Adaptive Sentence Boundary Disambiguation

Labeling of sentence boundaries is a necessary prerequisite for many natural language processing tasks, including part-of-speech tagging and sentence alignment. End-of-sentence punctuation marks are ambiguous; to disambiguate them most systems use brittle, special-purpose regular expression grammars and exception rules. As an alternative, we have developed an efficient, trainable algorithm that uses a lexicon with part-of-speech probabilities and a feed-forward neural network. After training for less than one minute, the method correctly labels over 98.5\% of sentence boundaries in a corpus of over 27,000 sentence-boundary marks. We show the method to be efficient and easily adaptable to different text genres, including single-case texts.Comment: This is a Latex version of the previously submitted ps file (formatted as a uuencoded gz-compressed .tar file created by csh script). The software from the work described in this paper is available by contacting [email protected]

The Intensive Margin of Technology Adoption

We present a tractable model for analyzing the relationship between economic growth and the intensive and extensive margins of technology adoption. The "extensive" margin refers to the timing of a country's adoption of a new technology; the "intensive" margin refers to how many units are adopted (for a given size economy). At the aggregate level, our model is isomorphic to a neoclassical growth model, while at the microeconomic level it features adoption of firms at the extensive and the intensive margin. Based on a data set of 15 technologies and 166 countries our estimations of the model yield four main findings: (i) there are large cross-country differences in the intensive margin of adoption; (ii) differences in the intensive margin vary substantially across technologies; (iii) the cross-country dispersion of adoption lags has declined over time while the cross-country dispersion in the intensive margin has not; (iv) the cross- country variation in the intensive margin of adoption accounts for more than 40% of the variation in income per capita.Economic Growth, Technology Adoption, Cross-country studies.

Simulations of the relativistic parsec-scale jet in 3C273

We present a hydrodynamical 3D simulation of the relativistic jet in 3C273, in comparison to previous linear perturbation analysis of Kelvin-Helmholtz instability developing in the jet. Our aim is to assess advantages and limitations of both analytical and numerical approaches and to identify spatial and temporal scales on which the linear regime of Kelvin-Helmholtz instability can be applied in studies of morphology and kinematics of parsec-scale jets.Comment: 4 pages, 3 figures; to be published in Proceedings of the workshop "Multiband Approach to AGN", held on Sep.30-Oct.2 in Bonn. Publication: Memorie della Societa Astronomica Italiana, v. 26, No.1 (2005). Reduced figure resolution! Version with original figures is availavble at http://www.mpifr-bonn.mpg.de/bonn04/proceedings/perucho.pd

Dynamical analysis of the Gliese-876 Laplace resonance

The existence of multiple planetary systems involved in mean motion conmensurabilities has increased significantly since the Kepler mission. Although most correspond to 2-planet resonances, multiple resonances have also been found. The Laplace resonance is a particular case of a three-body resonance where the period ratio between consecutive pairs is n_1/n_2 near to n_2/n_3 near to 2/1. It is not clear how this triple resonance can act in order to stabilize (or not) the systems. The most reliable extrasolar system located in a Laplace resonance is GJ876 because it has two independent confirmations. However best-fit parameters were obtained without previous knowledge of resonance structure and no exploration of all the possible stable solutions for the system where done. In the present work we explored the different configurations allowed by the Laplace resonance in the GJ876 system by varying the planetary parameters of the third outer planet. We find that in this case the Laplace resonance is a stabilization mechanism in itself, defined by a tiny island of regular motion surrounded by (unstable) highly chaotic orbits. Low eccentric orbits and mutual inclinations from -20 to 20 degrees are compatible with the observations. A definite range of mass ratio must be assumed to maintain orbital stability. Finally we give constrains for argument of pericenters and mean anomalies in order to assure stability for this kind of systems.Comment: 7 pages, 7 figures, accepted in MNRA

Jet precession in the active nucleus of M81. Ongoing VLBI monitoring

In a recent publication, we reported results of a multi-frequency VLBI campaign of observations of the Active Galactic Nucleus (AGN) in galaxy M\,81, phase-referenced to the supernova SN\,1993J. We were able to extract precise information on the relative astrometry of the AGN radio emission at different epochs and frequencies. We found strong evidence of precession in the AGN jet (i.e., a systematic evolution in the jet inclination at each frequency) coupled to changes in the overall flux density at the different frequencies. In these proceedings, we summarise the main contents of our previous publication and we report on (preliminary) new results from our follow-up VLBI observations, now phase-referenced to the young supernova SN2008iz. We also briefly discuss how these results match the picture of our previously-reported precession model.Comment: 6 pages, 3 figures. Proceedings of EVN meeting 201

Generalized Integral Operators and Schwartz Kernel Theorem

In connection with the classical Schwartz kernel theorem, we show that in the framework of Colombeau generalized functions a large class of linear mappings admit integral kernels. To do this, we need to introduce news spaces of generalized functions with slow growth and the corresponding adapted linear mappings. Finally, we show that in some sense Schwartz' result is contained in our main theorem.Comment: 18 page

Front dynamics in turbulent media

A study of a stable front propagating in a turbulent medium is presented. The front is generated through a reaction-diffusion equation, and the turbulent medium is statistically modeled using a Langevin equation. Numerical simulations indicate the presence of two different dynamical regimes. These regimes appear when the turbulent flow either wrinkles a still rather sharp propagating interfase or broadens it. Specific dependences of the propagating velocities on stirring intensities appropriate to each case are found and fitted when possible according to theoretically predicted laws. Different turbulent spectra are considered.Comment: 8 pages, REVTEX, 6 postscript figures included. To appear in Phys. Fluids (1997

The exact solution of the Riemann problem with non-zero tangential velocities in relativistic hydrodynamics

We have generalised the exact solution of the Riemann problem in special relativistic hydrodynamics for arbitrary tangential flow velocities. The solution is obtained by solving the jump conditions across shocks plus an ordinary differential equation arising from the self-similarity condition along rarefaction waves, in a similar way as in purely normal flow. The dependence of the solution on the tangential velocities is analysed, and the impact of this result on the development of multidimensional relativistic hydrodynamic codes (of Godunov type) is discussed.Comment: 26 pages, 4 figures. Accepted for publication in Journal of Fluid Mechanic

Langevin approach to generate synthetic turbulence

We present an analytical scheme, easily implemented numerically, to generate synthetic Gaussian turbulent flows by using a linear Langevin equation, where the noise term acts as a stochastic stirring force. The characteristic parameters of the velocity field are well introduced, in particular the kinematic viscosity and the spectrum of energy. As an application, the diffusion of a passive scalar is studied for two different energy spectra. Numerical results are compared favorably with analytical calculations.Comment: 7 pages, REVTEX, 6 figures. To appear in Physics of Fluids (April 1997