Showing papers by "Brown University published in 1982"

Fundamentals of interactive computer graphics

Abstract: Fundamentals of interactive computer graphics , Fundamentals of interactive computer graphics , مرکز فناوری اطلاعات و اطلاع رسانی کشاورزی

Isotope signatures associated with early meteoric diagenesis

Abstract: The environments in which carbonate diagenesis proceeds have been documented in previous studies of Holocene and late Pleistocene sediments and limestones on Barbados, West Indies. Variations in the carbon and oxygen isotopic composition of limestones, produced during early freshwater diagenesis, have been observed in this study to occur in specific patterns. Six potentially useful patterns emerge when one views stable isotope data within a stratigraphic framework: (1) the subaerial exposure surface is characterized by strongly 12C-enriched limestones. δ13C compositions of underlying limestones grow progressively heavier with increasing depth; (2) the subaerial exposure surface may also be marked by slight 18O-enrichment; (3) an abrupt shift in δ18O values may differentiate sediments above the exposure surface from those below; (4) sediments altered in the marine-meteoric mixing zone may be characterized by positive covariance between their δ18O and δ13C compositions; (5) the vadose-phreatic boundary may be marked by a sharp increase in δ13C values in the seaward portions of a fresh groundwater system; and (6) samples contemporaneously altered in a single fresh groundwater system within an areally restricted region should display a narrow range of δ18O and a wide range of δ13C compositions. Analysis of samples from five Palaeozoic and Mesozoic formations, which contained petrographic evidence of early freshwater diagenesis, showed that isotope patterns similar to those observed in Barbados limestones have been preserved in rocks as old as Mississippian. These isotope patterns could prove to be useful for identifying diagenetically induced porosity trends in carbonate rocks. They might be used to identify limestones diagenetically altered in meteoric environments, to identify mixing zone cements and dolomites, and to trace the regional and vertical distributions of early meteoric groundwater systems in ancient carbonate formations.

Lifetime of a quasiparticle in a two-dimensional electron gas

Abstract: We have investigated the inelastic Coulomb lifetime ${\ensuremath{\tau}}_{\mathrm{ee}}$ of a quasiparticle near to the Fermi surface of a two-dimensional electron gas. Within a perturbative approach based upon the random-phase approximation, we find that at low temperature $\frac{1}{{\ensuremath{\tau}}_{\mathrm{ee}}}$ behaves like ${T}^{2}\mathrm{ln}T$. Furthermore at small quasiparticle excitation energy, the leading contribution to $\frac{1}{{\ensuremath{\tau}}_{\mathrm{ee}}}$ is inversely proportional to the electronic density and does not depend upon the electric charge. Although the plasmon frequency goes to zero at long wavelength, plasmon emission contributes to the quasiparticle decay only when the quasiparticle excitation energy exceeds a certain threshold. The threshold becomes a small fraction of the Fermi energy in the high-density limit.

Collective excitations in semiconductor superlattices

Abstract: Electronic collective modes of a system of large numbers of equally spaced, parallel two-dimensional electron layers are discussed within a self-consistent-field approach. Plasmon dispersion relations for the finite system as well as for the infinite periodic system are obtained. It is shown that the optical-plasmon frequency of the periodic system goes into the known two- or three-dimensional limit, respectively, depending on whether $\mathrm{qa}\ensuremath{\gg}1$ or $\mathrm{qa}\ensuremath{\ll}1$, where $q$ is the wave number in the two-dimensional plane and $a$ is the layer spacing. Effect of a uniform static external magnetic field oriented normal to the two-dimensional layers, on the collective-mode spectrum, is discussed with the use of the self-consistent-field and hydrodynamic approximations. It is shown that magnetoplasmons, helicon, and Alfv\'en waves can all exist in such a periodic system under suitable conditions. The theory is generalized to a system where the alternate layers are electrons and holes. The relevance of these results to semiconductor superlattice systems (both types I and II) is pointed out.

Long-Term Submergence at 3°C of the Turtle, Chrysemys Picta Bellii, in Normoxic And Severely Hypoxic Water: I. Survival, Gas Exchange And Acid-Base Status

Abstract: Survival and blood acid-base status were measured on freshwater turtles ( Chrysemys picta bellii Gray) that were submerged at 3 °C in either aerated (high O 2 ) or N 2 -equilibrated (low O 2 ) water. Results from catheterized turtles, without access to air under these conditions, and from non-catheterized turtles which were either apnoeic (in high O 2 and low O 2 water) or in high O 2 water with access to air, are compared. Under the most adverse conditions (catheterized, submerged, low O 2 ), survival duration was 126±14 ( X ±S.E.) days, and 2 of the 10 turtles so treated were still alive after 177 days, although their condition was poor. Apnoeic, high O 2 turtles generally survived longer and were in better condition despite a skin fungus condition that selectively affected these animals. Six of ten non-catheterized high O 2 turtles were still alive after 189 days without breathing. All apnoeic turtles developed an acidosis which, except for a transient hypercapnia in low O 2 turtles, was a metabolic acidosis attributable to elevated lactic acid. Acidosis was most severe in low O 2 turtles in which peak plasma lactates exceeded 200 mM. High O 2 turtles, as judged by higher blood P O2 and lower lactate concentrations, were able to extract dissolved O 2 from the water and support a significant portion of their metabolic requirements by aerobic metabolism. Our data indicate that wintering turtles can remain alive for up to 6 months, even while totally anoxic and severely acidotic, but that the acid-base state and probably the recovery potential are significantly improved if dissolved O 2 is available for extrapulmonary uptake.

