiraud for purely diffusive reflection.
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    Mathematical Methods in the Applied Sciences 15 (1992), S. 1-10 
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    Keywords: Mathematics and Statistics ; Applied Mathematics
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    Topics: Mathematics
    Notes: We consider the scattering of plane, time-harmonic electromagnetic waves by a perfect conductor D. We first show that the set Fλ consisting of the span of a fixed linear combination of the electric and magnetic far-field patterns is dense in the space of square-integrable tangential vector fields defined on the unit sphere for all values of the wave number k ≠ 0 provided that Im k ≥0. We next consider the affine hull Aλ of Fλ and characterize the set Fλ⊥ in terms of electric Herglotz fields. This result is then used to derive an optimization scheme for solving the inverse scattering problem of determining D from a knowledge of Fλ and it is shown that this (unconstrained) optimization scheme has zero as its greatest lower bound.
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    Mathematical Methods in the Applied Sciences 15 (1992) 
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    Mathematical Methods in the Applied Sciences 15 (1992), S. 73-78 
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    Keywords: Mathematics and Statistics ; Applied Mathematics
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    Topics: Mathematics
    Notes: This paper presents two different methods for the construction of exact solutions to the combined KdV and mKdV equation. The first method is a direct one based on a general form of solution to both the KdV and the modified KdV (mKdV) equations. The second method is a leading order analysis method. The method was devised by Jeffrey and Xu. Each of these methods is capable of solving the combined KdV and mKdV equation exactly.
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    Mathematical Methods in the Applied Sciences 15 (1992) 
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    Mathematical Methods in the Applied Sciences 15 (1992), S. 287-297 
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    Keywords: Mathematics and Statistics ; Applied Mathematics
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    Topics: Mathematics
    Notes: We introduce the concept of exponential attractor for non-autonomous systems. Then we prove the existence and finite dimensionality of the attractor for the model equation where K and f are quasiperiodic in time.
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    Mathematical Methods in the Applied Sciences 15 (1992), S. 331-343 
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    Keywords: Mathematics and Statistics ; Applied Mathematics
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    Topics: Mathematics
    Notes: We prove the existence of infinitely many non-zero time-periodic solutions (breathers) to the dispersive wave equation of the form which are localized in the spatial variable, that is The main tool employed is the concentration compactness principle of P. L. Lions.
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    Mathematical Methods in the Applied Sciences 15 (1992), S. 345-363 
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    Keywords: Mathematics and Statistics ; Applied Mathematics
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    Topics: Mathematics
    Notes: In this paper we derive a mixed variational formulation for the exterior Stokes problem in terms of the vorticity and stream function, or the vector potential in three dimensions. The main steps are the construction of the stream function (or vector potential) and the proof of the Babuška-Brezzi ‘inf-sup’ condition. The two- and three-dimensional cases are treated separately because the structure of the stream function differs substantially according to the number of dimensions considered. The conclusion of this work is that if the problem is set in the weighted Sobolev spaces of Hanouzet and Giroire, the analysis of the exterior Stokes problem is quite the same as if the domain were bounded.
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    Mathematical Methods in the Applied Sciences 15 (1992), S. 411-419 
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    Keywords: Mathematics and Statistics ; Applied Mathematics
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    Topics: Mathematics
    Notes: Regularized inner products, r(s, e1, e2,) of the distorted plane waves fór the plasma wave equation uu - Δu + qu = 0 are introduced for potentials with compact support. The regularized inner product may be represented in terms of the far-field pattern. The use of the WKB approximation shows that the behaviour of r as e2 → - e1 is closely related to the Radon transform of q. This observation indicates the possibility of finding the Radon transform of q in terms of r(s, e1, e2) and then of recovering q by means of the inverse Radon transform.
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    Mathematical Methods in the Applied Sciences 15 (1992), S. 123-143 
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    Keywords: Mathematics and Statistics ; Applied Mathematics
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    Notes: In this paper we obtain necessary and sufficient conditions for the validity of the estimate The constant c does not depend on u. u is a vector field with values in ∝3. It is defined on a bounded set G of ∝3 or an unbounded one, denoted by Ĝ. The boundary conditions are as follows: either the normal component of u vanishes or the tangential one does. Our conditions are expressed in terms of the Betti numbers of G or Ĝ.
