Showing papers by "Tel Aviv University published in 1993"

Geographic Localization of Knowledge Spillovers as Evidenced by Patent Citations

Abstract: We compare the geographic location of patent citations to those of the cited patents, as evidence of the extent to which knowledge spillovers are geographically localized. We find that citations to U.S. patents are more likely to come from the U.S., and more likely to come from the same state and SMSA as the cited patents than one would expect based only on the preexisting concentration of related research activity. These effects are particularly significant at the local (SMSA) level, and are particularly apparent in early citations.

International R&D Spillovers

Abstract: Investment in research and development (R&D) affects a country's total factor productivity. Recently new theories of economic growth have emphasized this link and have also identified a number of channels through which a country's R&D affects total factor productivity of its trade partners. Following these theoretical developments we estimate the effects of a country's R&D capital stock and the R&D capital stocks of its trade partners on the country's total factor productivity. We find large effects of both domestic and foreign R&D capital stocks on total factor productivity. The foreign R&D capital stocks have particularly large effects on the smaller countries in our sample (that consists of 22 countries). Moreover, we find that about one-quarter of the worldwide benefits of investment in R&D in the seven largest economies are appropriated by their trade partners.

Broadcast encryption

Abstract: We introduce new theoretical measures for the qualitative and quantitative assessment of encryption schemes designed for broadcast transmissions. The goal is to allow a central broadcast site to broadcast secure transmissions to an arbitrary set of recipients while minimizing key management related transmissions. We present several schemes that allow a center to broadcast a secret to any subset of privileged users out of a universe of size n so that coalitions of k users not in the privileged set cannot learn the secret. The most interesting scheme requires every user to store O(klog klog n) keys and the center to broadcast O(k2 log2 k log n) messages regardless of the size of the privileged set. This scheme is resilient to any coalition of k users. We also present a scheme that is resilient with probability p against a random subset of k users. This scheme requires every user to store O(log k log(l/p)) keys and the center to broadcast O(klog2 fclog(l/p)) messages.

Mitochondrial ribosomal RNA mutation associated with both antibiotic-induced and non-syndromic deafness.

Abstract: Maternally transmitted non-syndromic deafness was described recently both in pedigrees with susceptibility to aminoglycoside ototoxicity and in a large Arab-Israeli pedigree. Because of the known action of aminoglycosides on bacterial ribosomes, we analysed the sequence of the mitochondrial rRNA genes of three unrelated patients with familial aminoglycoside-induced deafness. We also sequenced the complete mitochondrial genome of the Arab-Israeli pedigree. All four families shared a nucleotide 1555 A to G substitution in the 12S rRNA gene, a site implicated in aminoglycoside activity. Our study offers the first description of a mitochondrial rRNA mutation leading to disease, the first cases of non-syndromic deafness caused by a mitochondrial DNA mutation and the first molecular genetic study of antibiotic-induced ototoxicity.

Fractional Fourier transforms and their optical implementation. II

Abstract: The linear transform kernel for fractional Fourier transforms is derived. The spatial resolution and the space–bandwidth product for propagation in graded-index media are discussed in direct relation to fractional Fourier transforms, and numerical examples are presented. It is shown how fractional Fourier transforms can be made the basis of generalized spatial filtering systems: Several filters are interleaved between several fractional transform stages, thereby increasing the number of degrees of freedom available in filter synthesis.

Long-Term Stabilizing Effect of Angiotensin-Converting Enzyme Inhibition on Plasma Creatinine and on Proteinuria in Normotensive Type II Diabetic Patients

Abstract: Objective: To evaluate the long-term effect of angiotensin-converting enzyme inhibition on proteinuria and on the rate of decline in kidney function in patients with type II diabetes mellitus and m...

Constant depth circuits, Fourier transform, and learnability

Abstract: In this paper, Boolean functions in ,4C0 are studied using harmonic analysis on the cube. The main result is that an ACO Boolean function has almost all of its "power spectrum" on the low-order coefficients. An important ingredient of the proof is Hastad's switching lemma (8). This result implies several new properties of functions in -4C(': Functions in AC() have low "average sensitivity;" they may be approximated well by a real polynomial of low degree and they cannot be pseudorandom function generators. Perhaps the most interesting application is an O(n POIYIOg(n ')-time algorithm for learning func- tions in ACO. The algorithm observes the behavior of an AC'" function on O(nPO'Y'Og(n)) randomly chosen inputs, and derives a good approximation for the Fourier transform of the function. This approximation allows the algorithm to predict, with high probability, the value of the function on other randomly chosen inputs.

