scispace - formally typeset
Search or ask a question

Showing papers by "University of Maryland, College Park published in 1969"


Journal ArticleDOI
TL;DR: New methods of automatically extracting documents for screening purposes, i.e. the computer selection of sentences having the greatest potential for conveying to the reader the substance of the document, indicate that the three newly proposed components dominate the frequency component in the production of better extracts.
Abstract: This paper describes new methods of automatically extracting documents for screening purposes, i.e. the computer selection of sentences having the greatest potential for conveying to the reader the substance of the document. While previous work has focused on one component of sentence significance, namely, the presence of high-frequency content words (key words), the methods described here also treat three additional components: pragmatic words (cue words); title and heading words; and structural indicators (sentence location).The research has resulted in an operating system and a research methodology. The extracting system is parameterized to control and vary the influence of the above four components. The research methodology includes procedures for the compilation of the required dictionaries, the setting of the control parameters, and the comparative evaluation of the automatic extracts with manually produced extracts. The results indicate that the three newly proposed components dominate the frequency component in the production of better extracts.

1,506 citations


01 Jan 1969
TL;DR: The field of picture processing by computer is reviewed from a technique-oriented standpoint and the processing of given pictures (as opposed to computer-synthesized pictures) is considered.
Abstract: : The field of picture processing by computer is reviewed from a technique-oriented standpoint. Only the processing of given pictures (as opposed to computer-synthesized pictures) is considered. Specific areas covered include: (a) Pictures as information sources and their efficient encoding; (b) Approximation of pictures - sampling and quantization techniques; (c) Position-invariant operations on pictures and their implementation (digital, electro-optical, optical); applications to matched filtering (template matching), spatial frequency filtering and image restoration, measurement of image quality, and image enhancement ('smoothing' and 'sharpening'); (d) Picture properties (linear; local and 'textural'; random) useful for pictorial pattern recognition; (e) 'Figure extraction' from pictures; figure properties (topology, size, shape); (f) Picture description and 'picture languages.' (Author)

712 citations


Journal ArticleDOI
TL;DR: The Hamiltonian methods of Arnowitt, Deser, and Misner can be applied to homogeneous cosmological models, and prove to be an efficient way both of constructing the Einstein equations and of studying their solutions as mentioned in this paper.
Abstract: The Hamiltonian methods of Arnowitt, Deser, and Misner can be applied to homogeneous cosmological models, and prove to be an efficient way both of constructing the Einstein equations and of studying their solutions. By using an appropriate form for the metric, one finds that the constraint equations for these models can be solved explicitly, and the resulting problem in Hamiltonian mechanics resembles that of a particle in a potential well. The most unusual feature of the Hamiltonian is that it is explicitly time dependent. There is an easy and attractive choice of factor orderings which allows one to pass on to a quantum theory (by imposing canonical commutation relations on the independent canonical variables) while maintaining the signature of the quantized metric. For the closed-space cosmological model (Bianchi type IX) which is studied in most detail, a classical (high-quantum-number) state remains classical as the wave function is followed back in time toward the initial singularity. There is no tendency for significant contributions from states of low quantum number to develop even when the radius of the universe is much less than ${(\frac{G\ensuremath{\hbar}}{{c}^{3}})}^{\frac{1}{2}}={10}^{\ensuremath{-}33}$ cm.

451 citations


Journal ArticleDOI
TL;DR: In this paper, the authors show that the probability that all of these coincidences were accidental is extremely small and imply that electromagnetic and seismic effects can be ruled out with a high level of confidence.
Abstract: Coincidences have been observed on gravitational-radiation detectors over a base line of about 1000 km at Argonne National Laboratory and at the University of Maryland. The probability that all of these coincidences were accidental is incredibly small. Experiments imply that electromagnetic and seismic effects can be ruled out with a high level of confidence. These data are consistent with the conclusion that the detectors are being excited by gravitational radiation.

444 citations


Journal ArticleDOI
TL;DR: Interstellar formaldehyde has been detected in absorption against numerous galactic and extragalactic radio sources by means of the ground-state rotational transition at 4830 MHz as mentioned in this paper.
Abstract: Interstellar formaldehyde (${\mathrm{H}}_{2}$CO) has been detected in absorption against numerous galactic and extragalactic radio sources by means of the ${1}_{11}$-${1}_{10}$ ground-state rotational transition at 4830 MHz. The absorbing regions often correspond in velocity with 18-cm OH features. ${\mathrm{H}}_{2}$CO is the first organic polyatomic molecule ever detected in the interstellar medium and its widespread distribution indicates that processes of interstellar chemical evolution may be much more complex than previously assumed.