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    Mathematical Methods in the Applied Sciences 15 (1992), S. 479-493 
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    Keywords: Mathematics and Statistics ; Applied Mathematics
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    Topics: Mathematics
    Notes: The existence of a weak solution of a two-dimensional non-stationary free-boundary problem related to flame propagation is established. The main feature of the problem is that the nonlinear coupling of the two parabolic equations occur on the free boundary and has exponential growth.
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    Mathematical Methods in the Applied Sciences 15 (1992), S. 537-545 
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    Notes: The non-characteristic Cauchy problem for the heat equation uxx(x,t) = u1(x,t), 0 ≤ x ≤ 1, - ∞ 〈 t 〈 ∞, u(0,t) = ϕ(t), ux(0, t) = ψ(t), - ∞ 〈 t 〈 ∞ is regularizèd when approximate expressions for ϕ and ψ are given. Properties of the exact solution are used to obtain an explicit stability estimate.
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    Mathematical Methods in the Applied Sciences 15 (1992), S. 525-535 
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    Notes: Our recent paper about some new fundamental solutions is complemented by a representation of the fundamental solution of certain evolution operators of fourth order in terms of a family of fundamental solutions of operators of second order. By applying this to the operators of vibrating beams and plates we deduce representations of their fundamental solutions as simple definite integrals over tabulated functions.
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    Mathematical Methods in the Applied Sciences 15 (1992), S. 559-570 
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    Keywords: Mathematics and Statistics ; Applied Mathematics
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    Notes: Let D ⊂ ∝n be a bounded domain with piecewise-smooth boundary, and q(x,t) a smooth function on D × [0, T]. Consider the time-like Cauchy problem Given g, h for which the equation has a solution, we show how to approximate u(x,t) by solving a well posed fourth-order elliptic partial differential equation (PDE). We use the method of quasi-reversibility to construct the approximating PDE. We derive error estimates and present numerical results.
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    Mathematical Methods in the Applied Sciences 15 (1992), S. 571-591 
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    Keywords: Mathematics and Statistics ; Applied Mathematics
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    Topics: Mathematics
    Notes: This paper is concerned with non-linear parabolic problems. This particular type of probiem arose from the study of the instability of transverse detonation waves in channels. A fairly complete account of the equilibrium states is given. A global approach is adopted for the upper and lower solutions and the energy estimates. Finally, the invariant manifolds that describe the asymptotic behaviour of the solutions near the equilibrium states are discussed.
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    Mathematical Methods in the Applied Sciences 15 (1992), S. 631-645 
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    Keywords: Mathematics and Statistics ; Applied Mathematics
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    Topics: Mathematics
    Notes: In this paper we study the questions of the existence and uniqueness of the solutions for a thermoelastic system of equations in a two-dimensional domain, where both the viscosity v and the rigidity D are positive. It seems that such a system has up to now been considered in a one-dimensional setting only. The change of dimensions enforces the growth conditions with respect to θ and the additional regularity of the data. The existence of solutions in the case of the Neumann boundary conditions for θ and some weak regularity of data is proved. Under stronger regularity conditions the uniqueness is also established. The system has an interpretation as a plate reinforced by a shape memory alloy (SMA) wire mesh.
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    Notes: The so-called Timoshenko beam equation is a good linear model for the transverse vibrations of a homogeneous beam. Following the variational approach of Washizu, the governing equation is deduced in the case when the physical/geometrical parameters of the beam vary along its axis. The equation may not be studied by means of the iterated use of Fourier series. However, a convenient change of variables permits us to prove the well-posedness of the associated Cauchy problem for a beam with sliding ends (the solution is intended in a mild sense). The proof is given in an abstract framework.
