3,204 research outputs found
A Practically Efficient Algorithm for Generating Answers to Keyword Search over Data Graphs
In keyword search over a data graph, an answer is a non-redundant subtree
that contains all the keywords of the query. A naive approach to producing all
the answers by increasing height is to generalize Dijkstra's algorithm to
enumerating all acyclic paths by increasing weight. The idea of freezing is
introduced so that (most) non-shortest paths are generated only if they are
actually needed for producing answers. The resulting algorithm for generating
subtrees, called GTF, is subtle and its proof of correctness is intricate.
Extensive experiments show that GTF outperforms existing systems, even ones
that for efficiency's sake are incomplete (i.e., cannot produce all the
answers). In particular, GTF is scalable and performs well even on large data
graphs and when many answers are needed.Comment: Full version of ICDT'16 pape
Does binding of synesthetic color to the evoking grapheme require attention?
The official published version can be accessed from the link below.The neural mechanisms involved in binding features such as shape and color are a matter of some debate. Does accurate binding rely on spatial attention functions of the parietal lobe or can it occur without attentional input? One extraordinary phenomenon that may shed light on this question is that of chromatic-graphemic synesthesia, a rare condition in which letter shapes evoke color perceptions. A popular suggestion is that synesthesia results from cross-activation between different functional regions (e.g., between shape and color areas of the ventral pathway). Under such conditions binding may not require parietal involvement and could occur preattentively. We tested this hypothesis in two synesthetes who perceived grayscale letters and digits in color. We found no evidence for preattentive binding using a visual search paradigm in which the target was a synesthetic inducer. In another experiment involving color judgments, we show that the congruency of target color and the synesthetic color of irrelevant digits modulates performance more when the digits are included within the attended region of space. We propose that the mechanisms giving rise to this type of synesthesia appear to follow at least some principles of normal binding, and even synesthetic binding seems to require attention.This work has been supported by a Veterans Administration Senior Research Career Scientist Award and NINDS grant #MH62331 to LCR and the Elizabeth Roboz Einstein fellowship in Neuroscience and Human Development to NS
Relativistic Radiation Mediated Shocks
The structure of relativistic radiation mediated shocks (RRMS) propagating
into a cold electron-proton plasma is calculated and analyzed. A qualitative
discussion of the physics of relativistic and non relativistic shocks,
including order of magnitude estimates for the relevant temperature and length
scales, is presented. Detailed numerical solutions are derived for shock
Lorentz factors in the range , using a novel
iteration technique solving the hydrodynamics and radiation transport equations
(the protons, electrons and positrons are argued to be coupled by collective
plasma processes and are treated as a fluid). The shock transition
(deceleration) region, where the Lorentz factor drops from to , is characterized by high plasma temperatures and highly anisotropic radiation, with characteristic
shock-frame energy of upstream and downstream going photons of a few~ and , respectively.Photon scattering is dominated
by e pairs, with pair to proton density ratio reaching
. The width of the deceleration region, in terms of
Thomson optical depths for upstream going photons, is large,
( neglecting the contribution of
pairs) due to Klein Nishina suppression of the scattering cross section. A high
energy photon component, narrowly beamed in the downstream direction, with a
nearly flat power-law like spectrum, , and an energy
cutoff at carries a fair fraction of the energy flux
at the end of the deceleration region. An approximate analytic model of RRMS,
reproducing the main features of the numerical results, is provided
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