329 citations


Journal ArticleDOI
TL;DR: In this paper, the eikonal approximation for high-energy collisions, long familiar in the theory of potential scattering, is considered from the viewpoint of relativistic quantum field theory.
Abstract: The eikonal approximation for high-energy collisions, long familiar in the theory of potential scattering, is considered from the viewpoint of relativistic quantum field theory. We study, in particular, the Feynman amplitude $M(s, t)$ describing the scattering of two spin-0 particles, $a$ and $b$, interacting by the exchange of spin-0 mesons. We show that if ${M}_{n}(s, t)$, the contribution to $M(s, t)$ arising from all $n\mathrm{th}$-order Feynman diagrams in which exactly $n$ mesons are exchanged between $a$ and $b$, is written in an appropriately symmetrized way, and if the terms in any $a$ or $b$ particle propagator which are quadratic in the internal momenta are then dropped, the resulting expression, ${{M}_{n}}^{\mathrm{eik}}(s, t)$, may be evaluated in closed form, and the sum over $n$, which defines ${M}^{\mathrm{eik}}(s, t)$, may be carried out. The representation of ${M}^{\mathrm{eik}}(s, t)$ found in this way involves the exponential of a function $\ensuremath{\chi}$ of a relative space-time variable $x=({x}^{0}, \mathrm{x})$ and the external momenta; $\ensuremath{\chi}$ is a relativistic generalization of the eikonal ${\ensuremath{\chi}}_{\mathrm{pot}}$ familiar from the theory of high-energy potential scattering. ${M}^{\mathrm{eik}}(s, t)$ is both crossing-symmetric and time-reversal-invariant. In the static limit (${m}_{b}\ensuremath{\rightarrow}\ensuremath{\infty}$), $\ensuremath{\chi}$ tends to ${\ensuremath{\chi}}_{\mathrm{pot}}$ for the appropriate Yukawa potential and ${f}^{\mathrm{eik}}=\ensuremath{-}\frac{{M}^{\mathrm{eik}}}{8\ensuremath{\pi}\ensuremath{\surd}s}$ has a limiting form ${{f}_{\mathrm{pot}}}^{\mathrm{eik}}$, which we also derive directly from the theory of potential scattering; for small scattering angles, ${{f}_{\mathrm{pot}}}^{\mathrm{eik}}$ coincides with the standard result. The amplitude for particle-antiparticle scattering is studied in the same model. It is shown that the eikonal ${\ensuremath{\chi}}_{2}(x)$ associated with the contribution of all annihilation-type diagrams has a logarithmic singularity at $x=0$ whose coefficient is proportional to $\ensuremath{\alpha}(t)+1$, where $\ensuremath{\alpha}(t)$ is the Regge-trajectory function obtained from the asymptotic behavior of the ladder-type diagrams alone. Another connection with Regge behavior is made by showing that the summation of a certain infinite class of radiative corrections to the lowest-order $\ensuremath{\gamma}\ensuremath{-}e$ Compton amplitude gives rise, in our eikonal approximation, to an eikonal $\ensuremath{\chi}(x)$ which has a similar logarithmic singularity with strength $1+\ensuremath{\beta}(t)$; here $\ensuremath{\beta}(t)$ is the trajectory function, introduced less directly in earlier work, which reproduces the major part of the spectrum of positronium on setting $\ensuremath{\beta}(t)=l=n\ensuremath{-}1$. A generalization of a simple algebraic identity used in the derivation of the above results, in the form of an integral representation, permits their extension to the case where one or more particles are off the mass shell. This is illustrated by a computation of an eikonal-type approximation to the Green's function for a relativistic particle moving in an external scalar field and by the summation of an infinite class of contributions to the vertex function in the model referred to above. The possibility of applying an off-shell eikonal approximation to the analysis of production processes is emphasized.

