Showing papers by "Brown University published in 1978"

A dichromatic framework for balanced trees

Abstract: In this paper we present a uniform framework for the implementation and study of balanced tree algorithms. We show how to imbed in this framework the best known balanced tree techniques and then use the framework to develop new algorithms which perform the update and rebalancing in one pass, on the way down towards a leaf. We conclude with a study of performance issues and concurrent updating.

Mechanics of thin-skinned fold-and-thrust belts

Abstract: The essential characteristics of thin-skinned fold-and-thrust belts include the following: a wedge-shaped deforming region, thicker at the back end from which the thrusts come; a weak layer at the base of the wedge; and large amounts of shortening and thickening within the wedge. All these characteristics are incorporated into an analytical model of a perfectly plastic wedge, underlain by a weak basal layer and yielding in compressive flow. The model is sufficiently flexible to be applied to a variety of tectonic situations. In each situation the model shows that shortening of the wedge and sliding over its base can occur for reasonable values of the yield stress of the wedge and of the weak basal layer, of the surface topographic slope, of the back slope of the basal layer, and of the thickness of the wedge. The present model differs markedly from the conventional concept of gravity gliding in that it shows that horizontal compressive stresses can play a major role in overcoming the resistance to basal sliding. In addition, the model suggests that the topographic slope, when necessary, can be a result of the compression within the wedge itself; no additional outside agency for producing the topographic slope need be postulated. Thus, the model suggests that a surface topographic slope is in general neither a necessary nor a sufficient condition for the formation of a thin-skinned fold-and-thrust belt.

The McMaster Model of Family Functioning.

Abstract: The model of family functioning being presented is the product of over twenty years of research in clinical work with family units. A method of family therapy has been developed based upon this model and will be described in a future issue. The model utilizes a general systems theory approach in an attempt to describe the structure, organization, and transactional patterns of the family unit. It allows examination of families along the total spectrum ranging from healthy to severely pathological in their functioning. It is based upon a Judaeo-Christian value set, and allows attention to be paid to cultural differences and other issues of cultural relativity.

Nuclear magnetic resonance studies of the glasses in the system Na2OB2O3SiO2

Abstract: The 11B NMR spectra have been used to study the structure of glasses in the system Na2OB2O3SiO2. The fraction of BO4 units, and the fraction of BO3 units with one or two nonbridging oxygens, are measured and analyzed according to a structural model. The results indicate that: (1) for a sodium oxide to boron oxide ratio of 0.5 or less, the Na+1 ions are attracted primarily by the borate network; therefore, the ternary glasses can be viewed as binary sodium borate glasses diluted by SiO2; (2) when the sodium oxide to boron oxide ratio exceeds 0.5, the additional Na2O results in the formation of [BSi4O10]−1 units at the expense of diborate and SiO4 units. In this process, Na+1 ions are still taken up only by the borate network. After all the available SiO4 units are consumed to form [BSi4O10]−1 units, additional Na+1 ions are proportionally shared between the borate and silicate networks.

Limits to ductility set by plastic flow localization

Abstract: The theory of strain localization is reviewed with reference both to local necking in sheet metal forming processes and to more general three dimensional shear band localizations that sometimes mark the onset of ductile rupture. Both bifurcation behavior and the growth of initial imperfections are considered. In addition to analyses based on classical Mises-like constitutive laws, we discuss approaches to localization based on constitutive models that may more accurately model processes of slip and progressive rupturing on the microscale in structural alloys. Among these non-classical constitutive features are the destabilizing roles of yield surface vertices and of non-normality effects, arising, for example, from slight pressure sensitivity of yield. We also discuss analyses based on a constitutive model of a progressively cavitating dila- tional plastic material which is intended to model the process of ductile void growth in metals. A variety of numerical results are presented. In the context of the three dimensional theory of localization, we show that a simple vertex model predicts ratios of ductility in plane strain tension to ductility in axisymmetric tension qualitatively consistent with experiment.. We also illustrate the destabilizing influence of a hydrostatic stress dependent void nucleation criterion. In the sheet necking context, and focussing on positive biaxial stretching, it is shown that forming limit curves based on a simple vertex model and those based on a simple void growth model are qualitatively in accord, although attributing instability to very different physical mechanisms. These forming limit curves are compared with those obtained from the Mises material model and employing various material and geometric imperfections.

