2,205 research outputs found
When the path is never shortest: a reality check on shortest path biocomputation
Shortest path problems are a touchstone for evaluating the computing
performance and functional range of novel computing substrates. Much has been
published in recent years regarding the use of biocomputers to solve minimal
path problems such as route optimisation and labyrinth navigation, but their
outputs are typically difficult to reproduce and somewhat abstract in nature,
suggesting that both experimental design and analysis in the field require
standardising. This chapter details laboratory experimental data which probe
the path finding process in two single-celled protistic model organisms,
Physarum polycephalum and Paramecium caudatum, comprising a shortest path
problem and labyrinth navigation, respectively. The results presented
illustrate several of the key difficulties that are encountered in categorising
biological behaviours in the language of computing, including biological
variability, non-halting operations and adverse reactions to experimental
stimuli. It is concluded that neither organism examined are able to efficiently
or reproducibly solve shortest path problems in the specific experimental
conditions that were tested. Data presented are contextualised with biological
theory and design principles for maximising the usefulness of experimental
biocomputer prototypes.Comment: To appear in: Adamatzky, A (Ed.) Shortest path solvers. From software
to wetware. Springer, 201
Temporal Correlations and Persistence in the Kinetic Ising Model: the Role of Temperature
We study the statistical properties of the sum , that is the difference of time spent positive or negative by the
spin , located at a given site of a -dimensional Ising model
evolving under Glauber dynamics from a random initial configuration. We
investigate the distribution of and the first-passage statistics
(persistence) of this quantity. We discuss successively the three regimes of
high temperature (), criticality (), and low temperature
(). We discuss in particular the question of the temperature
dependence of the persistence exponent , as well as that of the
spectrum of exponents , in the low temperature phase. The
probability that the temporal mean was always larger than the
equilibrium magnetization is found to decay as . This
yields a numerical determination of the persistence exponent in the
whole low temperature phase, in two dimensions, and above the roughening
transition, in the low-temperature phase of the three-dimensional Ising model.Comment: 21 pages, 11 PostScript figures included (1 color figure
Arzner, Dorothy (1897â1979)
Dorothy Arzner was the only American woman that was successful in both the silent and the sound era. She was the only woman director working for Hollywood in the 1930s and one of the very few female Hollywood directors that have been successful from the 1920s to the 1940s. Arzner in fact came to symbolize everything that women could achieve in cinema and her work had a great influence on later women directors such as Ida Lupino, Joan Micklin Silver, and Elaine May. Her films were reclaimed by feminist scholars during the 1970s as texts that subvert patriarchal constructions of gender. In particular, feminist film critics often quote Arzner\u27s Dance, Girl, Dance (1940) as a film that actively critiques the culture of the spectacle where the woman is objectified by the gaze of the camera. Overall, although Arzner uses the âmaster\u27s tools,â she still manages to dismantle âthe master\u27s houseâ by denaturalizing the gender conventions of her era
Geometric quantum computation using fictitious spin- 1/2 subspaces of strongly dipolar coupled nuclear spins
Geometric phases have been used in NMR, to implement controlled phase shift
gates for quantum information processing, only in weakly coupled systems in
which the individual spins can be identified as qubits. In this work, we
implement controlled phase shift gates in strongly coupled systems, by using
non-adiabatic geometric phases, obtained by evolving the magnetization of
fictitious spin-1/2 subspaces, over a closed loop on the Bloch sphere. The
dynamical phase accumulated during the evolution of the subspaces, is refocused
by a spin echo pulse sequence and by setting the delay of transition selective
pulses such that the evolution under the homonuclear coupling makes a complete
rotation. A detailed theoretical explanation of non-adiabatic geometric
phases in NMR is given, by using single transition operators. Controlled phase
shift gates, two qubit Deutsch-Jozsa algorithm and parity algorithm in a
qubit-qutrit system have been implemented in various strongly dipolar coupled
systems obtained by orienting the molecules in liquid crystal media.Comment: 37 pages, 17 figure
No evidence for intense, cold accretion on to YSOs from measurements of Li in T-Tauri stars
We have used medium-resolution spectra to search for evidence that proto-stellar objects accrete at high rates during their early 'assembly phase'. Models predict that depleted lithium and reduced luminosity in T-Tauri stars are key signatures of 'cold' high-rate accretion occurring early in a star's evolution. We found no evidence in 168 stars in NGC 2264 and the Orion nebula cluster for strong lithium depletion through analysis of veiling-corrected 6708Ă
lithium spectral line strengths. This suggests that 'cold' accretion at high rates (M = 5 Ă 10-4 Mâ yr-1) occurs in the assembly phase of fewer than 0.5 per cent of 0.3 = Mâ = 1.9Mâ stars. We also find that the dispersion in the strength of the 6708Ă
lithium line might imply an age spread that is similar in magnitude to the apparent age spread implied by the luminosity dispersion seen in colour-magnitude diagrams. Evidence for weak lithium depletion (<10 per cent in equivalent width) that is correlated with luminosity is also apparent, but we are unable to determine whether age spreads or accretion at rates less than 5 Ă 10-4 Mâ yr-1 are responsible. ©2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.DJS is funded by a UK Science and Technology Facilities Council
(STFC) studentship. The authors wish to thank Isabelle Baraffe for
providing cold accretion models and useful discussions. Spectra
were extracted and calibrated using the AF2 pipeline developed by
Richard Jackson. This research is based on observations made with
the William Herschel Telescope operated on the island of La Palma
by the Isaac Newton Group (ING) in the Spanish Observatorio del
Roque de los Muchachos of the Instituto de Astrofisica de Canarias.