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    Numerical Methods for Partial Differential Equations 8 (1992) 
    ISSN: 0749-159X
    Keywords: Mathematics and Statistics ; Numerical Methods
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    Numerical Methods for Partial Differential Equations 8 (1992), S. 77-95 
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    Keywords: Mathematics and Statistics ; Numerical Methods
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    Notes: In this paper the optimal control of uncertain parabolic systems of partial differential equations is investigated. In order to search for controllers that are insensitive to uncertainties in these systems, an iterative optimization procedure is proposed. This procedure involves the solution of a set of operator valued parabolic partial differential equations. The existence and uniqueness of solutions to these operator equations is proved, and a stable numerical algorithm to approximate the uncertain optimal control problem is proposed. The viability of the proposed algorithm is demonstrated by applying it to the control of parabolic systems having two different types of uncertainty.
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    Numerical Methods for Partial Differential Equations 8 (1992), S. 203-220 
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    Keywords: Mathematics and Statistics ; Numerical Methods
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    Topics: Mathematics
    Notes: Coupling techniques are essential to combining different numerical methods together for the purpose of solving an elliptic boundary value problem. By means of nonconforming constraints, the combinations of various Lagrange finite element methods often cause reduced rates of convergence. In this article, we present a method using penalty plus hybrid technique to match different finite element methods such that the optimal convergence rates in the ‖ · ‖h and zero norms of errors of the solution can always be achieved. Also, such a coupling technique will lead to an optimal asymptotic condition number for the associated coefficient matrix. Moreover, this study can easily be extended for combining the finite difference method with the finite element method to also yield the optimal rate of convergence.
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    Numerical Methods for Partial Differential Equations 8 (1992), S. 221-266 
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    Keywords: Mathematics and Statistics ; Numerical Methods
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    Topics: Mathematics
    Notes: An important class of problems in mathematical physics involves equations of the form -∇ · (A∇φ) = f. In a variety of problems it is desirable to obtain an accurate approximation of the flow quantity u = -A∇φ. Such an accurate approximation can be determined by the mixed finite element method. In this article the lowest-order mixed method is discussed in detail. The mixed finite element method results in a large system of linear equations with an indefinite coefficient matrix. This drawback can be circumvented by the hybridization technique, which leads to a symmetric positive-definite system. This system can be solved efficiently by the preconditioned conjugate gradient method. After approximating u by the lowest-order mixed finite element method, streamlines and residence times can be determined easily and accurately by computations at the element level.
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    Numerical Methods for Partial Differential Equations 8 (1992), S. 277-290 
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    Keywords: Mathematics and Statistics ; Numerical Methods
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    Topics: Mathematics
    Notes: We derive an algorithm for solving the initial value problem for ut = ½σ2uxx + f(u)ux. The approach is based on the representation of the solution to the above equation in the form of the functional of Brownian motion. For small σ we get the approximation for ut = f(u)ux. A comparison with the random choice method is illustrated by the numerical example.
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    Numerical Methods for Partial Differential Equations 8 (1992) 
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    Numerical Methods for Partial Differential Equations 8 (1992), S. 357-379 
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    Keywords: Mathematics and Statistics ; Numerical Methods
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    Topics: Mathematics
    Notes: Spurious or kinematic modes have posed a major obstacle to the implementation of the mixed finite element method. This research shows that spurious modes resulting from the approximation spaces not satisfying the LBB condition do not prevent a well posed problem. When the LBB condition is not satisfied, the resulting matrix equations are singular. A direct solution method is presented for the efficient solution of the possibly singular equations. Orthogonal flux basis functions are introduced to simplify the problem. Then the solution procedure is based on nested domain decomposition. This solution procedure is shown to be competitive with direct solution methods for the displacement finite element method. Examples are included to demonstrate various aspects of the LBB condition and the solution procedure.
