Showing papers by "University of Maryland, College Park published in 1970"
711 citations
TL;DR: It is shown that every simply-connected object in such a picture has elements which can be deleted without destroying its simple- connectedness, which makes it easy to prove that a well-known "shrinking" algorithm always works--that is, shrinks any simply- connected object down to a single element.
Abstract: Natural concepts of connectedness and simple-connectedness are defined for subsets of a digital picture. It is shown that every simply-connected object (with more than one element) in such a picture has elements which can be deleted without destroying its simple- connectedness. This makes it easy to prove that a well-known "shrinking" algorithm always works--that is, shrinks any simply-connected object down to a single element. It also becomes easy to show that the natural "edge-following" algorithm, in which one "keeps one's hand on the wall," follows completely around the edge of any simply-connected object; this result in turn can be used to show that a well-known "border-following" algorithm (in which one follows the border elements of the object rather than the "cracks" between the object and its comple- ment) always works. Various related questions are also treated, among them, that of whether there can exist a "parallel" shrinking algorithm.
672 citations
TL;DR: In this article, the velocity correlation function of an atom in a simple liquid is calculated using a frequency-dependent version of the Stokes-Einstein formula, and good agreement is obtained with the velocities determined by Rahman using computer experiments.
Abstract: The velocity correlation function of an atom in a simple liquid is calculated using a frequency-dependent version of the Stokes-Einstein formula. Stokes's law for the frictional force on a moving sphere is generalized to arbitrary frequency, compressibility, and visco-elasticity, with arbitrary slip of the fluid on the surface of the sphere. This frequency-dependent friction coefficient is then used in a generalized Stokes-Einstein formula, and the velocity correlation function is found by Fourier inversion. By using physically reasonable values for viscoelastic parameters, good agreement is obtained with the velocity correlation function determined by Rahman using computer experiments.
456 citations
TL;DR: The stability of the Schwarzschild exterior metric against small perturbations is investigated in this article, where it is shown that the perturbation with imaginary frequencies that grow exponentially with time are physically unacceptable and hence the metric is stable.
Abstract: The stability of the Schwarzschild exterior metric against small perturbations is investigated. The perturbations superposed on the Schwarzschild background metric are the same as those given by Regge and Wheeler, consisting of odd- and even-parity classes, and with the time dependence $\mathrm{exp}(\ensuremath{-}ikt)$, where $k$ is the frequency. An analysis of the Einstein field equations computed to first order in the perturbations away from the Schwarzschild background metric shows that when the frequency is made purely imaginary, the solutions that vanish at large values of $r$, conforming to the requirement of asymptotic flatness, will diverge near the Schwarzschild surface in the Kruskal coordinates. Since the background metric itself is finite at this surface, the above behavior of the perturbation clearly contradicts the basic assumption that the perturbations are small compared to the background metric. Thus perturbations with imaginary frequencies that grow exponentially with time are physically unacceptable, and hence the metric is stable. Perturbations with real values of $k$ representing gravitational waves are also examined. It is shown that the only nontrivial stationary perturbation that can exist is one that is due to the rotation of the source, which is given by the odd perturbation with the angular momentum $l=1$. The significance of solutions with complex frequencies is pointed out, as is the lack of a theorem (completeness of the eigenfunction) for the even-parity case to parallel the Sturm-Liouville theory, which is applicable to the odd-parity case. Such a theorem would be required to convert the computations indicating stability as given here into a fully rigorous stability theorem.
418 citations
TL;DR: The proposed approach on a model problem — the Dirichlet problem with an interface for Laplace equation with sufficient condition for the smoothnees can be determined, and the boundary of the domain and the interface will be assumed smooth enough.
Abstract: Numerical solutions of boundary value problems for elliptic equations with discontinuous coefficients are of special interest In the case when the interface (ie the surface of the discontinuity of the coefficients) is smooth enough, then also the solution is usually very smooth (except on the interface) To obtain a high order of accuracy presents some difficulty, especially if the interface does not fit with the elements (in the finite element method) In this case the norm of the error in the spaceW1/2 is of the orderh
1/2 (see eg [1]) and on one dimensional case it is easy to see that the accuracy cannot be improved In this paper we shall show an approach which avoids this difficulty The idea is similar to [2] We shall show the proposed approach on a model problem — theDirichlet problem with an interface forLaplace equation; this will avoid pure technical difficulties The boundary of the domain and the interface will be assumed smooth enough The sufficient condition for the smoothnees can be determined
413 citations
260 citations
TL;DR: Several graph theoretic cluster techniques aimed at the automatic generation of thesauri for information retrieval systems are explored and two algorithms have been tested that find maximal complete subgraphs.
