Showing papers by "National Technical University of Athens published in 1982"
TL;DR: This clearly written, mathematically rigorous text includes a novel algorithmic exposition of the simplex method and also discusses the Soviet ellipsoid algorithm for linear programming; efficient algorithms for network flow, matching, spanning trees, and matroids; the theory of NP-complete problems; approximation algorithms, local search heuristics for NPcomplete problems, more.
Abstract: This clearly written , mathematically rigorous text includes a novel algorithmic exposition of the simplex method and also discusses the Soviet ellipsoid algorithm for linear programming; efficient algorithms for network flow, matching, spanning trees, and matroids; the theory of NP-complete problems; approximation algorithms, local search heuristics for NPcomplete problems, more All chapters are supplemented by thoughtprovoking problems A useful work for graduate-level students with backgrounds in computer science, operations research, and electrical engineering Mathematicians wishing a self-contained introduction need look no further—American Mathematical Monthly 1982 ed
7,221 citations
05 May 1982
TL;DR: Many important combinatorial optimization problems, including the traveling salesman problem (TSP), the clique problem and many others, call for the optimization of a linear functional over some discrete set of vectors.
Abstract: Many important combinatorial optimization problems, including the traveling salesman problem (TSP), the clique problem and many others, call for the optimization of a linear functional over some discrete set of vectors.
224 citations
TL;DR: It is shown that the complexity of the foUowmg class of problems depends explicitly on the rate of growth of a sLmple parameter of the family of prototypes.
Abstract: The complexity of the foUowmg class of problems Is investigated: Given a distance matrix, fred the shortest spanning tree that is isomorphic to a given prototype. Several classical combinatorial problems, both easy and hard, fall into this category for an appropriate choice of the family of prototypes, for example, taking the family to be the set of all paths gives the traveling salesman problem or taking the family to be the set of all 2-stars gives the weighted matching problem It is shown that the complexity of these problems depends explicitly on the rate of growth of a sLmple parameter of the family of prototypes.
184 citations
TL;DR: In this paper, the Mises elastic-plastic boundary is introduced to define the core region, instead of the respective circle, and the minimum value of the strain-energy density is replaced by the minimum values of the radius of the elastic boundary in the role of the decisive quantity defining the direction of crack propagation.
159 citations
145 citations
TL;DR: Fast methods for the determination of the autoregressive (AR) portion of the ARMA model are presented and the definition of two broad classes of matrices, called diagonal innovation matrices (DIM) and peripheral innovation Matrices (PIM), for which fast schemes can be developed are introduced.
Abstract: In many signal processing applications, one often seeks the solution of a linear system of equations by means of fast algorithms. The special form of the matrix associated with the linear system may permit the development of algorithms requiring 0 (p2) or fewer operations. Hankel and Toeplitz matrices provide well known examples and various fast schemes have been developed in the literature to cover these cases. These techniques have common characteristics so that they may be generalized to cover a wider class of linear systems. The purpose of this paper is to develop fast algorithms that cover this wider set of systems. An important feature of the general scheme introduced here is that it leads to the definition of two broad classes of matrices, called diagonal innovation matrices (DIM) and peripheral innovation matrices (PIM), for which fast schemes can be developed. The class of PIM matrices includes many structures appearing in signal processing applications. Most of them are extensively studied in this paper and Fortran coding is provided. Finally, ARMA modeling is considered and within the general framework already introduced, fast methods for the determination of the autoregressive (AR) portion of the ARMA model are presented.
90 citations
TL;DR: In this article, a set of typical components of the vector of objective functionals for copolymerization reactor problems are selected for detailed examination employing multiobjective function optimization techniques, and non-inferior, or Pareto, sets of optimal solutions have been determined for the dual objectives of narrowing bothcopolymer composition and molecular weight distributions in the styrene-acrylonitrile system.
78 citations
29 Mar 1982
TL;DR: The limit of the parallelism achievable by the multiversion approach is characterized and the resulting space-parallelism trade-off is demonstrated.
Abstract: We examine the problem of concurrency control when the database management system supports multiple versions of the data. We characterize the limit of the parallelism achievable by the multiversion approach and demonstrate the resulting space-parallelism tradeoff.
71 citations
53 citations
TL;DR: A critical review of some of the more recent models, and drawing some conclusions on the actual and potential contribution of energy modelling in energy policy formulation, is presented in this paper, where a conceptual framework for energy-economy interactions is presented and key parameters are identified.