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    Numerical Methods for Partial Differential Equations 8 (1992), S. 341-355 
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    Keywords: Mathematics and Statistics ; Numerical Methods
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    Topics: Mathematics
    Notes: Advection-dominated flows occur widely in the transport of groundwater contaminants, the movements of fluids in enhanced oil recovery projects, and many other contexts. In numerical models of such flows, adaptive local grid refinement is a conceptually attractive approach for resolving the sharp fronts or layers that tend to characterize the solutions. However, this approach can be difficult to implement in practice. A domain decomposition method developed by Bramble, Ewing, Pasciak, and Schatz, known as the BEPS method, overcomes many of the difficulties. We demonstrate the applicability of BEPS ideas to finite element collocation on trial spaces of piecewise Hermite cubics. The resulting scheme allows one to refine selected parts of a spatial grid without destroying algebraic efficiencies associated with the original coarse grid. We apply the method to steady-state problems with boundary and interior layers and a time-dependent advection-diffusion problem.
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    Numerical Methods for Partial Differential Equations 8 (1992), S. 303-323 
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    Topics: Mathematics
    Notes: In the mixed finite element approximation of Stokes-like problems, stability considerations normally restrict the choice of the finite element subspaces to families satisfying the so-called inf-sup condition. However, in the case of the generalized Stokes problems that one encounters when solving either the nonstationary compressible or incompressible Navier-Stokes equations by means of some operator splitting, the contribution of the time derivative is adding a term in the continuity equations which, at least theoretically, leads to a problem that is always stable. On the other hand, the numerical solution of the discrete problems built with approximations not satisfying the inf-sup condition are generally plagued with oscillations that look pretty much like the checkerboard phenomenon occurring in the classical context. To clarify this apparent contradiction, we undertook a theoretical study of the condition number of the linear problem involving the sensitive variable, which for such a saddle point problem is the dual problem. In the course of this study, we have exhibited the relationship between that condition number and the inf-sup constant, thus providing a different point of view on the results of Brezzi and Babuska. Finally, we conducted a parametric study of the behavior of that condition number with respect to the mesh size, the viscosity, and the time step. That study showed that, even in this context, the use of “stable element” was adding to the quality of the approximation by reducing the condition number to an acceptable level.
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    Numerical Methods for Partial Differential Equations 8 (1992), S. 381-394 
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    Keywords: Mathematics and Statistics ; Numerical Methods
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    Topics: Mathematics
    Notes: We study the effect of numerical integration when the p-version of the finite element method is used to approximate the eigenpairs of elliptic partial differential operators. We obtain optimal orders of convergence for approximate eigenvalues and eigenvectors under a certain set of requirements on the quadrature rules employed. This is the same set of conditions that has been shown (in an earlier work) to be sufficient for the optimal approximation of the solutions of the corresponding source problems.
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    Numerical Methods for Partial Differential Equations 8 (1992), S. 405-421 
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    Keywords: Mathematics and Statistics ; Numerical Methods
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    Topics: Mathematics
    Notes: We present a priori and a posteriori estimates for the error between the Galerkin and a discretized Galerkin method for the boundary integral equation for the single layer potential on the square plate. Using piecewise constant finite elements on a rectangular mesh we study the error coming from numerical integration. The crucial point of our analysis is the estimation of some error constants, and we demonstrate that this is necessary if our methods are to be used. After the determination of these constants we are in the position to prove invertibility and quasioptimal convergence results for our numerical scheme, if the chosen numerical integration formulas are sufficiently precise. © 1992 John Wiley & Sons, Inc.
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    Numerical Methods for Partial Differential Equations 8 (1992) 
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    Numerical Methods for Partial Differential Equations 8 (1992), S. 493-503 
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    Notes: This work develops a procedure for representing multimaterial interfaces in finite difference models. The boundary separating the two materials can be on or between a row of nodes. The development is validated by embedding the boundary between two regions in a single, larger region and comparing the results. The development is facilitated by the use of a physically based notation that represents the displacement approximations in terms of rigid-body rotations and strain gradient quantities that produce the displacements. Four example problems are presented.
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    Numerical Methods for Partial Differential Equations 8 (1992), S. 505-514 
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    Keywords: Mathematics and Statistics ; Numerical Methods
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    Topics: Mathematics
    Notes: A numerical algorithm is given for solving a class of infinite-order differential equations previously discussed by the authors. These equations yield solutions to certain time-dependent boundary-value problems for the heat equation. An extrapolation process is given for increasing the speed of convergence of the sequence generated by the method. © 1992 John Wiley & Sons, Inc.