Clustering instability in dissipative gases

Abstract: It is shown that a gas composed of inelastically colliding particles is unstable to the formation of high density clusters. A possible physical mechanism underlying this instability is proposed. A theoretical analysis, based on the Jenkins-Richman equations, as well as a numerical simulation of the dynamics of an unforced system of hard disks in a periodic rectangular enclosure, renders support to the proposed mechanism. In particular, a simple formula for the characteristic intercluster distance is derived and found to be in agreement with the numerical results. Applications to granular systems of engineering interest as well as to astrophysics are briefly outlined.

Quantum mechanical interaction-free measurements

Abstract: A novel manifestation of nonlocality of quantum mechanics is presented. It is shown that it is possible to ascertain the existence of an object in a given region of space without interacting with it. The method might have practical applications for delicate quantum experiments.

Oscillatory Blowing: A Tool to Delay Boundary-Layer Separation

Abstract: The effects of oscillatory blowing as a means of delaying separation are discussed. Experiments were carried out on a follow, flapped NACA 01115 airfoil equipped with a two-dimensional slot over the hinge of the flap. The flap extended over 25% of the chord and was detected at angles as high as 40 deg. The steady blowing momentum coefficients could be varied independently of the amplitudes and frequencies of the superimposed oscillations. The modulated blowing was a major factor in improving the performance of the airfoil at much lower energy inputs than was hitherto known. Optimum benefits in performance were obtained at reduced frequencies, based on the flap chord, of an order of unity. Significant increase in lift as well as cancellation of form drag were observed

Chapter 8 New-product diffusion models

Abstract: Preface. Introduction. Part I: Overview. 1. New Product Diffusion Models: From Theory to Practice V. Mahajan, et al. Part II: Strategic, Global and Digital Environment for Diffusion Analysis. 2. Firm Strategy and Speed of Diffusion S. Kuester, et al. 3. Multi-Market and Global Diffusion M. Dekimpe, et al. 4. Innovation Adoption and Diffusion in the Digital Environment: Some Research Opportunities A. Rangaswamy, S. Gupta. Part III: Diffusion Models. 5. Modeling the Marketing Mix Influence in New Product Diffusion F. Bass, et al. 6. Diffusion Models with Replacement and Multiple Purchases B. Ratchford, et al. 7. Growth Models for Multi-Product Interactions: Current Status and New Directions B. Bayus, et al. 8. Dynamic Models Incorporating Competition R. Chatterjee, et al. 9. Disaggregate Level Diffusion Models J. Roberts, J. Lattin. 10. Operations Planning in the Presence of Innovation Diffusion Dynamics M.A. Cohen, et al. Part IV: Estimation. 11. Estimation Techniques for Macro Diffusion Models W.P. Putsis, V. Srinivasan. Part V: Applications and Software. 12. Diffusion Models: Managerial Applications and Software G.L. Lilien, et al. List of Contributors. About The Editors. References. Index.

Challenges to theory development in entrepreneurship research

Abstract: Why do some new ventures succeed while others fail? What is the essence of entrepreneurship? Who is most likely to become a successful entrepreneur and why? How do entrepreneurs make decisions? What market, regulatory, and organizational environments foster the most successful entrepreneurial activities? Entrepreneurship research is plagued by these and other fundamental unanswered questions, for which there does not exist a cohesive explanatory, predictive, or normative theory. In this article we identify major challenges for entrepreneurship theory development, and offer insights into promising directions for future research. Our conclusion suggests that it may be too ambitious to expect a complete and robust theory due to the interdisciplinary nature of entrepreneurship. However, we show that by integrating perspectives and by applying analytic, empirical and experimental tools from a range of fields, some of the fundamental questions can be answered.

Foveal ganglion cell loss is size dependent in experimental glaucoma.

Abstract: Thirty-two areas located in the temporal midperipheral retina were evaluated in whole-mount preparations from four monkeys with monocular experimental glaucoma. Diameter frequency distributions of remaining ganglion cells in the glaucomatous eye were compared with corresponding areas in the normal fellow eye. Large cells were significantly more vulnerable at each stage of cell damage as determined by linear-regression analysis. The magnitude of size-dependent loss was moderate at an early stage (20% loss), peaked at 50% total cell loss, and decreased in advanced damage (70% loss). In glaucomatous eyes, the lower retina had significantly more large cell loss than the corresponding areas of the upper retina. In optic nerve zones that matched the retinal areas studied, large axons selectively were damaged first. Psychophysical testing aimed at functions subserved by larger ganglion cells is recommended for detection and follow-up of early glaucoma; however, assessment of functions unique to small cells is more appropriate for detecting change in advanced glaucoma.