The general multimodal network equilibrium problem with elastic demand

Abstract: In this article, we consider the general multimodal traffic equilibrium model with elastic demands. The link travel costs may depend upon the entire load pattern and the travel demands associated with each origin-destination pair and mode may depend upon travel costs associated with every origin-destination pair and every mode of transportation. For this model, we define the concepts of user-optimality and equilibrium. We establish, by means of a constructive proof motivated by the theory of variational inequalities, the existence of a unique equilibrium under appropriate monotonicity conditions. We then show that the existence proof induces an algorithm for the computation of equilibrium traffic patterns. The algorithm proceeds by iteration, each step of which amounts to computing the equilibrium pattern for a single modal linear traffic equilibrium problem with elastic demands. We derive estimates for the speed of convergence.

Void nucleation effects on shear localization in porous plastic solids

Abstract: The influence of void nucleation occurring during the deformation history on shear localization is investigated by employing a constitutive model of a rate independent porous plastic solid. Both plastic strain controlled and stress controlled nucleation processes are simulated. Two deformation histories are considered, one corresponding to uniaxial tension and the other to plane strain tension. The enhanced triaxiality at the center of a neck is simulated by application of Bridgman's solution for the stress and deformation state at the minimum section of a necked bar. The destabilizing effect that arises when void nucleation is stress controlled and nucleation occurs over a narrow range of stress is illustrated. Results are also presented employing parameter values representative of spheroidized carbon steels employed in a recent experimental study carried out by Fisher [23] and the predictions of the model are discussed in light of the experimental observations.

A parameter adaptive control structure for linear multivariable systems

Abstract: This paper presents an adaptive control structure which can be used to assign all poles and zeros of a continuous-time linear multivariable system represented by an ( m \times m ) strictly proper transfer matrix T(s) , provided T(s) has no right-half plane zeros. The controller parameters can be directly estimated from input-output data. The paper also serves to point out the type of a priori information necessary for multivariable adaptive controller design. This information is a natural extension of that required in the scalar case.

Evolution of the Tharsis Province of Mars: The importance of heterogeneous lithospheric thickness and volcanic construction

Abstract: The Tharsis province of Mars is a broad region characterized by anomalously elevated topography, a positive free-air gravity anomaly, and extensive volcanic and tectonic activity. The evolution of this region has spanned up to 4 b.y. of Martian history. The traditional explanation of the Tharsis province is that uplift of the lithosphere caused by a thermal, chemical, or dynamical anomaly in the Martian mantle or crust led to lithospheric fracturing and to the later volcanic emplacement of thin plains units and large shields. By this explanation the majority of the topographic anomaly is due to uplift. The stress field predicted for lithospheric uplift, however, does not match the generally radial trends of observed extensional fractures. Strictly thermal or compositional explanations for the support of the Tharsis rise encounter problems with the magnitude and duration of the required lateral variations in mantle density. For these reasons, we propose a new model for the origin and evolution of the Tharsis province which emphasizes that the topography of Tharsis may be produced largely by construction, rather than uplift. The model is based on the premise that the elastic lithosphere of Mars was laterally heterogeneous early in Martian history; such heterogeneity is discernible later in Martian history from the variable tectonic response of the lithosphere to local surface loads. Stress due to both global and local causes was concentrated in zones of thin lithosphere, including at least a portion of the Tharsis region. As a result, fracturing was also concentrated in such zones and favored localization of volcanism by providing access to the surface for mantle-derived magma. The heating associated with volcanism maintained the lithosphere locally thin, so that further fracturing and volcanism were concentrated in the same area. The Tharsis rise was thus built primarily by volcanic construction. Most visible tectonic features, by this model, were produced by the response of the lithosphere to loading, not by uplift. Early in the history of Tharsis the lithosphere responded to volcanic loading by nearly local isostatic compensation, while later additional loads have been partly supported by the finite strength of the globally thick elastic lithosphere of Mars. This mechanism for the evolution of Tharsis led to permanent topographic and gravity highs and a greatly thickened crust in the Tharsis region. A major advantage of the model is that no anomalous dynamical or chemical properties need to be sustained in the Martian mantle beneath Tharsis for billions of years. The mantle beneath Tharsis would thus play a passive rather than an active role in the regional volcanic and tectonic activity, much like the role of the mantle beneath major midocean ridges on earth. This and other models for the origin and evolution of the Tharsis province can be further tested by establishing the detailed chronology of tectonic features in the region.

