Showing papers by "Massachusetts Institute of Technology published in 1975"

The entity-relationship model: toward a unified view of data

Abstract: A data model, called the entity-relationship model, is proposed. This model incorporates some of the important semantic information about the real world. A special diagrammatic technique is introduced as a tool for database design. An example of database design and description using the model and the diagrammatic technique is given. Some implications for data integrity, information retrieval, and data manipulation are discussed.The entity-relationship model can be used as a basis for unification of different views of data: the network model, the relational model, and the entity set model. Semantic ambiguities in these models are analyzed. Possible ways to derive their views of data from the entity-relationship model are presented.

Optimum Consumption and Portfolio Rules in a Continuous-Time Model*

Abstract: Publisher Summary A common hypothesis about the behavior of limited liability asset prices in perfect markets is the random walk of returns or in its continuous-time form the geometric Brownian motion hypothesis, which implies that asset prices are stationary and log-normally distributed. A number of investigators of the behavior of stock and commodity prices have questioned the accuracy of the hypothesis. In an earlier study described in the chapter, it was examined that the continuous-time consumption-portfolio problem for an individual whose income is generated by capital gains on investments in assets with prices assumed to satisfy the “geometric Brownian motion” hypothesis. Under the additional assumption of a constant relative or constant absolute risk-aversion utility function, explicit solutions for the optimal consumption and portfolio rules were derived. The changes in these optimal rules with respect to shifts in various parameters such as expected return, interest rates, and risk were examined by the technique of comparative statics. This chapter presents an extension of these results for more general utility functions, price behavior assumptions, and income generated also from noncapital gains sources. If the geometric Brownian motion hypothesis is accepted, then a general separation or mutual fund theorem can be proved such that, in this model, the classical Tobin mean-variance rules hold without the objectionable assumptions of quadratic utility or of normality of distributions for prices. Hence, when asset prices are generated by a geometric Brownian motion, the two-asset case can be worked on without loss of generality.

Spectral asymmetry and Riemannian geometry. III

Abstract: 1. Introduction. The main purpose of this paper is to present a generalization of Hirzebruch's signature theorem for the case of manifolds with boundary. Our result is in the framework of Riemannian geometry and can be viewed as analogous to the Gauss–Bonnet theorem for manifolds with boundary, although there is a very significant difference between the two cases which is, in a sense, the central topic of the paper. To explain this difference let us begin by recalling that the classical Gauss–Bonnet theorem for a surface X with boundary Y asserts that the Euler characteristic E(X) is given by a formula:where K is the Gauss curvature of X and σ is the geodesic curvature of Y in X. In particular if, near the boundary, X is isometric to the product Y x R+, the boundary integral in (1.1) vanishes and the formula is the same as for closed surfaces. Similar remarks hold in higher dimensions. Now if X is a closed oriented Riemannian manifold of dimension 4, there is another formula relating cohomological invariants with curvature in addition to the Gauss–Bonnet formula. This expresses the signature of the quadratic form on H2(X, R) by an integral formulawhere p1 is the differential 4-form representing the first Pontrjagin class and is given in terms of the curvature matrix R by p1 = (2π)−2Tr R2. It is natural to ask if (1.2) continues to hold for manifolds with boundary, provided the metric is a product near the boundary. Simple examples show that this is false, so that in general

The protection of information in computer systems

Abstract: This tutorial paper explores the mechanics of protecting computer-stored information from unauthorized use or modification. It concentrates on those architectural structures-whether hardware or software-that are necessary to support information protection. The paper develops in three main sections. Section I describes desired functions, design principles, and examples of elementary protection and authentication mechanisms. Any reader familiar with computers should find the first section to be reasonably accessible. Section II requires some familiarity with descriptor-based computer architecture. It examines in depth the principles of modern protection architectures and the relation between capability systems and access control list systems, and ends with a brief analysts of protected subsystems and protected objects. The reader who is dismayed by either the prerequisites or the level of detail in the second section may wish to skip to Section III, which reviews the state of the art and current research projects and provides suggestions for further reading.