Abstract: Several graph theoretic cluster techniques aimed at the automatic generation of thesauri for information retrieval systems are explored. Experimental cluster analysis is performed on a sample corpus of 2267 documents. A term-term similarity matrix is constructed for the 3950 unique terms used to index the documents. Various threshold values, T, are applied to the similarity matrix to provide a series of binary threshold matrices. The corresponding graph of each binary threshold matrix is used to obtain the term clusters.Three definitions of a cluster are analyzed: (1) the connected components of the threshold matrix; (2) the maximal complete subgraphs of the connected components of the threshold matrix; (3) clusters of the maximal complete subgraphs of the threshold matrix, as described by Gotlieb and Kumar.Algorithms are described and analyzed for obtaining each cluster type. The algorithms are designed to be useful for large document and index collections. Two algorithms have been tested that find maximal complete subgraphs. An algorithm developed by Bierstone offers a significant time improvement over one suggested by Bonner.For threshold levels T ≥ 0.6, basically the same clusters are developed regardless of the cluster definition used. In such situations one need only find the connected components of the graph to develop the clusters.
241 citations
TL;DR: On the local scale man-made influences on climate are substantial but that on the global scale natural forces still prevail, and activation of an adequate worldwide monitoring system to permit early assessment of these changes is urgent.
Abstract: Natural climatic fluctuations, even those of recent years, cover a considerable range. They can be characterized as a "noise" spectrum which masks possible global effects of man-caused increases of atmospheric CO2 and particulates. Local modifications, either deliberate or inadvertent, measurably affect the microclimate. Some artificial alterations of the microlimate are beneficial in agriculture. Among the unplanned effects, those produced by urbanization on local temperature and on wind field are quite pronounced. The influences on rainfall are still somewhat controversial, but effects may extend considerably beyond the confines of metropolitan areas. They are the result of water vapor released by human activity and of the influence of condensation and freezing nuclei produced in overabundance by motor vehicles and other combustion processes. Therefore it appears that on the local scale man-made influences on climate are substantial but that on the global scale natural forces still prevail. Obviously this should not lead to complacency. The potential for anthropogenic changes of climate on a larger and even a global scale is real. At this stage activation of an adequate worldwide monitoring system to permit early assessment of these changes is urgent. This statement applies particularly to the surveillance of atmospheric composition and radiation balance at sites remote from concentrations of population, which is now entirely inadequate. In my opinion, man-made aerosols, because of their optical properties and possible influences on cloud and precipitation processes, constitute a more acute problem than CO2. Many of their effects are promptly reversible; hence, one should strive for elimination at the source. Over longer intervals, energy added to the atmosphere by heat rejection and CO2 absorption remain matters of concern.
238 citations
217 citations
01 May 1970-Metallurgical and Materials Transactions B-process Metallurgy and Materials Processing Science
TL;DR: In this article, the authors used the Hall-Petch relations to describe the complete stress-strain behavior for polycrystals and provided a reference for understanding the manner in which these other properties should also depend on the grain size.
Abstract: The ductile-brittle transition, hardness, fatigue, and creep behavior of polycrystalline materials are known to be influenced under certain conditions by the polycrystal grain size. These properties have been correlated, historically, with the material stress-strain behavior. The (Hall-Petch) stress-grain size relations are useful for describing the complete stress-strain behavior for polycrystals and, therefore, these relations provide a reference for understanding the manner in which these other properties should also depend on the grain size. In some cases, the grain size dependence of a particular property follows directly from this connection.
212 citations
TL;DR: Two-dimensional eye movements were recorded for long periods in total darkness while 2 Ss attempted to keep their eyes in the primary position or 10 deg arc to the left or right: effective position control was observed in the absence of a visual error signal.
TL;DR: In this article, the authors studied the van der Waals interaction between two-photon exchange between neutral spinless systems and obtained a generalized form of the Casimir-Polder potential, which includes both electric and magnetic effects.