The theory of figural concepts

Abstract: The main thesis of the present paper is that geometry deals with mental entities (the so-called geometrical figures) which possess simultaneously conceptual and figural characters. A geometrical sphere, for instance, is an abstract ideal, formally determinable entity, like every genuine concept. At the same time, it possesses figural properties, first of all a certain shape. The ideality, the absolute perfection of a geometrical sphere cannot be found in reality. In this symbiosis between concept and figure, as it is revealed in geometrical entities, it is the image component which stimulates new directions of thought, but there are the logical, conceptual constraints which control the formal rigour of the process. We have called the geometrical figuresfigural concepts because of their double nature. The paper analyzes the internal tensions which may appear in figural concepts because of this double nature, development aspects and didactical implications.

Atomic snapshots of shared memory

Abstract: This paper introduces a general formulation of atomic snapshot memory, a shared memory partitioned into words written (updated) by individual processes, or instantaneously read (scanned) in its entirety. This paper presents three wait-free implementations of atomic snapshot memory. The first implementation in this paper uses unbounded (integer) fields in these registers, and is particularly easy to understand. The second implementation uses bounded registers. Its correctness proof follows the ideas of the unbounded implementation. Both constructions implement a single-writer snapshot memory, in which each word may be updated by only one process, from single-writer, n-reader registers. The third algorithm implements a multi-writer snapshot memory from atomic n-writer, n-reader registers, again echoing key ideas from the earlier constructions. All operations require Θ(n2) reads and writes to the component shared registers in the worst case. —Authors' Abstract

Factor Separation in Numerical Simulations.

Abstract: A simple method is developed for computing the interactions among various factors influencing the atmospheric circulations. It is shown how numerical simulations can be utilized to obtain the pure contribution of any factor to any predicted field, as well as the contributions due to the mutual interactions among two or more factors. The mathematical basis for n factors is developed, and it is shown that 2n simulations are required for the separation of the contributions and their possible interactions. The method is demonstrated with two central factors, the topography and surface fluxes, and their effect on the rainfall distribution for a cyclone evolution in the Mediterranean.

Multi-channel signal separation by decorrelation

Abstract: Identification of an unknown system and recovery of the input signals from observations of the outputs of an unknown multiple-input, multiple-output linear system are considered. Attention is focused on the two-channel case, in which the outputs of a 2*2 linear time invariant system are observed. The approach consists of reconstructing the input signals by assuming that they are statistically uncorrelated and imposing this constraint on the signal estimates. In order to restrict the set of solutions, additional information on the true signal generation and/or on the form of the coupling systems is incorporated. Specific algorithms are developed and tested. As a special case, these algorithms suggest a potentially interesting modification of Widrow's (1975) least-squares method for noise cancellation, where the reference signal contains a component of the desired signal. >

Super-exponential methods for blind deconvolution

Abstract: A class of iterative methods for solving the blind deconvolution problem, i.e. for recovering the input of an unknown possibly nonminimum-phase linear system by observation of its output, is presented. These methods are universal do not require prior knowledge of the input distribution, are computationally efficient and statistically stable, and converge to the desired solution regardless of initialization at a very fast rate. The effects of finite length of the data, finite length of the equalizer, and additive noise in the system on the attainable performance (intersymbol interference) are analyzed. It is shown that in many cases of practical interest the performance of the proposed methods is far superior to linear prediction methods even for minimum phase systems. Recursive and sequential algorithms are also developed, which allow real-time implementation and adaptive equalization of time-varying systems. >

Sensitive dependence on initial conditions

Abstract: It is shown that the property of sensitive dependence on initial conditions in the sense of Guckenheimer follows from the other two more technical parts of one of the most common recent definitions of chaotic systems. It follows that this definition applies to a broad range of dynamical systems, many of which should not be considered chaotic. We investigate the implications of sensitive dependence on initial conditions and its relation to dynamical properties such as rigidity, ergodicity, minimality and positive topological entropy. In light of these investigations and several examples which we exhibit, we propose a natural family of dynamical systems- chi -systems-as a better abstract framework for a general theory of chaotic dynamics.