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    Numerical Methods for Partial Differential Equations 8 (1992), S. 33-57 
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    Topics: Mathematics
    Notes: Penlty coupling techniques on an interface boundary, artificial or material, are first presented for combining the Ritz-Galerkin and finite element methods. An optimal convergence rate first is proved in the Sobolev norms. Moreover, a significant coupling strategy, L + 1 = O(|ln h|), between these two methods are derived for the Laplace equation with singularities, where L + 1 is the total number of particular solutions used in the Ritz-Galerkin method, and h is the maximal boundary length of quasiuniform elements used in the linear finite element method. Numreical experiments have been carried out for solving the benchmark model: Motz's problem. Both theoretical analysis and numreical experiments clearly display the importance of penalty-combined methods is solving elliptic equations with singularities.
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    Numerical Methods for Partial Differential Equations 8 (1992), S. 1-19 
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    Topics: Mathematics
    Notes: A modified version of the method of fundamental solutions which incorporates the singular behavior of the problem under consideration is introduced. The method is tested on potential and biharmonic problems and its performance is compared to the performance of the standard method of fundamental solutions.
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    Numerical Methods for Partial Differential Equations 8 (1992), S. 21-31 
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    Notes: We attempt to obtain a two-level implicit finite difference scheme using nine spatial grid points of O(k2 + kh2 + h4) for solving the 2D nonlinear parabolic partial differential equation v1uxx + v2uyy = f(x, y, t, u, ux, uy, u1) where v1 and v2 are positive constants, with Dirichlet boundary conditions. The method, when applied to a linear diffusion-convection problem, is shown to be unconditionally stable. Computational efficiency and the results of numerical experiments are discussed.
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    Numerical Methods for Partial Differential Equations 8 (1992), S. 59-76 
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    Keywords: Mathematics and Statistics ; Numerical Methods
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics
    Notes: We compare the relative performances of two iterative schemes based on projection techniques for the solution of large sparse nonsymmetric systems of linear equations, encountered in the numerical solution of partial differential equations. The Block-Symmetric Successive Over-Relaxation (Block-SSOR) method and the Symmetric-Kaczmarz method are derived from the simplest of projection methods, that is, the Kaczmarz method. These methods are then accelerated using the conjugate gradient method, in order to improve their convergence. We study their behavior on various test problems and comment on the conditions under which one method would be better than the other. We show that while the conjugate-gradient-accelerated Block-SSOR method is more amenable to implementation on vector and parallel computers, the conjugate-gradient accelerated Symmetric-Kaczmarz method provides a viable alternative for use on a scalar machine.
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    Numerical Methods for Partial Differential Equations 8 (1992), S. 97-111 
    ISSN: 0749-159X
    Keywords: Mathematics and Statistics ; Numerical Methods
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics
    Notes: A simple nonconforming quadrilateral Stokes element based on “rotated” multi-linear shape functions is analyzed. On strongly nonuniform meshes the usual parametric version of this element suffers from a lack of consistency, while its nonparametric counterpart turns out to be convergent with optimal orders. This theoretical result is confirmed by numerical tests.
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    Numerical Methods for Partial Differential Equations 8 (1992), S. 143-147 
    ISSN: 0749-159X
    Keywords: Mathematics and Statistics ; Numerical Methods
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics
    Notes: Some mathematical aspects of the variational approach to grid generation are discussed. Existence and regularity results for 2D are presented.
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    Numerical Methods for Partial Differential Equations 8 (1992), S. 171-202 
    ISSN: 0749-159X
    Keywords: Mathematics and Statistics ; Numerical Methods
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics
    Notes: A fully Sinc-Galerkin method in both space and time is presented for fourth-order time-dependent partial differential equations with fixed and cantilever boundary conditions. The sine discretizations for the second-order temporal problem and the fourth-order spatial problems are presented. Alternate formulations for variable parameter fourth-order problems are given, which prove to be especially useful when applying the forward techniques of this article to parameter recovery problems. The discrete system that corresponds to the time-dependent partial differential equations of interest are then formulated. Computational issues are discussed and an accurate and efficient algorithm for solving the resulting matrix system is outlined. Numerical results that highlight the method are given for problems with both analytic and singular solutions as well as fixed and cantilever boundary conditions.