Abstract: We study the van der Waals interaction ${V}_{2\ensuremath{\gamma}}^{\mathrm{AB}}(R)$ arising from two-photon exchange between neutral spinless systems $A$ and $B$. By using the analytic properties of the two-photon contribution to the scattering amplitude for $A+B\ensuremath{\rightarrow}A+B$ and of the full amplitudes for $\ensuremath{\gamma}+A\ensuremath{\rightarrow}\ensuremath{\gamma}+A$ and $\ensuremath{\gamma}+B\ensuremath{\rightarrow}\ensuremath{\gamma}+B$, we show that it is possible to express ${V}_{2\ensuremath{\gamma}}^{\mathrm{AB}}(R)$ entirely in terms of measurable quantities, the elastic scattering amplitudes for photons of various frequencies $\ensuremath{\omega}$. This approach includes relativistic corrections, higher multipoles, and retardation effects from the outset and thus avoids any $\frac{v}{c}$ expansion or any direct reference to the detailed structure of the systems involved. We obtain a generalized form of the Casimir-Polder potential, which includes both electric and magnetic effects, and, correspondingly, a generalized asympotic form ${V}_{2\ensuremath{\gamma}}^{\mathrm{AB}}(r)\ensuremath{\sim}\ensuremath{-}\frac{D}{{R}^{7}}$, where $D=\frac{[23({\ensuremath{\alpha}}_{E}^{A}{\ensuremath{\alpha}}_{E}^{B}+{\ensuremath{\alpha}}_{M}^{A}{\ensuremath{\alpha}}_{M}^{B})\ensuremath{-}7({\ensuremath{\alpha}}_{E}^{A}{\ensuremath{\alpha}}_{M}^{B}+{\ensuremath{\alpha}}_{M}^{A}{\ensuremath{\alpha}}_{E}^{B})]}{4\ensuremath{\pi}}$ and the $\ensuremath{\alpha}$'s denote static polarizabilities. In addition, we show that the potential may be written as a single integral over $\ensuremath{\omega}$, involving products of the dynamical polarizabilities ${\ensuremath{\alpha}}_{X}(\ensuremath{\omega})$ evaluated at real frequencies, in contrast to the familiar integral over imaginary frequencies; for the case of interacting atoms, the domain of applicability of the various formulas is clarified, and the problem of evaluating ${V}_{2\ensuremath{\gamma}}^{\mathrm{AB}}(R)$ from present experimental information is discussed. Some simple interpolation formulas are presented, which may accurately describe ${V}_{2\ensuremath{\gamma}}^{\mathrm{AB}}(R)$ in terms of a few constants.
TL;DR: In this paper, it was shown that for a nonlinear one-dimensional differential delay equation a(t) = F;(t, x1(.)) (DDE) one can frequently determine (almost by inspection) if the 0 solution is asymptotically stable and give a region of attraction.
TL;DR: A number of primitive models have been designed to estimate effects on global temperature and none of them indicate changes of more than 2°C, a moderate amount gaged by natural changes even since the last Pleistocene glaciation.
Abstract: The climatic elements, such as temperature and precipitation, fluctuate naturally on a global, regional, and local scale. These fluctuations are of shorter or longer duration. Their causes are only partially known. They constitute a ‘noise’ pattern against which man-made changes, if any, have to be evaluated. Anthropogenic changes on a global scale are projected by some to cause rise in atmospheric carbon dioxide from cumbustion of fossil fuels, increase in atmospheric dust content from human activities, and clouds produced by aircraft exhaust. All of these interfere with the radiation balance but there is no present indication that they have up to now caused any significant changes. This may not hold in the future when it is foreseen that CO2, e.g., may readily double in a few decades. A number of primitive models have been designed to estimate effects on global temperature. None of them indicate changes of more than 2°C, a moderate amount gaged by natural changes even since the last Pleistocene glaciation.
TL;DR: In this article, the authors explored the relationship between the importance of a job aspect (value) to an individual and his degree of satisfaction and dissatisfaction with that aspect, and the results provided a plausible explanation for the seemingly inconsistent results of previous studies in this area.
TL;DR: The paper shows that proper refinement of the elements around the corners leads to the rate of convergence which is the same as it would be on domain with smooth boundary.