About Coding without Restrictions for the Awgn Channel

Abstract: Many coded modulation constructions, such as lattice codes, are visualized as restricted subsets of an infinite constellation (IC) of points in the n-dimensional Euclidean space. The author regards an IC as a code without restrictions employed for the AWGN channel. For an IC the concept of coding rate is meaningless and the author uses, instead of coding rate, the normalized logarithmic density (NLD). The maximum value C/sub /spl infin// such that, for any NLD less than C/sub /spl infin//, it is possible to construct an PC with arbitrarily small decoding error probability, is called the generalized capacity of the AWGN channel without restrictions. The author derives exponential upper and lower bounds for the decoding error probability of an IC, expressed in terms of the NLD. The upper bound is obtained by means of a random coding method and it is very similar to the usual random coding bound for the AWGN channel. The exponents of these upper and lower bounds coincide for high values of the NLD, thereby enabling derivation of the generalized capacity of the AWGN channel without restrictions. It is also shown that the exponent of the random coding bound can be attained by linear ICs (lattices), implying that lattices play the same role with respect to the AWGN channel as linear-codes do with respect to a discrete symmetric channel. >

On the use of wavelet expansions in the method of moments (EM scattering)

Abstract: An approach which incorporates the theory of wavelet transforms in method-of-moments solutions for electromagnetic wave interaction problems is presented. The unknown field or response is expressed as a twofold summation of shifted and dilated forms of a properly chosen basis function, which is often referred to as the mother wavelet. The wavelet expansion can adaptively fit itself to the various length scales associated with the scatterer by distributing the localized functions near the discontinuities and the more spatially diffused ones over the smooth expanses of the scatterer. The approach is thus best suited for the analysis of scatterers which contain a broad spectrum of length scales ranging from a subwavelength to several wavelengths. Using a Galerkin method and subsequently applying a threshold procedure, the moment-method matrix is rendered sparsely populated. The structure of the matrix reveals the localized scale-fitting distribution long before the matrix equation is solved. The performance of the proposed discretization scheme is illustrated by a numerical study of electromagnetic coupling through a double-slot aperture. >

Growth factor function of vasoactive intestinal peptide in whole cultured mouse embryos

Abstract: Factors controlling central nervous system (CNS) growth immediately after neurulation are mostly unknown Vasoactive intestinal peptide (VIP) receptors are widely distributed in the embryonic nervous system, and VIP has trophic and mitogenic properties on embryonic neural tissues but inhibits growth and mitosis in certain tumours To address the potential effects of VIP on embryonic growth, we used whole postimplantation embryo cultures After a 4-h incubation, VIP stimulated growth, increasing somite number, embryonic volume, DNA and protein content, and number of cells in S-phase A VIP antagonist substantially inhibited these VIP-mediated increments in growth The VIP antagonist completely suppressed VIP-stimulated mitosis in the CNS while decreasing the same in non-neuronal tissues by 38% In vitro autoradiography revealed GTP-sensitive and GTP-insensitive VIP receptors which were differentially regulated in VIP antagonist-treated embryos The present study suggests that VIP acts as a growth factor on early postimplantation embryos through multiple VIP receptors that exhibit tissue-specific responses

Scale-invariant motion in intermittent chaotic systems

Abstract: We investigate the dynamics generated from iterated maps and analyze the motion in terms of the probabilistic continuous-time random-walk (CTRW) approach. Two different CTRW models are considered: (i) Particles jump between sites (turning points) or (ii) particles move at a constant velocity between sites and choose a new direction at random. For both models we study the mean-squared displacement 〈${\mathit{r}}^{2}$(t)〉 and the propagator P(r,t), the probability to be at location r at time t having started at the origin at t=0. Iterated maps are used to generate both dispersive and enhanced diffusion and the results are analyzed using the CTRW framework and scaling arguments. For the case of dispersive motion we discuss the problem of the stationary state and point out its relevance.

A primal-dual infeasible-interior-point algorithm for linear programming

Abstract: As in many primal—dual interior-point algorithms, a primal—dual infeasible-interior-point algorithm chooses a new point along the Newton direction towards a point on the central trajectory, but it does not confine the iterates within the feasible region. This paper proposes a step length rule with which the algorithm takes large distinct step lengths in the primal and dual spaces and enjoys the global convergence.

Factors predictive of long-term coronary heart disease mortality among 10,059 male Israeli civil servants and municipal employees. A 23-year mortality follow-up in the Israeli Ischemic Heart Disease Study.

Abstract: Over 10,000 male civil servants and municipal employees in Israel, aged 40 years and above, underwent an extensive clinical, biochemical, anthropometric, sociodemographic and psychosocial evaluation i