This research has made use of archival data products from the
Two-Micron All-Sky Survey (2MASS), which is a joint project of
the University of Massachusetts and the Infrared Processing and
Analysis Center, funded by the National Aeronautics and Space
Administration (NASA) and the National Science Foundation
A lithium depletion boundary age of 22 Myr for NGC 1960
We present a deep Cousins RI photometric survey of the open cluster NGC 1960, complete to R_C \simeq 22, I_C \simeq 21, that is used to select a sample of very low-mass cluster candidates. Gemini spectroscopy of a subset of these is used to confirm membership and locate the age-dependent "lithium depletion boundary" (LDB) --the luminosity at which lithium remains unburned in its low-mass stars. The LDB implies a cluster age of 22 +/-4 Myr and is quite insensitive to choice of evolutionary model. NGC 1960 is the youngest cluster for which a LDB age has been estimated and possesses a well populated upper main sequence and a rich low-mass pre-main sequence. The LDB age determined here agrees well with precise age estimates made for the same cluster based on isochrone fits to its high- and low-mass populations. The concordance between these three age estimation techniques, that rely on different facets of stellar astrophysics at very different masses, is an important step towards calibrating the absolute ages of young open clusters and lends confidence to ages determined using any one of them.Based on observations made with the INT operated on the island of La Palma by the Isaac Newton Group in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias.
Based on observations obtained at the Gemini Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (United States), the Science and Technology Facilities Council (United Kingdom), the National Research Council (Canada), CONICYT (Chile), the Australian Research Council (Australia), MinistĂ©rio da CiĂȘncia, Tecnologia e Inovação (Brazil) and Ministerio de Ciencia, TecnologĂa e InnovaciĂłn Productiva (Argentina).
CPB acknowledges receipt of a Science and Technology Facilities Council postgraduate studentship. SPL is supported by a RCUK fellowship
Pre-main-sequence isochrones - II. Revising star and planet formation time-scales
archiveprefix: arXiv primaryclass: astro-ph.SR keywords: techniques: photometric, stars: evolution, stars: formation, stars: fundamental parameters, Hertzsprung-Russell and colour-magnitude diagrams, stars: pre-main-sequence adsurl: http://adsabs.harvard.edu/abs/2013MNRAS.434..806B adsnote: Provided by the SAO/NASA Astrophysics Data SystemWe have derived ages for 13 young (<30âMyr) star-forming regions and find that they are up to a factor of 2 older than the ages typically adopted in the literature. This result has wide-ranging implications, including that circumstellar discs survive longer (â 10â12 Myr) and that the average Class I lifetime is greater (â1âMyr) than currently believed. For each star-forming region, we derived two ages from colourâmagnitude diagrams. First, we fitted models of the evolution between the zero-age main sequence and terminal-age main sequence to derive a homogeneous set of main-sequence ages, distances and reddenings with statistically meaningful uncertainties. Our second age for each star-forming region was derived by fitting pre-main-sequence stars to new semi-empirical model isochrones. For the first time (for a set of clusters younger than 50âMyr), we find broad agreement between these two ages, and since these are derived from two distinct mass regimes that rely on different aspects of stellar physics, it gives us confidence in the new age scale. This agreement is largely due to our adoption of empirical colourâTeff relations and bolometric corrections for pre-main-sequence stars cooler than 4000âK. The revised ages for the star-forming regions in our sample are: âŒ2âMyr for NGC 6611 (Eagle Nebula; M 16), IC 5146 (Cocoon Nebula), NGC 6530 (Lagoon Nebula; M 8) and NGC 2244 (Rosette Nebula); âŒ6âMyr for Ï Ori, Cep OB3b and IC 348; â10âMyr for λ Ori (Collinder 69); â11âMyr for NGC 2169; â12âMyr for NGC 2362; â13âMyr for NGC 7160; â14âMyr for ÏâPer (NGC 884); and â20âMyr for NGC 1960 (M 36).CPMB is funded by a UK Science and Technology Facilities Council
(STFC) studentship. SPL is supported by an RCUK fellowship.