48 citations
TL;DR: In this paper, the authors investigated the thermomechanical behavior of particle composites in their transition region, and the value of the glass-transition temperature Tg, which constitutes an upper limit for the structurally important glassy region, was examined.
Abstract: The thermomechanical behavior of particle composites was investigated in their transition region. In particular, the value of the glass-transition temperature Tg, which constitutes an upper limit for the structurally important glassy region, was examined. According to experimental evidence existing in the literature the introduction of a reinforcing filler in a polymeric matrix causes Tg of the latter to increase, unless mechanical imperfections counterbalance the reinforcing effect or even produce a Tg for the composite which is lower than that of the matrix. Based on mechanical theories, valid for the mechanical moduli of viscoelastic particle composites, a model was introduced that explains why the glass transition of composite materials may be reduced in some cases, whereas it may be increased in others. The concept of interphase between inclusions and matrix was used for the development of the model. Interphase is assumed to be a region, which is created between the matrix material and the filler particles, both considered as homogeneous and isotropic, whose thermomechanical properties and volume fraction may be determined from the overall thermomechanical behavior of the composite.
TL;DR: In this paper, some invariant algebraic properties of the chemical reactions in the space of ΔG (free energy change), T (temperature) are developed, and based on these properties, a search strategy is proposed for the creation and screening of alternative reaction paths, leading to a desired chemical product.
TL;DR: In this paper, a volume integral equation technique based on Green's function theory in regions with planar boundaries is used to formulate the problem of scattering from cylindrical inhomogeneities immersed inside a dielectric-slab waveguide, where analytical expressions for the reflection and transmission coefficients in the slab, when a guided surface mode is incident upon the inhomogeneity from the left, are derived up to order [b(k1 − k2)]3.
Abstract: Scattering from cylindrical inhomogeneities immersed inside a dielectric-slab waveguide is investigated analytically. A volume integral equation technique based on Green’s function theory in regions with planar boundaries is used to formulate the problem. For the case of circularly inhomogeneous shapes, an analytical solution is developed when |b(k1 − k2) < 1, where k1 − k2 is the difference between the wave numbers of the slab and the circular inhomogeneity whose radius is b. Analytical expressions for the reflection and transmission coefficients in the slab, when a guided surface mode is incident upon the inhomogeneity from the left, are derived up to order [b(k1 − k2)]3. Numerical results are computed and plotted for several cases.
03 Nov 1982
TL;DR: In this paper, it was shown that the problem of deciding whether an instance of the traveling salesman problem has a uniquely optimal solution is complete for Δ2P, which is the case for all instances of the problem.
Abstract: We show that the problem of deciding whether an instance of the traveling salesman problem has a uniquely optimal solution is complete for Δ2P.
TL;DR: In this paper, a closed-form solution of the differential equation − λ 1 y″ (1+y ′2 ) 3 2 − √ (y + bx + c) related to the elastic line of a straight thin rod due to a terminal co-planar loading was obtained.
Abstract: In this paper a closed-form solution of the differential equation − λ 1 y″ (1+y ′2 ) 3 2 − λ 2 y′y″ (1+y ′2 ) 3 2 −= α(y + bx + c) related to the elastic line of a straight thin rod due to a terminal co-planar loading, is obtained. Based on this solution, the problem of eccentric buckling of the same rod, taking into account the influence of transverse shear deformations, is examined.
TL;DR: In this paper, a methodological approach is presented, relating the level of strategic petroleum reserves, the decision variable, with the costs associated with their storage and the economic losses incurred by an energy shortage.
Abstract: Energy constitutes a small fraction of the production inputs in the economy, yet the rigidity imposed by the short and medium term inability to substitute energy with other production factors in case of an energy shortage, requires the establishment of safeguard mechanisms to ensure sufficient energy supply during an emergency. These safeguards usually take the form of strategic energy reserves. A methodological approach is presented here, relating the level of strategic petroleum reserves, the decision variable, with the costs associated with their storage and the economic losses incurred by an energy shortage. This approach is based on the use of a decision tree, to portray emergency scenarios. Each scenario, a branch of the decision tree, can be evaluated in terms of a cost function which includes the inventory procurement and maintenance cost and the shortage cost inflicted by a petroleum shortfall.