Implementing Quicksort programs

Abstract: This paper is a practical study of how to implement the Quicksort sorting algorithm and its best variants on real computers, including how to apply various code optimization techniques. A detailed implementation combining the most effective improvements to Quicksort is given, along with a discussion of how to implement it in assembly language. Analytic results describing the performance of the programs are summarized. A variety of special situations are considered from a practical standpoint to illustrate Quicksort's wide applicability as an internal sorting method which requires negligible extra storage.

Continuing crack-tip deformation and fracture for plane-strain crack growth in elastic-plastic solids

Abstract: Analysis of the deformation field consistent with a Prandtl stress distribution travelling with an advancing plane-strain crack reveals the functional form of the near tip crack profile in an elastic-plastic solid. The crack opening δ is shown to have the form δ ∼ r In (const./r) at a distance r from the tip. This observation coupled with data generated from finite element investigations of growing cracks in small-scale yielding permits the construction of a relation characterizing the deformation at an extending crack tip. A ductile crack-growth criterion consisting of the attainment of a critical opening at a small characteristic material distance from the tip is adopted. Predictions of the stability of a growing crack for both small-scale yielding specimens and those subject to general yielding are discussed.

Thermodynamics of the quasi-static growth of Griffith cracks

Abstract: R estrictions on the quasi-static extension, or healing, of Griffith cracks are developed in the framework of irreversible thermodynamics. It is emphasized that thermodynamics requires that (G − 2γ)ι ⩾ 0, where ι is crack speed, G the Irwin energy release rate, and 2γ the work of reversible separation of the surfaces to be fractured. Implications for ‘lattice trapping’ models of cracks and for thermally-activated crack motion are discussed, as are the effects on crack growth and healing of a surface-reactive environment, in which case γ must be given a definition appropriate to adsorption-altered surface properties.

On feedback stabilization of time-varying discrete linear systems

Abstract: Results are given for stabilizing time-varying discrete linear systems by means of a feedback control stemming from a receding-horizon concept and a minimum quadratic cost with a fixed terminal constraint. The results parallel those recently obtained for continuous-time systems [8] and extend a well-known method of Kleinman for stabilizing discrete fixed linear systems [7].

Planar Geometric Projections and Viewing Transformations

Abstract: In computer graphics one is often concerned with representing three-dimensional objects on a two-dimensional display surface. The choice of such a representation depends on several factors, including the purpose for which the representation is intended, the visual effects tha t are desired, and the shape of the object. This paper describes how twodimensional views can be obtained using planar geometric projections such as perspective and parallel projections. It discusses how these projections can be generated from a threedimensional representation of an object in a manner suitable for computer graphics systems. In particular, it shows how these projections can be generated using the viewing transformations of the Core Graphics System. The factors tha t affect the choice of projection are also discussed, and some guidelines for making such a choice are given.

Psoriasis and occlusive vascular disease

Abstract: To test the hypothesis that psoriasis is associated with an increased incidence of occlusive vascular disease (thrombophlebitis, myocardial infarction, pulmonary embolization, and cerebrovascular accident), the clinical records of 323 psoriatic and 325 non-psoriatic patients admitted to the dermatology service of the Roger Williams General Hospital were examined. The data obtained in this study suggest that (1) the occurrence rate of occlusive vascular disease is significantly greater in the psoriatic than in the non-psoriatic dermatological patient. This is particularly true in the male population; (2) psoriasis predisposes to occlusive vascular disease; and (3) the psoriatic patient with certain predisposing factors is at greater risk of experiencing an occlusive vascular episode than both the non-predisposed psoriatic and the non-psoriatic dermatological patient.