Three-dimensional structure of a photosynthetic membrane

Abstract: The photosynthetic membranes in a variety of organisms are involved in the conversion of energy from sunlight into chemically useful forms1–3. Rhodopseudomonas viridis is a purple non-sulphur photosynthetic bacterium offering unique advantages for the study of photosynthetic membranes. Its internal photosynthetic membranes form large flat sheets which, as first described by Giesbrecht and Drews4, contain subunits organized into a regular repeating pattern. The membranes are ideally suited to the use of Fourier image analysis techniques, and have been studied in two dimensions5,6. We have now determined the three-dimensional structure of photosynthetic membranes of R. viridis by electron microscopy. A large central structure protrudes from both surfaces of the membrane, and is surrounded by six smaller centres of mass. Comparison of these data with studies on the polypeptide composition of the membrane suggests a membrane in which a photosynthetic reaction centre is surrounded by light-harvesting bacteriochlorophyll protein complexes.

Scales of chiral symmetry breaking in quantum chromodynamics

Abstract: Computer simulations indicate that the range of the force responsible for chiral symmetry breaking in quantum chromodynamics is relatively short, independent of confinement, and depends strongly on the color representation of the quarks. Chiral-symmetry restoration temperatures are obtained for quarks in either the fundamental or the adjoint representations of color SU(2). Results on higher representations support the symmetry-breaking pattern ("tumbling") suggested by Raby, Dimopoulos, and Susskind.

Unbound bilirubin and kernicterus in low-birth-weight infants.

Abstract: Unbound bilirubin, bilirubin binding capacity, and bilirubin binding affinity were determined by the horseradish peroxidase method at the time of maximum hyperbilirubinemia and/or before exchange transfusions in 13 preterm infants who later died and had autopsies performed. Five of the 13 infants had kernicterus at autopsy. There were no significant differences in weight, gestational age, highest indirect bilirubin level, albumin concentration, severity of acidosis, use of assisted ventilation, sepsis, or other major clinical complications between the five infants with kernicterus and the eight infants without kernicterus. Compared with the eight nonkernicteric infants, the five kernicteric infants had significantly higher unbound bilirubin concentrations (13 +/- vs 27 +/- 9 nmoles/liter, respectively, P less than .05) and significantly lower bilirubin binding capacity and affinity. The data suggest an association between low bilirubin binding capacity and affinity, increased unbound bilirubin, and kernicterus in preterm infants with severe clinical complications.

Interactive Editing Systems: Part II

Abstract: Many daily tasks, whether done with conventional tools or with computers, can be wowed as editing tasks: tasks in which the state of some target entity is changed by the user. This article, Par t I of a two-part series, examines computer-based interactwe editing systems, which allow users to change the state of targets such as manuscripts and programs. This paper is a tutorial tha t defines terms and introduces issues for the novice, and provides a reference for the more knowledgeable reader The aun is to provide a comprehensive and systematic view of the features of typical systems, highlighting substantive similarities and differences. User and system views of the editing process are provided, a historical perspective is presented, and the functional capabilities of editors are discussed, with emphasis on user-level rather than implementation-level considerations.

Forskolin as an activator of cyclic AMP accumulation and lipolysis in rat adipocytes.

Abstract: Forskolin increased cyclic AMP accumulation in isolated adipocytes and markedly potentiated the elevation of cyclic AMP due to isoproterenol. In adipocyte membranes, forskolin stimulated adenylate cyclase activity at concentrations of 0.1 microM or greater. Forskolin did not affect the EC50 for activation of adenylate cyclase but did increase the maximal effect of isoproterenol. Neither the soluble nor particulate low-Km cyclic AMP phosphodiesterase activity was affected by forskolin. Low concentrations of forskolin (0.1-1.0 microM), which significantly elevated cyclic AMP levels, did not increase lipolysis, whereas similar increases in cyclic AMP levels due to isoproterenol elevated lipolysis. Forskolin did not inhibit the activation of triacylglycerol lipase by cyclic AMP-dependent protein kinase or the subsequent hydrolysis of triacylglycerol. Higher concentrations of forskolin (10-100 microM) did increase lipolysis. Both the increased cyclic AMP production and lipolysis due to forskolin were inhibited by the antilipolytic agents insulin and N6-(phenylisopropyl)adenosine. Hypothyroidism reduced the ability of forskolin to stimulate cyclic AMP production and lipolysis. These results indicate that forskolin increases cyclic AMP production in adipocytes through an activation of adenylate cyclase. Lipolysis is activated by forskolin but at higher concentrations of total cyclic AMP than for catecholamines.