Abstract: The rate of convergence of the finite element method is greatly influenced by the existence of corners on the boundary. The paper shows that proper refinement of the elements around the corners leads to the rate of convergence which is the same as it would be on domain with smooth boundary.
TL;DR: In this paper, the authors propose an alternative to the usual dual resonance model by considering charges and electrostatics on a sphere, and the four point Born term is shown to give the Virasoro model with a particular value of the intercept ( a = 2).
TL;DR: In this article, the authors studied the fine structure of the spectrum of a closed linear operator T in terms of the ascent, descent, nullity and defect of the operator T. Their notation will be used throughout this paper.
Abstract: In this paper the fine structure of the spectrum of a closed linear operator T is studied in terms of the ascent, descent, nullity and defect of the operators )~- T. Several characterizations of poles of the resolvent operator Ra(T) are obtained:and these are used to characterize certain classes of operators, e.g., the class of meromorphic operators. Much of the underlying algebraic theory was developed by A. E. Taylor [17] and M. A. Kaashoek [8]. Their notation will be used throughout this paper. 1. Algebraic Properties of Ascent, Descent, Nullity and Defect Let X be a linear space over the field of complex numbers and let T be a linear operator with domain ~(T) and range ~(T) in X. The ascent of T, ~(T), is the smallest nonnegative integer p such that the null manifolds Jf(T p) and A/,(Tp+ i) are equal. If no such p exists, set ~(T) = ~. The descent of T, 6(T), is the smallest nonnegative integer q such that the range spaces ~(T q) and ~(Tq+ 1) are equal. If no such q exists, set 6(T)= ~. The nullity of T, n(T), is the dimension of A/'(T). The defect of T, d(T), is the dimension of the quotient space X/~t(T). Let r(T)= {2~CI ~(2- T )=6(2- T )=0}. When X is a Banach space and T is a closed linear operator, r(T)= 0(T), the resolvent set of T.
01 May 1970
TL;DR: Using a product of differences of averages taken over pairs of adjacent nonoverlapping neighborhoods tends to yield sharply localized edges.
Abstract: Major "edges" in a noisy signal or picture can be detected by subtracting averages taken over pairs of adjacent nonoverlapping neighborhoods, but this method does not locate the edges precisely. It has been found that using a product of such differences of averages tends to yield sharply localized edges.
TL;DR: Evidence is presented that interspecific competition and microclimatic conditions may interact in these salamanders to limit the distributions of the species and that extinction may become a potential threat to one of thespecies when the other is more adept at exploiting these conditions.
Abstract: The research presented here concerns the relationship between the distributions of the plethodontid salamanders Plethodon cinereus cinereus (Green) and P. richmondi shenandoah Highton and Worthington. The purpose of this study is to present evidence that interspecific competition and microclimatic conditions may interact in these salamanders to limit the distributions of the species and that extinction may become a potential threat to one of the species when the other is more adept at exploiting these conditions. This paper describes the microgeographic distributions of the two species. Subsequent papers will give experimental data on interspecific competition and microclimatic conditions as factors limiting distributions. The distributions of closely related animal species often present difficult problems for interpretation. Lack (1944) felt that when two such species come into contact, interspecific competition has the potential to force them to one of four stable states: (1) one species will eliminate the other, (2) the two may occupy allopatric, but often contiguous, geographic regions, (3) they may live in sympatry but occupy different habitats or (4) they may occupy the same habitat but differ in food, or other resource, utilization. That any such arrangements do evolve through interspecific competition is poorly documented, although often inferred from indirect evidence (Miller, 1967). Indeed, Andrewartha and Birch (1954) question the biological importance of interspecific competition itself. Studies on the distributions of salamanders have been particularly unrevealing, and only two have critically attempted to define the factors influencing the interactions of related species-pairs. Hairston (1949, 1951) described the altitudinal separation of Plethodon jordani and P. glutinosus. The former usually occurs at high altitudes and the latter at lower elevations in the southern Appalachian Mountains, although there are areas where altitudinal separation breaks down and the two species occur sympatrically. P. jordani is apparently limited in distribution by moisture, since it fails to drop to the drier low elevations even in the absence of P. glutinosus. The distribution of P. glutinosus is possibly limited in some areas by competition, since it is able to exist at high elevations in the absence of P. jordani but often not in its presence. Hairston felt that there is geographic variation in the interspecific competition of these two species. Dumas (1956), from his study of two sympatric species of Plethodon from western North America (P. vehiculum and P. dunni), arrived at a conclusion somewhat different than that of Hairston. Dumas felt that the two species he studied are sympatric in geographic distribution with little or no competition for food due to the adverse physical environment limiting population size. There is, however, competition on a microgeographic scale for sites that are favorable with respect to moisture, with P. dunni displacing P. vehiculum from preferred sites. Hairston's study exemplifies state (2) and Dumas' study illustrates state (3) in Lack's system, although it is not clear whether or not these states are stable or are transitory indications of impending competitive exclusion. State (4) of Lack's system is demonstrated by two studies on populations of salamanders. Worthington (1968, 1969) showed that three species of eastern North American salamanders lay their eggs in the
TL;DR: The convergence condition of the Gauss-Seidel and Jacobi iterations for nonlinear elliptic boundary value problems has been studied by various authors, and no historical survey shall be attempted here as discussed by the authors.