On the Relative Importance of the Driving Forces of Plate Motion

Abstract: Summary A number of possible mechanisms have recently been proposed for driving the motions of the lithospheric plates, such as pushing from mid-ocean ridges, pulling by downgoing slabs, suction toward trenches, and coupling of the plates to flow in the mantle. We advance a new observational method of testing these theories of the driving mechanism. Our basic approach is to solve the inverse problem of determining the relative strength of the plausible driving forces, given the observed motions and geometries of the lithospheric plates. Since the inertia of the plates is negligible, each plate must be in dynamic equilibrium, so that the sum of the torques acting on a plate must be zero. Thus, our problem is to determine the relative sizes of the forces that minimize the components of net torque on each plate. The results indicate that the forces acting on the downgoing slab control the velocity of the oceanic plates and are an order of magnitude stronger than any other force. Namely, all the oceanic plates attached to substantial amounts of downgoing slabs move with a ' terminal velocity ' at which the gravitational body force pulling the slabs downward is nearly balanced with the resistance acting on the slab; regardless of the other features of the trailing horizontal part of the plates. The drag on the bottom of the plates which resist motion is stronger under the continents than under the oceans.

Dynamics of deserts and drought in the Sahel

Abstract: It is suggested that the high albedo of a desert contributes to a net radiative heat loss relative to its surroundings and that the resultant horizontal temperature gradients induce a frictionally controlled circulation which imports heat aloft and maintains thermal equilibrium through sinking motion and adiabatic compression. In the subtropics this sinking motion is superimposed on the descending branch of the mean Hadley circulation but is more intense. Thus the desert feeds back upon itself in an important manner. If one takes into account the biosphere, this feedback mechanism could conceivably lead to instabilities or metastabilities in desert border regions. It is argued that a reduction of vegetation, with consequent increase in albedo, in the Sahel region at the southern margin of the Sahara would cause sinking motion, additional drying, and would therefore perpetuate the arid conditions. Numerical integrations with the general circulation model of NASA's Goddard Institute for Space Studies appear to substantiate this hypothesis. Increasing the albedo north of the ITCZ from 14% to 35% had the effect of shifting the ITCZ several degrees of latitude south and decreasing the rainfall in the Sahel about 40% during the rainy season.

Universal codeword sets and representations of the integers

Abstract: Countable prefix codeword sets are constructed with the universal property that assigning messages in order of decreasing probability to codewords in order of increasing length gives an average code-word length, for any message set with positive entropy, less than a constant times the optimal average codeword length for that source. Some of the sets also have the asymptotically optimal property that the ratio of average codeword length to entropy approaches one uniformly as entropy increases. An application is the construction of a uniformly universal sequence of codes for countable memoryless sources, in which the n th code has a ratio of average codeword length to source rate bounded by a function of n for all sources with positive rate; the bound is less than two for n = 0 and approaches one as n increases.

Kinetics of strain-induced martensitic nucleation

Abstract: Intersections of shear bands in metastable austenites have been shown to be effective sites for strain-induced martensitic nucleation. The shear bands may be in the form of e’ (hcp) martensite, mechanical twins, or dense bundles of stacking faults. Assuming that shear-band intersection is the dominant mechanism of strain-induced nucleation, an expression for the volume fraction of martensite vs plastic strain is derived by considering the course of shear-band formation, the probability of shear-band intersections, and the probability of an intersection generating a martensitic embryo. The resulting transformation curve has a sigmoidal shape and, in general, approaches saturation below 100 pct. The saturation value and rate of approach to saturation are determined by two temperature-dependent parameters related to the fee-bee chemical driving force and austenite stacking-fault energy. Fitting the expression to available data on 304 stainless steels gives good agreement for the shape of individual transformation curves as well as the temperature dependence of the derived parameters. It is concluded that the temperature dependence of the transformation kinetics (an important problem in the development of TRIP steels) may be minimized by decreasing the fee, bec, and hep entropy differences through proper compositional control.

The spectrum of positive elliptic operators and periodic bicharacteristics

Abstract: Let X be a compact boundaryless C ∞ manifold and let P be a positive elliptic self-adjoint pseudodifferential operator of order m>0 on X. For technical reasons we will assume that P operates on half-densities rather than functions.