233 citations


Journal ArticleDOI
TL;DR: In this paper, it was shown that the statistical uncertainty due to this kind of average is of the order of 1/the square root of T. The coefficient of the square root is related to a characteristic relaxation time of the correlation function.
Abstract: : In using a computer experiment to calculate the time correlation function of some dynamical variable, the ensemble average over an equilibrium distribution in phase space often is replaced by a time average over a finite interval T. It is shown here that the statistical uncertainty due to this kind of average is of the order of 1/the square root of T. The coefficient of the square root is related to a characteristic relaxation time of the correlation function. If only time averaging is performed, the resulting statistical uncertainty in typical computer experiments may be of the order of twenty percent. (Author)

227 citations


Journal ArticleDOI
TL;DR: In this paper, the Planck radiation law for the blackbody radiation spectrum is derived without the formalism of quantum theory without any assumptions of discrete or discontinuous processes, assuming the existence, at the absolute zero of temperature, of classical homogeneous fluctuating electromagnetic radiation with a Lorentz-invariant spectrum, and assuming that classical electrodynamics holds for a dipole oscillator.
Abstract: The Planck radiation law for the blackbody radiation spectrum is derived without the formalism of quantum theory. The hypotheses assume (a) the existence, at the absolute zero of temperature, of classical homogeneous fluctuating electromagnetic radiation with a Lorentz-invariant spectrum; (b) that classical electrodynamics holds for a dipole oscillator; (c) that a free particle in equilibrium with blackbody radiation has the classical mean kinetic energy $\frac{1}{2}\mathrm{kT}$ per degree of freedom. The Lorentz invariance of the spectrum of zero-temperature radiation is used to derive the zero-point electromagnetic energy-density spectrum, found to be linear in frequency, $\frac{1}{2}\ensuremath{\hbar}\ensuremath{\omega}$ per normal mode. The procedures based on classical theory employed by Einstein and Hopf, which were formerly regarded as giving a rigorous derivation of the Rayleigh-Jeans radiation law, are modified and corrected for electromagnetic zero-point energy to allow a rigorous derivation of the full blackbody spectrum from classical theory without any assumptions of discrete or discontinuous processes.

201 citations


Book
01 Jan 1969
TL;DR: The field of picture processing by computer is reviewed from a technique-oriented standpoint in this article, where the processing of given pictures (as opposed to computer-synthesized pictures) is considered.
Abstract: : The field of picture processing by computer is reviewed from a technique-oriented standpoint. Only the processing of given pictures (as opposed to computer-synthesized pictures) is considered. Specific areas covered include: (a) Pictures as information sources and their efficient encoding; (b) Approximation of pictures - sampling and quantization techniques; (c) Position-invariant operations on pictures and their implementation (digital, electro-optical, optical); applications to matched filtering (template matching), spatial frequency filtering and image restoration, measurement of image quality, and image enhancement ('smoothing' and 'sharpening'); (d) Picture properties (linear; local and 'textural'; random) useful for pictorial pattern recognition; (e) 'Figure extraction' from pictures; figure properties (topology, size, shape); (f) Picture description and 'picture languages.' (Author)

193 citations


Proceedings Article
07 May 1969
TL;DR: A method of defining "languages" whose "sentences" are directed graphs with symbols at their vertices is described, showing that if the graphs permitted in a "context-sensitive" web grammar are required to be acyclic, and the parsing rules are assumed to be graph homomorphisms, then any sub-graph which is parsed by a rule must be "convex".
Abstract: Classes of "phrase-structure grammars" are defined whose "languages" consist, not of strings of symbols, but of directed graphs with symbols at their vertices ("webs"). Examples of such "web grammars" are given, having languages consisting of trees, of two-terminal series-parallel networks, and of "triangular" networks. It is shown that if the graphs permitted in a "context-sensitive" web grammar are required to be acyclic, and the parsing rules are assumed to be graph homomorphisms, then any sub-graph which is parsed by a rule must be "convex", and any rule is a composite of rules each of which parses a subgraph having just two points. Foreword Since the early 1960's, considerable effort has been devoted to the problem of developing formal "picture languages" whose "sentences" are pictures of various types. This work (see^ for a recent survey) can be thought of as generalizing the concepts and methods of mathematical linguistics from conventional languages, in which sentences are strings formed by concatenating symbols, to "languages" in which "sentences" are formed by combining symbols in more general ways. The present paper describes a method of defining "languages" whose "sentences" are directed graphs with symbols at their vertices.

187 citations



Journal ArticleDOI
TL;DR: The Walsh functions are of great practical interest since they lead to equipment that is easily implemented by semiconductor technology, and some interesting applications of electromagnetic Walsh waves have been found theoretically.
Abstract: Communication theory was founded on the system of sine-cosine functions. A more general theory has become known more recently; it replaces the sine-cosine functions by other systems of orthogonal functions, and the concept of frequency by that of sequency. Of these systems, the Walsh functions are of great practical interest since they lead to equipment that is easily implemented by semiconductor technology. Filters, multiplexing equipment, and a voice analyzer/synthesizer have been built successfully for Walsh functions. Some interesting applications of electromagnetic Walsh waves have been found theoretically.