TL;DR: In this article, a review of the calculations involving electromagnetic theory, where the forces are termed long-ranged van der Waals forces, or retarded dispersion forces, is presented, along with a heuristic connection between zero-point energy and semiclassical fluctuation phenomena.
TL;DR: In this article, the authors describe measurements of the reflectivity, scattering, and transmission of light by metals with rough surfaces, and compare the experimental results with the scalar scattering theory and make some general comments about the properties of surface dipoles and currents.
Abstract: This paper describes measurements of the reflectivity, scattering, and transmission of light by metals with rough surfaces. For surfaces whose roughness is very short ranged, the ratio of rough-surface reflectivity to smooth-surface reflectivity varies exponentially as ${\ensuremath{\lambda}}^{\ensuremath{-}2}$ both above the plasmon frequency and in regions where ${\ensuremath{\epsilon}}_{2}\ensuremath{\gg}|{\ensuremath{\epsilon}}_{1}|$. In the latter regions for these rough surfaces, the scattered intensity follows a ${\ensuremath{\lambda}}^{\ensuremath{-}4}$ wavelength variation. For surfaces which are more wavy, the reflectivity and scattered light vary less rapidly with wavelength. Well below the plasmon frequency, additional fields not present on smooth surfaces, but coming from induced extra currents and dipoles on rough surfaces, add coherently to the specular beam, with a resonant wavelength variation. Near the plasmon frequency there is extra absorption. We have studied the angular variation of the scattered light, and have observed additional incoherent light associated with these extra dipoles and currents. We compare the experimental results with the scalar scattering theory and make some general comments about the properties of surface dipoles and currents.
TL;DR: Steady state potential distributions near photoemitting plate immersed in plasma, noting overshoot effects role for sheaths around satellites in interplanetary plasma, were found in this paper.
Abstract: Steady state potential distributions near photoemitting plate immersed in plasma, noting over-shoot effects role for sheaths around satellites in interplanetary plasma
TL;DR: It is proved that, in general, this extension of web grammar does not increase the generative power of the grammar, but it is useful, for otherwise it is not possible to incorporate negative contextual conditions into the rules, since the context of a given vertex can be unbounded.
Abstract: This paper is concerned with the class of “web grammars,≓ introduced by Pfaltz and Rosenfeld, whose languages are sets of labelled graphs. A slightly modified definition of web grammar is given, in which the rewriting rules can have an applicability condition, and it is proved that, in general, this extension does not increase the generative power of the grammar. This extension is useful, however, for otherwise it is not possible to incorporate negative contextual conditions into the rules, since the context of a given vertex can be unbounded. A number of web grammars are presented which define interesting classes of graphs, including unseparable graphs, unseparable planar graphs and planar graphs. All the grammars in this paper use “normal embeddings≓ in which the connections between the web that is written and the host web are conserved, so that any rewriting rule affects the web only locally.
TL;DR: This article found that syllables preceding a judged-pause location were usually longer than those following, whether or not a silent interval was present, but syllable length governed judgments independently of juncture cues.
TL;DR: In this paper, chemical shifts for thirty-one compounds of the type, cis-and trans-L2PdCl2, sym-L 2Pd2Cl4 and trans-AmPdCL2, where L is a tertiary phosphine and Am is a nitrogen ligand.