Survey of eye movement recording methods

Abstract: This paper reviews most of the known techniques for measuring eye movements, explaining their principle of operation and their primary advantages and disadvantages. The five sections of the paper cover the following topics: (1) types of eye movement, (2) characteristics of the eye which lend themselves to measurement and the principal approaches to the measurement of eye movement, (3) practical methods of measurement with especial attention to the new techniques, (4) general considerations guiding a selection of method, and (5) summarizing of the major findings in a concise table.

The Analysis of Sentence Production1

Abstract: Publisher Summary This chapter presents an analysis of sentence production, by distinguishing between the general problem of language production, which must include message formulation, and the specific problem of sentence production, which may be viewed as translation process. The chapter assumes that somewhere in the recesses of the central nervous system, an interaction takes place among the current motor and perceptual experiences, stored information, motivational systems, and various other variables. This interaction gives rise to a communicative intention, also termed as message. This message has to be translated into a set of instructions sufficient to guide the articulatory apparatus. The final point that should be made from the analysis of sentence production is that even if one to has good evidence about the vocabulary of the computational system that mediates the translation from messages into their realization as instructions to the articulatory system, one would still be unsatisfied. The reason of this dissatisfaction is that one would want to be able to characterize the information flow in the system in terms of interactions between the various structural types represented in the computational vocabulary.

Cavity formation from inclusions in ductile fracture

Abstract: The previously proposed conditions for cavity formation from equiaxed inclusions in ductile fracture have been examined. Critical local elastic energy conditions are found to be necessary but not sufficient for cavity formation. The interfacial strength must also be reached on part of the boundary. For inclusions larger than about 100A the energy condition is always satisfied when the interfacial strength is reached and cavities form by a critical interfacial stress condition. For smaller cavities the stored elastic energy is insufficient to open up interfacial cavities spontaneously. Approximate continuum analyses for extreme idealizations of matrix behavior furnish relatively close limits for the interfacial stress concentration for strain hardening matrices flowing around rigid non-yielding equiaxed inclusions. Such analyses give that in pure shear loading the maximum interfacial stress is very nearly equal to the equivalent flow stress in tension for the given state of plastic strain. Previously proposed models based on a local dissipation of deformation incompatibilities by the punching of dislocation loops lead to rather similar results for interfacial stress concentration when local plastic relaxation is allowed inside the loops. At very small volume fractions of second phase the inclusions do not interact for very substantial amounts of plastic strain. In this regime the interfacial stress is independent of inclusion size. At larger volume fractions of second phase, inclusions begin to interact after moderate amounts of plastic strain, and the interfacial stress concentration becomes dependent on second phase volume fraction. Some of the many reported instances of inclusion size effect in cavity formation can thus be satisfactorily explained by variations of volume fraction of second phase from point to point.

Masses and Other Parameters of the Light Hadrons

Abstract: The masses and static parameters of the light hadrons (the pseudoscalar and vector meson nonets and the baryon octet and decuplet) are calculated using the bag model. The effects of quark kinetic energy, bag energy, strange-quark mass, colored-gluon exchange in lowest order, and energy associated with certain quantum fluctuations are included. These are parameterized by four constants which have fundamental significance and do not change from multiplet to multiplet. The fit to the spectrum is good. The ordering of all the states is given correctly. The intramultiplet splittings are very accurate for the baryon decuplet and reasonably good for the other multiplets. The isosinglet pseudoscalar mesons eta and eta' are treated separately in light of the special role that gluon intermediate states play in these states. Magnetic moments, weak decay constants, and charge radii are calculated. Where comparison with experiment is possible, we generally obtain improvement over the naive quark model. It is shown that exotic baryons do not exist as narrow resonances in this approximation to the bag theory. (AIP)

Police organization: A longitudinal examination of job attitudes in an urban police department.

Abstract: This research was supported in part by the Organizational Behavior Research Center at the University of California, Irvine and the Office of Naval Research (Contract No. N00014-69-A-0200-9001 NR 151-315). I would like to gratefully acknowledge my academic colleagues, in particular Lyman W. Porterand Mason Haire fortheir insightful suggestions and assistance during various phases of this research.