Asymptotic Behavior of Solutions of Evolution Equations

Abstract: Publisher Summary This chapter presents the methods of investigation of the asymptotic behavior of solutions of evolution equations, endowed with a dissipative mechanism, based on the study of the structure of the ω-limit set of trajectories of the evolution operator generated by the equation. The dissipative mechanism usually manifests itself by the presence of a Liapunov functional, which is constant on ω -limit sets; the central idea of the approach presented in the chapter is to use this information in conjunction with properties of ω -limit sets such as invariance and minimality. The chapter discusses two examples of wave equations with weak damping for which the scheme set up by Hale applies. In particular, this requires that the Liapunov functional be continuous on phase space. The chapter explores the case of a hyperbolic conservation law that generates a semigroup on space. It also presents a survey of various applications and extensions of these ideas that may serve as a guide to those interested in learning more about the method.

Corticosteroid effect on immunoglobulins

Abstract: The corticosteroid (prednisone) effect on serum immunoglobulins in 9 atopic asthmatic patients who required corticosteroids for the control of asthma was evaluated. Serum immunoglobulins were determined before corticosteroids were administered, an average of 15 days while on corticosteroids, and again an average of 22 days after corticosteroids were discontinued. While on corticosteroids (averaging 16.8 mg prednisone daily) for 15 days, mean serum IgG was significantly decreased (-22%, p less than or equal to 0.01), mean serum IgA tended to be decreased (-10%), and mean serum IgM was essentially unchanged. Serum IgE was initially significantly increased (p less than 0.01) when compared to levels of other serum immunoglobulins (IgG,A,M). An average of 22 days after corticosteroids were discontinued, mean serum IgG was still significantly decreased (p less than 0.05), and mean serum IgA again tended to be decreased. Serum IgM remained unchanged and mean serum IgE now was significantly decreased (p less than 0.01). Corticosteroids appear to have a significant effect on levels of some serum immunoglobulins.

Measurement of surface recombination velocity in semiconductors by diffraction from picosecond transient free-carrier gratings

Abstract: Direct measurement of surface recombination velocity has been achieved for n‐InP, p‐InP, and n‐GaAs by a novel technique which is based on picosecond optical resolution of a transient diffraction grating, formed by an excess electron‐hole plasma near the surface of the semiconductor The influence of diffusion, bulk recombination, and surface recombination on the carrier density are included in the analysis of the experiment, which shows that the method is specifically sensitive to surface recombination velocity Effects of the plasma density on the diffusion coefficient and the carrier lifetime are discussed

Mixed alkali glass spectra and structure

Abstract: The far infrared and Raman spectra of several series of mixed alkali metaphosphate glasses have been investigated as a function of the mole fraction x of the network-modifying ionic oxides in xM2O(1−x)M2′O · P2O5 The frequencies of the cation-motion bands in the far infrared spectra, which correspond to cationsite vibrations, do not shift with x, indicating that the vibrationally significant local geometry and forces associated with a particular cation are unaffected by the introduction of the second cation into the glass structure Each Raman-active band due to vibrations of the metaphosphate network occurs at a different frequency for each pure glass (x = 0 or 1), but for mixed alkali glasses only one band occurs for each type of mode and it varies linearly with x This indicates that the cations in these mixed alkali glasses are homogeneously distributed, there is no significant molecular-level domain formation and the phosphate chains are associated with an averaged cation environment whose effect on the chain modes varies with x A simple vibrational model is presented which shows that the cation-dependent shifts are due to small changes in network bond angles and variation of the cationsite forces

Distributed Processing

Abstract: This issue of Computer is based on two workshops in distributed processing held at Brown University August 17-19, 1976, and August 3-5, 1977. Sponsored by the Army Research Office, the National Science Foundation, and the Office of Naval Research, the workshops attempted to define what distributed processing means and to develop a taxonomy of distributed processing applications and techniques. Achievements to date and outstanding research problems were examined in an attempt to find either commonality of problems and solutions or substantial differences.