Journal ArticleDOI
TL;DR: In this paper, the binary-collision expansion for potentials consisting of a hard core and a soft tail is discussed and the restrictions to their applicability are determined, and the different expressions so far proposed are critically examined.

Journal ArticleDOI
TL;DR: In this paper, the one-particle distribution function which satisfies the Boltzmann equation is interpreted as the average of the phase-space function, and the second moments of the fluctuating pressure tensor and the heat-flow vector are evaluated by using the first Chapman-Enskog approximation.
Abstract: The one-particle distribution function which satisfies the Boltzmann equation is interpreted as the average of the phase-space function $\ensuremath{\Sigma}{i=1}^{N}\ensuremath{\delta}(\stackrel{\ensuremath{\rightarrow}}{\mathrm{r}}\ensuremath{-}{\stackrel{\ensuremath{\rightarrow}}{\mathrm{r}}}_{i})\ensuremath{\delta}(\stackrel{\ensuremath{\rightarrow}}{\mathrm{v}}\ensuremath{-}{\stackrel{\ensuremath{\rightarrow}}{\mathrm{v}}}_{i})$. The equation of motion for this function is a generalized Langevin equation. This equation is the linear Boltzmann equation to which a fluctuating force term is added. An expression for the second moment of this force, in terms of the Boltzmann kernel and the equilibrium second moment of the distribution function, is derived in analogy with the known procedure involving the Langevin equation. The second moments of the fluctuating pressure tensor and the heat-flow vector are evaluated by using the first Chapman-Enskog approximation. They are equal to the expressions derived by Landau and Lifshitz, using thermodynamic fluctuation theory in relation to the linearized hydrodynamic equations.

Journal ArticleDOI
TL;DR: Power spectra studies of interplanetary magnetic field fluctuations based on data from Pioneer 6 space probe were performed by as mentioned in this paper, where the Pioneer 6 was used for power spectra analysis.
Abstract: Power spectra studies of interplanetary magnetic field fluctuations based on data from Pioneer 6 space probe


Journal ArticleDOI
TL;DR: In this paper, a singularity involving infinite densities at a finite proper time in the past is strongly suggested for the beginnings of the Universe by Einstein's general relativity theory, and is consistent with the few relevant observational data.
Abstract: A singularity involving infinite densities at a finite proper time in the past is strongly suggested for the beginnings of the Universe by Einstein's general relativity theory, and is consistent with the few relevant observational data. There is no reasonable point at which to anticipate a failure of the theory, especially since a simplified quantum calculation in the accompanying paper predicts that quantum effects do not change the nature of the singularity. Therefore, we suggest that the singularity be treated as an essential element of cosmological theory, and indicate how this can be made more palatable by refining our concepts of time.

Journal ArticleDOI
TL;DR: Eye position stability when the target was stationary, before and after movements, did not require saccades either when thetarget was in the preferred retinal fiixation locus or when it fell as much as 2½ deg arc from this position.


Journal ArticleDOI
TL;DR: In this article, a newly designed high-stability sing-around circuit ultrasonic velocities in aqueous salt solutions down to concentrations of 2'×'10−4mole/liter have been measured.
Abstract: Using a newly designed high‐stability sing‐around circuit ultrasonic velocities in aqueous salt solutions down to concentrations of 2 × 10−4mole/liter have been measured. Adiabatic apparent molal compressibilities at infinite dilution, and where possible standard molal compressibilities, have been determined for some aqueous solutions of tetra‐alkylammonium bromides, alkali‐metal chlorides and sulfates, and MgSO4. The determination of these quantities from precise sound velocity, volumetric, and calorimetric measurements is discussed. The practice of obtaining φKs0 from empirical extrapolation of φKs data is shown to be unreliable and unnecessary.

Journal ArticleDOI
TL;DR: In this article, a model based on perturbations between rotational levels of different vibrational states of the water-vapor molecule was proposed to explain the laser action and to predict new laser lines.
Abstract: Water-vapor lasers oscillate on more than a hundred pulsed lines and over a dozen CW lines over the wavelength range from 7 to 220 μ (1300-45 cm-1). Nearly eighty of these lines are now identified as vibrational-rotational and pure rotational transition in the low-lying vibrational states of the water-vapor molecule. The structure of the water-vapor molecule is reviewed, and the mechanisms responsible for creating population inversions in water-vapor discharges are discussed. It is shown how a model, based on perturbations between rotational levels of different vibrational states, can be used to explain the laser action and to predict new laser lines. Detailed summaries of data on all observed laser lines in H 2 O16, H 2 O18, and D 2 O16and their identifications are presented. Some of the lines are newly reported, and more accurate wavelength measurements have been made for many of the lines. The observation of predicted laser lines in H 2 O18is shown to be a strong confirmation of the perturbation model. The list of references contains a fairly complete bibliography of previous work on the water-vapor laser, and a representative bibliography of work on the energy-level structure of the water-vapor molecule.