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    Numerical Methods for Partial Differential Equations 8 (1992), S. 149-169 
    ISSN: 0749-159X
    Keywords: Mathematics and Statistics ; Numerical Methods
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics
    Notes: The theory of optimal control and the semianalytical method of elliptic partial differential equation (PDE) in a prismatic domain are mutually simulated issues. The simulation of discrete-time linear quadratic (LQ) control with the substructural chain problem in static structural analysis is given first. From the minimum potential energy variational principle of substructural chain, the generalized variational principle with two kinds of variables and the dual equations are derived. The simulation relation is then recognized by comparing the variational principle and dual equations of the LQ control theory. The simulation between elliptic PDE in the prismatic domain and continuous-time LQ control is established in the same way, and the interval energy is naturally introduced, as in the case of substructural chain. The assembling and condensation equations can help one to derive the differential equations of the submatrices of potential energy and mixed energy. The well known Riccati equation is one of them. The interval assembling and condensation algorithm can be used to solve the Riccati equation. Some numerical examples are given to illustrate the method.
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    Numerical Methods for Partial Differential Equations 8 (1992), S. 267-276 
    ISSN: 0749-159X
    Keywords: Mathematics and Statistics ; Numerical Methods
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics
    Notes: The combined approach of linearization and splitting up is used for devising new algorithms to solve a one-dimensional Burgers' equation. Two schemes are discussed and the computed solutions are compared with the exact solution. For this problem it is found that the schemes proposed yield excellent numerical results for Reynolds number R ranges from 50 up to 1500. The schemes were also tested for another problem whose R = 10000. In this case a filtering technique is used to overcome the nonlinear instability.
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    Numerical Methods for Partial Differential Equations 8 (1992), S. 291-301 
    ISSN: 0749-159X
    Keywords: Mathematics and Statistics ; Numerical Methods
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics
    Notes: A multivariate box spline framework for the formulation of numerical methods for partial differential equations has been constructed. In particular, a fourth-order Galerkin method and a second-order collocation method were derived and applied to a test problem (classical Poisson equation on a square). The examples indicate that accuracy compares favorably with standard methods and the success of iterative schemes suggests an underlying stabilizing effect.
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    Numerical Methods for Partial Differential Equations 8 (1992), S. 451-467 
    ISSN: 0749-159X
    Keywords: Mathematics and Statistics ; Numerical Methods
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics
    Notes: This article concerns the development of energy-based variational formulations and their corresponding finite element-boundary element Rayleigh-Ritz approximations for solving the time-harmonic vibration and scattering problem of an inhomogeneous penetrable fluid or solid object immersed in a compressible, inviscid, homogeneous fluid. The resulting coupled finite element and boundary integral methods (FEM-BEM) have the following attractive features: (1) Separate direct and complementary variational principles lead naturally to several alternative structure variable and fluid variable methodologies. (2) The solution in the exterior region is represented by a combined single- and double-layer potential which ensures the validity of the methods for all wave numbers; even though this representation introduces hypersingular integrals, for actual computations the hypersingular operator may be rewritten in terms of single-layer potentials, which can be integrated by standard techniques. (3) Since the discretized equations for the interior region and for the boundary are derived from the first variation of bilinear functionals the resulting algebraic systems of equations are always symmetric. In addition, the transition conditions across the interface are natural. This allows one to approximate the solutions within the interior and exterior regions independently, without imposing any boundary constraints.