TL;DR: In this article, a simple and accurate algorithm for determining the propagation constants of cylindrical dielectric waveguides of arbitrary refractive index was proposed, which eliminates the use of Bessel or other complicated functions.
Abstract: From Maxwell's equations we derive a simple and accurate algorithm which can determine the propagation constants of the modes of cylindrical dielectric waveguides of arbitrary refractive index. It eliminates the use of Bessel or other complicated functions, and could be implemented in small microcomputers.
TL;DR: In this article, an exhaustive theoretical analysis of the distribution of the elastic-strain energy density at the tip of a crack for cracks in isotropic elastic media deformed under modes I and II was developed.
Abstract: The study of the distribution of the elastic-strain energy density at the tip of a crack presents a special interest for the understanding of the mechanism of fracture. In this investigation an exhaustive theoretical analysis of the distribution of the elastic-strain energy density at the tip of a crack for cracks in isotropic elastic media deformed under modes I and II was developed. This study was complemented by an experimental solution of this type of problem, based on the method of reflected caustics. According to this method the distribution of the elastic-strain energy density was evaluated along a circular closed curve, defining the singular core around the crack-tip, which was dominated by the singularity at the crack tip and whose diameter is very small, not exceeding a few millimeters. In this way the situation at the crack-tip singularity was directly depicted on the energy density distribution. Interesting results concerning the factors influencing the mode of the strain energy distribution around the crack and the type of the crack path were derived.
TL;DR: In this article, the effect of several parameters, such as the heating rate, the filler-volume fraction and the particle diameter on Tg was investigated and conclusions concerning the thickness of the boun dary interphase were derived.
Abstract: Glass-transition temperatures Tg for iron-epoxy composites were deter mined from heat-capacity measurements. The effect of several parameters, such as the heating rate, the filler-volume fraction and the particle diameter on Tg was investigated and conclusions concerning the thickness of the boun dary interphase were derived. Tg-values obtained from TMA-measurements were compared to the respective values obtained from DSC-measurements.
TL;DR: In this paper, a general method to construct path-independent integrals in the framework of plane elasticity is presented, which are valid not only for the case of a single straight crack, but also for any number of collinear cracks.
Abstract: A general method to construct path-independent integrals in the framework of plane elasticity is presented. The proposed path-independent integrals are valid not only for the case of a single straight crack, but also for the case of any number of collinear cracks. The integrands of these integrals are classified in three classes, which satisfy some well-defined restrictions. Therefore, it is possible to construct a large number of new path-independent integrals. As an illustration some of them are derived and are applied to the theoretical computation of the stress intensity factor in five particular cases of cracked elastic plates.
TL;DR: In this article, a theoretical model consisting of a series of infinite concentric cylinders surrounding a fiber in a composite material was introduced to give a quantitative account of interface phenomena, already experimentally observed.
Abstract: A theoretical model, consisting of a series of infinite concentric cylinders surrounding a fiber in a composite material, was introduced in this paper to give a quantitative account of interface phenomena, already experimentally observed. A series of specimens, conveniently designed to represent the theoretical model, were subjected to dynamic modes of loading to measure the amount of adhesion between fibers and matrices by means of an adhesion coefficient developed in the theory. It was found that theoretical results for the adhesion between matrix and filler were compatible with the structural characteristics of the specimens tested.
01 May 1982
TL;DR: This new scheme is the fastest existing method for the sequential solution of the general covariance equations when prewindowing is not desirable and leads to O(13p) multiplications per recursion only.
Abstract: This paper introduces a sequential fast scheme of the Kalman type to solve the covariance equations of linear prediction. The new algorithm constitutes a generalization of the scheme introduced recently by D. Falconer and L. Ljung, and applies to a more general situation. The algo rithm is characterized by conceptual simplicity and efficiency. Although it is based on matrix partitioning concepts and shifting properties of the signal vector, it does not have the restriction of prewindowing as it is the case with the previously mentioned algorithm, thus it can be used to model small frames of signals as well as to model signals generated by an AR model, giving in this case the exact solution. Finally it must be noted that the algorithm is strictly time recursive and leads to O(13p) multiplications per recursion only. Thus, this new scheme is the fastest existing method for the sequential solution of the general covariance equations when prewindowing is not desirable.
TL;DR: Using the law of reflection and catastrophe theory, conditions to predict caustic patterns in a 3-D space are established, which were created from the reflection of a light beam from an analytically known surface.