Journal ArticleDOI
TL;DR: In this paper, a brief survey of the transfer matrix method of analyzing the dynamic behavior of beam structures is presented for single rows of aircraft panels which are continuous over supporting stringers.


Journal ArticleDOI
TL;DR: Explorer 34 observations of hydrogen and He ions in solar wind, noting number density variations and tentative association with geomagnetic storms was made by as mentioned in this paper, who reported that the number of hydrogen ions in the solar wind varied with the number density.
Abstract: Explorer 34 observations of hydrogen and He ions in solar wind, noting number density variations and tentative association with geomagnetic storms

Journal ArticleDOI
TL;DR: A method for recording movements of bacteria in time and space on a single photograph is described, and illustrations of applications of the technique are presented.
Abstract: A method for recording movements of bacteria in time and space on a single photograph is described. Quantitative information on the behavior of various motile organisms may easily be obtained for comparative studies. The method possesses certain advantages over cinematography, and illustrations of applications of the technique are presented.

Journal ArticleDOI
TL;DR: In this article, rates of photosynthesis were used to evaluate the response of phytoplankton when passed through the cooling system of a power generating facility, where photosynthesis was measured as the uptake of carbon.
Abstract: Rates of photosynthesis were used to evaluate the response of phytoplankton when passed through the cooling system of a power generating facility. Photosynthesis was measured as the uptake of carbon. A factorial design permitted evaluation of heat input, total effect of passage, and degree of recovery. Six experiments measured response at a fixed time of day and at various times in a daily cycle.


Journal ArticleDOI
TL;DR: In this article, a classical statistical thermodynamics in the presence of electromagnetic radiation is reanalyzed, and is reformulated to give a natural classical description of the phenomena which originally led to the introduction of quanta.
Abstract: Classical statistical thermodynamics in the presence of electromagnetic radiation is reanalyzed, and is reformulated to give a natural classical description of the phenomena which originally led to the introduction of the idea of quanta. The traditional classical ideal gas fails to exist in principle for particles of finite mass which have electromagnetic interactions, and hence the classical proofs of energy equipartition are all erroneous. A consistently classical treatment of thermal radiation leads to the natural introduction of temperature-independent fluctuating radiation in the universe. The spectrum of this electromagnetic zero-point radiation may be obtained from the arguments for Wien's displacement law or from the requirement of Lorentz invariance of the radiation spectrum; this zero-point spectrum agrees with the $\frac{1}{2}\ensuremath{\hbar}\ensuremath{\omega}$ per normal mode familiar in quantum theory. The presence of temperature-independent disordered energy from zero-point radiation leads to a contribution to the entropy connected with thermodynamic probability distinct from the contribution of caloric entropy. The use of quanta in calculations of the thermodynamic probability is seen as a subterfuge to account for this mismatch between caloric entropy and probability. Several examples of statistical thermodynamics, which are generally regarded as having their explanation in terms of quanta, allow natural explanations within the context of classical theory with classical electromagnetic zero-point radiation.

Journal ArticleDOI
TL;DR: The behavior of the entropy h as a function of/z is studied, and it is shown that h achieves its maximum value for Haar measure m, if T is ergodic with respect to m, and if h(m) is finite, then m is uniquely characterized as that measure which maximizes the entropy.
Abstract: Introduction. Entropy is a numerical invariant attached to a quadruple (3, ~-, tz, T), where T is a measure-preserving transformation of the measure space (X, o~,/z) (/~ a probability). In this paper T is a continuous automorphism on a compact metrizable group X = G; -~ is the Borel field. We study the behavior of the entropy h as a function of/z. We show that h achieves its maximum value for Haar measure m. Moreover, if T is ergodic with respect to m, and if h(m) is finite, then m is uniquely characterized as that measure which maximizes the entropy. Thus Haar measure is retrievable from the measuretheoretic structure of the system. In [1], Adler and Weiss showed that ergodic Haar measure strictly maximizes the entropy for an automorphism of a twodimensional torus. They were able to use this result to establish that entropy is a complete invariant for automorphisms of the two-dimensional torus. In the last section we show that under certain circumstances, the convolution of two ergodic measures is ergodic.