The authors would like to thank Charles D. H. Williams
for maintaining the Xgrid facilities at the University of Exeter
which were used to reduce the photometric data presented in this
study. The authors thank Amelia Bayo for bringing to our attention
the important work on the λ Ori region published in Bayo
et al. (2011) and Bayo et al. (2012) which we overlooked in
our original submission. The inclusion of these works does not
change the results or conclusions of the paper. The authors also
thank the referee for useful comments and constructive suggestions
that have greatly improved this work. This research has made
use of data obtained at the Isaac Newton Telescope which is operated
on the island of La Palma by the Isaac Newton Group
(ING) in the Spanish Observatorio del Roque de los Muchachos
of the Institutio de Astrofisica de Canarias. This research has also
made use of archival data products from the Two-Micron All-Sky
Survey (2MASS), which is a joint project of the University of
Massachusetts and the Infrared Processing and Analysis Center,
funded by the National Aeronautics and Space Administration
(NASA) and the National Science Foundation
Eigenvectors, Circulation and Linear Instabilities for Planetary Science in 3 Dimensions (ECLIPS3D)
This is the final version. Available on open access from EDP Sciences via the DOI in this recordContext. The study of linear waves and instabilities is necessary to understand the physical evolution of an atmosphere, and can provide physical interpretation of the complex flows found in simulations performed using Global Circulation Models (GCM). In particular, the acceleration of superrotating flow at the equator of hot Jupiters has mostly been studied under several simplifying assumptions, the relaxing of which may impact final results. Aims. We develop and benchmark a publicly available algorithm to identify the eigenmodes of an atmosphere around any initial steady state. We also solve for linear steady states. Methods. We linearise the hydrodynamical equations of a planetary atmosphere in a steady state with arbitrary velocities and thermal profile. We then discretise the linearised equations on an appropriate staggered grid, and solve for eigenvectors and linear steady solutions with the use of a parallel library for linear algebra: ScaLAPACK. We also implement a posteriori calculation of an energy equation in order to obtain more information on the underlying physics of the mode. Results. Our code is benchmarked against classical wave and instability test cases in multiple geometries. The steady linear circulation calculations also reproduce expected results for the atmosphere of hot Jupiters. We finally show the robustness of our energy equation, and its power to obtain physical insight into the modes. Conclusions. We have developed and benchmarked a code for the study of linear processes in planetary atmospheres, with an arbitrary steady state. The calculation of an a posteriori energy equation provides both increased robustness and physical meaning to the obtained eigenmodes. This code can be applied to various problems, and notably to further study the initial spin up of superrotation of GCM simulations of hot Jupiter.European Union Horizon 2020Leverhulme TrustScience and Technology Facilities Council (STFC
Automating Vehicles by Deep Reinforcement Learning using Task Separation with Hill Climbing
Within the context of autonomous driving a model-based reinforcement learning
algorithm is proposed for the design of neural network-parameterized
controllers. Classical model-based control methods, which include sampling- and
lattice-based algorithms and model predictive control, suffer from the
trade-off between model complexity and computational burden required for the
online solution of expensive optimization or search problems at every short
sampling time. To circumvent this trade-off, a 2-step procedure is motivated:
first learning of a controller during offline training based on an arbitrarily
complicated mathematical system model, before online fast feedforward
evaluation of the trained controller. The contribution of this paper is the
proposition of a simple gradient-free and model-based algorithm for deep
reinforcement learning using task separation with hill climbing (TSHC). In
particular, (i) simultaneous training on separate deterministic tasks with the
purpose of encoding many motion primitives in a neural network, and (ii) the
employment of maximally sparse rewards in combination with virtual velocity
constraints (VVCs) in setpoint proximity are advocated.Comment: 10 pages, 6 figures, 1 tabl
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