Bad data analysis for power system state estimation

Abstract: The state estimation problem in electric power systems consists of four basic operations: hypothesize structure; estimate; detect; identify. This paper addresses the last two problems with respect to the bad data and structural error problem. The paper interrelates various detection and identification methods (sum of squared residuals, weighted and normalized residuals, nonquadratic criteria) and presents new results on bad data analysis (probability of detection, effect of bad data). The theoretical results are illustrated by means of a 25 bus network.

Meaning in visual search.

Abstract: Viewers briefly glimpsed pictures presented in a sequence at rates up to eight per second. They recognized a target picture as accurately and almost as rapidly when they knew only its meaning given by a name (for example, a boat) as when they had seen the picture itself in advance.

Voice onset time, frication, and aspiration in word-initial consonant clusters.

Abstract: The voice onset time (VOT) and the duration of the burst of frication noise at the release of a plosive consonant were measured from spectrograms of word-initial consonant clusters. Mean data from ...

Drought in the sahara: a biogeophysical feedback mechanism.

Abstract: Two integrations of a global general circulation model, differing only in the prescribed surface albedo in the Sahara, show that an increase in albedo resulting from a decrease in plant cover causes a decrease in rainfall. Thus any tendency for plant cover to decrease would be reinforced by a decrease in rainfall, and could initiate or perpetuate a drought.

Time to understand pictures and words

Abstract: WHEN an object such as a chair is presented visually, or is represented by a line drawing, a spoken word, or a written word, the initial stages in the process leading to understanding are clearly different in each case. There is disagreement, however, about whether those early stages lead to a common abstract representation in memory, the idea of a chair1–4, or to two separate representations, one verbal (common to spoken and written words), and the other image-like5. The first view claims that words and images are associated with ideas, but the underlying representation of an idea is abstract. According to the second view, the verbal representation alone is directly associated with abstract information about an object (for example, its superordinate category: furniture). Concrete perceptual information (for example, characteristic shape, colour or size) is associated with the imaginal representation. Translation from one representation to the other takes time, on the second view, which accounts for the observation that naming a line drawing takes longer than naming (reading aloud) a written word6,7. Here we confirm that naming a drawing of an object takes much longer than reading its name, but we show that deciding whether the object is in a given category such as ‘furniture’ takes slightly less time for a drawing than for a word, a result that seems to be inconsistent with the second view.

Shape of the 5 mm oxygen band in the atmosphere

Abstract: The problem of absorption of microwaves by molecular oxygen in the atmosphere is treated by means of a first-order approximation to the impact theory of overlapping spectral lines. By including only the coupling between adjacent rotational states in molecular collisions, we have devised a simple approximate method for computing the interference between lines from measurements on the resolved lines. The need for an empirically determined function describing the linewidth/ pressure ratio is eliminated. Comparisons with measurements at atmospheric pressures show that the first-order interference accounts for the low absorption at the band wings near 1 atm pressure. It also predicts the correct amount of asymmetry between high and low frequency wings. Improvement over previous models for the pressure broadening is obtained at frequencies \gsim 55 GHz. This approach is not specific to oxygen and could be adapted to other similar molecules.

The Early Structural Evolution and Anisotropy of the Oceanic Upper Mantle

Abstract: Summary The dispersion of Love and Rayleigh waves in the period range 17–167 s is used to detect the change in the structure of the upper mantle as the age of the sea-floor increases away from the mid-ocean ridge Using the single station method, the group and phase velocities of Rayleigh waves were measured for 78 paths in the east Pacific In order to describe the observed Rayleigh wave dispersion, both a systematic increase in velocities with the age of the sea-floor and anisotropy of propagation are required The maximum change in velocity with age is about 5 per cent, with the contrast between age zones decreasing with increasing period The greatest change occurs in the first few million years, due to the rapid cooling and solidification of the upper part of the lithosphere In the 0–5 My age zone, the average thickness of the lithosphere can be no greater than 30 km, including the water and crustal layers Within 10 My after formation, the lithosphere reaches a thickness of about 60 km As the mantle continues to cool, the shear velocity within the lithosphere increases Within the area of this study, no change occurs in the upper mantle deeper than about 80 km Rayleigh waves travel fastest in the direction of spreading The degree of anisotropy in Rayleigh wave propagation is frequency-dependent, reaching a maximum of 2–0±0–2 per cent at a period of about 70 s Several models are constructed which can reproduce this frequency-dependent anisotropy The regional phase velocities of the fundamental and first higher Love modes have been simultaneously measured using a new technique The Love wave data is inconsistent with the Rayleigh wave data unless SH velocity is higher than SV velocity within the uppermost 125 km of the mantle Anisotropy deeper than 250 km is suggested, but not required, by the data