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    Numerical Methods for Partial Differential Equations 8 (1992), S. 469-491 
    ISSN: 0749-159X
    Keywords: Mathematics and Statistics ; Numerical Methods
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics
    Notes: The defect correction technique, based on the Galerkin finite element method, is analyzed as a procedure to obtain highly accurate numerical solutions to second-order elliptic boundary value problems. The basic solutions, defined over a rectangular region Ω, are computed using continuous piecewise bilinear polynomials on rectangles. These solutions are O(h2) accurate globally in the second-order discrete Sobolev norm. Corrections to these basic solutions are obtained using higher-order piecewise polynomials (Lagrange polynomials or splines) to form defects. An O(h2) improvement is gained on the first correction. The lack of regularity of the discrete problems (beyond the second-order Sobolev norm) makes it impossible to retain this order of improvement, but for problems satisfying certain periodicity conditions, straightforward arbitrary accuracy is obtained, since these problems possess high-order regularity. © 1992 John Wiley & Sons, Inc.
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    Numerical Methods for Partial Differential Equations 8 (1992), S. 537-550 
    ISSN: 0749-159X
    Keywords: Mathematics and Statistics ; Numerical Methods
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics
    Notes: An adaptive refinement procedure consisting of a localized error estimator and a physically based approach to mesh refinement is developed for the finite difference method. The error estimator is a variation of a successful finite element error estimator. The errors are estimated by computing an error energy norm in terms of discontinuous and continuous stress fields formed from the finite difference results for plane stress problems. The error measure identifies regions of high error which are subsequently refined to improve the result. The local refinement procedure utilizes a recently developed approach for developing finite difference templates to produce a graduated mesh. The adaptive refinement procedure is demonstrated with a problem that contains a well-defined singularity. The results are compared to finite element and uniformly refined finite difference results.
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    Numerical Methods for Partial Differential Equations 8 (1992), S. 113-125 
    ISSN: 0749-159X
    Keywords: Mathematics and Statistics ; Numerical Methods
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics
    Notes: We implement and compare the Sawtooth (S) and V cycles in multigrid method (mgd) for the Poisson equation and for the fourth-order 13-point methods derived by Krishnaiah et al. Defect correction in the mgd context is also studied. Computations reveal that the S cycle is much cheaper compared to the V cycle in most of the situations. Defect correction is much cheaper than the direct application of the 13-point formula, giving almost the fourth-order accurate results.
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    Numerical Methods for Partial Differential Equations 8 (1992), S. 127-142 
    ISSN: 0749-159X
    Keywords: Mathematics and Statistics ; Numerical Methods
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics
    Notes: An accurate and efficient numerical method has been developed for a nonlinear diffusion convection-dominated problem. The scheme combines asymptotic methods with usual solution techniques for hyperbolic problems. After having localized shock or corner layers and rescaling, first terms of the inner expansion are computed. Using the same concepts gives a method to compute a very accurate solution of the nonlinear conservation law. Because our numerical scheme is based on a uniform approximation throughout the domain, the shock is localized very accurately and there is practically no smearing out. Numerical computations are presented. Another novel feature is the ability to break down the problem according to subdomains of different local behavior, based on asymptotic analysis, which may make it feasible to do computations with different processors.
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    Numerical Methods for Partial Differential Equations 8 (1992), S. 325-340 
    ISSN: 0749-159X
    Keywords: Mathematics and Statistics ; Numerical Methods
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics
    Notes: Random walk particle methods (RWPM) can be used in operator splitting schemes to simulate reactive solute transport in porous media. Projection functions are used to transfer particle location and mass information to concentrations at selected spatial points. Because of the stochastic nature of RWPM, concentration estimates made from particle distributions include a “noisy” error component. In some cases of reactive or density-dependent flows, this type of error may be propagated forward in time. It can be reduced by using larger numbers of particles or by using different projection functions. The effects of using different projection functions or numbers of particles in different flow regimes or dimensions are explored using concentration solutions for a set of one-, two-, and three-dimensional nonreactive test problems. Resulting solutions are compared with analytic results and classical random walk error estimates. A piecewise linear projection function provides a reasonable improvement in accuracy over the more convenient box methods at a modest increase in cost. The support of the projection functions should be O(Δx) to avoid excessive smearing. Multidimensional projection functions may be advantageously formed by products of different one-dimensional projection functions.