Abstract: To generalize the theory of far-field caustics, three theorems and several corollaries are presented in this paper. Using the law of reflection and catastrophe theory we have established conditions to predict caustic patterns in a 3-D space, which were created from the reflection of a light beam from an analytically known surface. The general theory was readily reduced to the already known cases of diffraction, indicating the validity of the general theory. Experimental evidence in two simple cases of reflectors, consisting of triangular and rectangular membranes, corroborated the results of the theory.
TL;DR: In this article, the influence of a biaxial loading on the form and orientation of reflected caustics was studied in connection with the orientation of the crack, and modified relations were given for the evaluation of the complex stress intensity factor K = K I iKri in terms of the angle ~v of the angular displacement of the caustic axis.
Abstract: Summary The optical method of reflected caustics was applied up-to-now to problems of cracked plates under uniaxial loading. Only the problem of the biaxial tension of the plate has been considered for the particular case where the crack is transverse to the longitudinal axis of the plate which coincided with the loading axis. In this paper the influence of a biaxial loading of the plate on the form and orientation of the caustic was studied in connection with the orientation of the crack. New modified relations were given for the evaluation of the complex stress intensity factor K = K I -- iKri in terms of the angle ~v of the angular displacement of the caustic axis. For the accurate evaluation of K I and K H nomograms of correction factors 6vmax, 6x max and dx rain were given in terms of the angle of inclination of the crack O9 = (90 -- fl) and the biaxiality factor k. Experimental evidence with PMMA internally cracked plates corroborated the results of theory.
TL;DR: In this paper, the neutrino oscillation length is related to the horizontal scale in a horizontal SU(3) H × SU(2) L × U(1) model.
TL;DR: In this article, the stress-strain behavior of metal-filled polymeric composites at various tensile deformation rates was studied at different tensile rates, and the dependency of the mechanical properties of iron-epoxy composites, such as elastic modulus, maximum load, breaking-strains and breaking energy, on the rate of deform ation, as well as on the filler-volume fraction was examined.
Abstract: The stress-strain behavior of metal-filled polymeric composites was studied at various tensile deformation rates. The dependency of the mechanical properties of iron-epoxy composites, such as elastic modulus, maximum load, breaking-strain and breaking-energy, on the rate of deform ation, as well as on the filler-volume fraction was examined. The results corroborate satisfactorily the theoretically predicted values.
TL;DR: A combination of the classical finite element and the singular integral equation method with the help of Schwarz's Alternating Method (S.A.M) is presented in this article, which is a powerful hybrid method for the efficient solution of problems of stress fields containing any type of singularity.
TL;DR: In this article, the authors deal with the efficient determination of FIR Wiener filters with linear phase by exploiting the Toeplitz-plus-Hankel structure which is imposed on the normal equations matrix by the phase linearity constraint and the stationarity assumption.
Abstract: The letter deals with the efficient determination of FIR Wiener filters with linear phase. This task is achieved by exploiting the Toeplitz-plus-Hankel structure which is imposed on the normal equations matrix by the phase linearity constraint and the stationarity assumption. It is shown that the design of these filters is ultimately reduced to the computation of optimum FIR Wiener filters by Levinson's algorithm.
TL;DR: In this article, an iterative method for the solution of singular integral equations is given by developing a recurrence formula, by using appropriate quadrature rules, and by giving the solution in an extremely simple form.
01 May 1982
TL;DR: In this article, the authors extended the concept of path independent integrals introduced previously for homogeneous media to the case of a crack in the interface between dissimilar half planes and obtained the stress intensity factor when the lips of the crack are loaded by identical distributions of stresses in both lips, or by concentrated forces or couples.
Abstract: The concept of path independent integrals introduced previously for homogeneous media is extended to the case of a crack in the interface between dissimilar half planes. Using any of the pathindependent integrals defined in this paper we obtain the stress intensity factor when the lips of the crack are loaded by identical distributions of stresses in both lips, or by concentrated forces or couples. The above integrals are extended in order to include the case of an arbitrary number of collinear cracks in the interface of two half planes. The case of periodic cracks may be derived from the above analysis. Finally, an interesting generalization may be obtained for the case where arbitrary cracks, inclusions or concentrated forces and couples, not necessarily applied at the lips of the crack, interact.