Optimum testing of multiple hypotheses in quantum detection theory

Abstract: The problem of specifying the optimum quantum detector in multiple hypotheses testing is considered for application to optical communications. The quantum digital detection problem is formulated as a linear programming problem on an infinite-dimensional space. A necessary and sufficient condition is derived by the application of a general duality theorem specifying the optimum detector in terms of a set of linear operator equations and inequalities. Existence of the optimum quantum detector is also established. The optimality of commuting detection operators is discussed in some examples. The structure and performance of the optimal receiver are derived for the quantum detection of narrow-band coherent orthogonal and simplex signals. It is shown that modal photon counting is asymptotically optimum in the limit of a large signaling alphabet and that the capacity goes to infinity in the absence of a bandwidth limitation.

Representation theory of artin algebras iii almost split sequences

Abstract: (1975). Representation theory of artin algebras iii almost split sequences. Communications in Algebra: Vol. 3, No. 3, pp. 239-294.

Trickle-bed reactors

Abstract: The term trickle bed is used here to mean a reactor in which a liquid phase and a gas phase flow concurrently downward through a fixed bed of catalyst particles while reaction takes place. The earliest antecedent probably was the so-called “trickling filter” which has long been used for removal of organic matter from wastewater streams by aerobic bacterial action. Biological growths are allowed to attach themselves to a bed of stone or other support over which the wastewater is allowed to trickle in contact with air. Another antecedent appeared in the 1930s when Duftschmid and co-workers at the I.G. Farbenindustrie used evaporative cooling of an inert oil which flowed upwards through a catalyst bed to remove the heat of reaction in a version of the FischerTropsch process for synthesis of liquid fuels from H2 and CO and a later version at the U.S. Bureau of Mines operated with concurrent upflow but without evaporative cooling. This was superceded by an ebulliating bed reactor because of difficulties with cementation of the catalyst in the fixed bed, but commercial scale operations of this process in Germany and elsewhere have all utilized completely vapor-phase reaction (Storch et al., 1951; Crowell et al., 1950; Benson et al., 1954; Kastens et al., 1952). Trickle-bed reactors have been used to a moderate extent in chemical processing, but most of the published information about their industrial applications concerns the processing with hydrogen of various petroleum fractions, in particular the hydrodesulfurization or hydrocracking of heavy or residual oil stocks and the hydrofinishing or hydrotreating of lubricating oils. Their commercial development during the 1950s by the Shell Companies and by the British Petroleum Company has been described by van Deemter (1964), Le Nobel and Choufoer ( 1959), and Lister (1964). The development of hydrocracking and hydrodesulfurization processes by Chevron, Esso (Exxon), Gulf, Union Oil, and others has been described in the proceedings of the various World Petroleum Congresses. Information is fragmentary on uses in chemical processing. A process for synthesis of butynediol (HOCH2C = CCH20H) from aqueous formaldehyde and acetylene uses trickle-bed flow over a copper acetylide catalyst and recycle of the product stream for heat removal (Brusie et al., 1963). Other trickle-bed studies of this reaction are given in a thesis by W. Bill, as reported by Bondi (1971), who also cites trickle-bed studies by H. Hofmann on the hydrogenation of glucose to sorbitol. Kronig (1963) describes a trickle-bed process used in one or more commercial plants for selective hydrogenation of acetylene to remove it in the presence of butadiene in Cq hydrocarbon streams. Operation at 10” to 20°C and 1.9 to 5.8 atm pressure allows liquid-phase processing which reportedly gives long catalyst life, unlike gas-phase processing in which polymers rapidly build up on the catalyst. The standard commercial process for manufacture of hydrogen peroxide utilizes a working solution of an alkyl