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    Numerical Methods for Partial Differential Equations 8 (1992) 
    ISSN: 0749-159X
    Keywords: Mathematics and Statistics ; Numerical Methods
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics
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    Numerical Methods for Partial Differential Equations 8 (1992), S. 395-404 
    ISSN: 0749-159X
    Keywords: Mathematics and Statistics ; Numerical Methods
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics
    Notes: We consider a class of mixed finite element methods for nonlinear parabolic problems over a plane domain. The finite element spaces taken are Raviart-Thomas spaces of index k, k ≥ 0. We obtain optimal order L2- and almost optimal order L∞-error estimates for the finite element solution and order optimal L2-error estimates for its gradient. We also derive the error estimates for the time derivatives of the solution. Our results extend those previously obtained by Johnson and Thomée for the corresponding linear problems with k ≥ 1.
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    Numerical Methods for Partial Differential Equations 8 (1992), S. 423-442 
    ISSN: 0749-159X
    Keywords: Mathematics and Statistics ; Numerical Methods
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics
    Notes: Some approximate methods for solving linear hyperbolic systems are presented and analyzed. The methods consist of discretizing with respect to time and solving the resulting hyperbolic system for fixed time by least squares finite element methods. An analysis of least squares approximations is given, including optimal order estimates for piecewise polynomial approximation spaces. Numerical results for the inviscid Burgers' equation are also presented. © 1992 John Wiley & Sons, Inc.
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    Numerical Methods for Partial Differential Equations 8 (1992), S. 443-450 
    ISSN: 0749-159X
    Keywords: Mathematics and Statistics ; Numerical Methods
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics
    Notes: We discuss multigrid methods and multilevel preconditioners for first kind boundary integral equations with weakly and hypersingular kernels. We find that the number of iterations needed is bounded or grows no worse than logarithmically in the numbers of unknowns. We also discuss the complexity for parallel implementations.
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    Numerical Methods for Partial Differential Equations 8 (1992), S. 515-535 
    ISSN: 0749-159X
    Keywords: Mathematics and Statistics ; Numerical Methods
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics
    Notes: A variational adjoint nodal method is proposed to yield an approximation theory for the eigenvalues of the multigroup neutron diffusion boundary value problem of a heterogeneous quasicritical one-dimensional slab reactor. Semianalytical eigensolutions are constructed for the associated whole-reactor group nodal fluxes. The method appears to be mathematically more consistent, computationally more straightforward, and practically more convenient than alternative nodal or finite element schemes. © 1992 John Wiley & Sons, Inc.
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    Numerical Methods for Partial Differential Equations 8 (1992), S. 551-574 
    ISSN: 0749-159X
    Keywords: Mathematics and Statistics ; Numerical Methods
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics
    Notes: A coefficient inverse problem of the one-dimensional hyperbolic equation with overspecified boundary conditions is solved by the finite difference method. The computation is carried out in the x direction instead of the usual t direction. The original boundary condition and the overspecified boundary data are used as the new initial conditions, and the original data at t = 0 are used to compute the coefficient directly. The computation time used by this scheme is almost equal to that for solving the hyperbolic equation in the same region once, even though the inverse problem is essentially nonlinear and hence more difficult to solve. An error estimate is obtained that guarantees the stability of the scheme marching in the x direction. Several numerical experiments are carried out to show the convergence and other properties of the scheme. © 1992 John Wiley & Sons, Inc.
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    Numerical Methods for Partial Differential Equations 8 (1992), S. 575-591 
    ISSN: 0749-159X
    Keywords: Mathematics and Statistics ; Numerical Methods
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mathematics
    Notes: We present a 19-point fourth-order finite difference method for the nonlinear second-order system of three-dimensional elliptic equations Auxx + Buyy + Cuzz = f, where A, B, C, are M × M diagonal matrices, on a cubic region R subject to the Dirichlet boundary conditions u(x, y, z) = u(0)(x, y, z) on ∂R. We establish, under appropriate conditions, O(h4) convergence of the difference method. Numerical examples are given to illustrate the method and its fourth-order convergence. © 1992 John Wiley & Sons, Inc.
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