255 research outputs found
Generalized bisimulation metrics
International audienceThe pseudometric based on the Kantorovich lifting is one of the most popular notion of distance between probabilistic processes proposed in the literature. However, its application in verification is limited to linear properties. We propose a generalization which allows to deal with a wider class of properties, such as those used in security and privacy. More precisely, we propose a family of pseudometrics, parametrized on a notion of distance which depends on the property we want to verify. Furthermore, we show that the members of this family still characterize bisimilarity in terms of their kernel, and provide a bound on the corresponding distance between trace distributions. Finally, we study the instance corresponding to differential privacy, and we show that it has a dual form, easier to compute. We also prove that the typical process-algebra constructs are non-expansive, thus paving the way to a modular approach to verification
Approximating a Behavioural Pseudometric without Discount for<br> Probabilistic Systems
Desharnais, Gupta, Jagadeesan and Panangaden introduced a family of
behavioural pseudometrics for probabilistic transition systems. These
pseudometrics are a quantitative analogue of probabilistic bisimilarity.
Distance zero captures probabilistic bisimilarity. Each pseudometric has a
discount factor, a real number in the interval (0, 1]. The smaller the discount
factor, the more the future is discounted. If the discount factor is one, then
the future is not discounted at all. Desharnais et al. showed that the
behavioural distances can be calculated up to any desired degree of accuracy if
the discount factor is smaller than one. In this paper, we show that the
distances can also be approximated if the future is not discounted. A key
ingredient of our algorithm is Tarski's decision procedure for the first order
theory over real closed fields. By exploiting the Kantorovich-Rubinstein
duality theorem we can restrict to the existential fragment for which more
efficient decision procedures exist
Stedelijke diversiteit: een introductie
Steden worden steeds diverser wat betreft hun bevolkingssamenstelling en de
functies van de stad (werken, wonen, vrije tijd). Wetenschappers spreken in
dit kader steeds vaker over de opkomst van superdiverse steden en wijken. In
dit themanummer onderzoeken we de betekenis van deze ontwikkeling voor
beleidsmakers, bewoners en ondernemers in diverse steden en wijken
First evidence for the Cenomanian-Turonian oceanic anoxic event (OAE2, Bonarelli event) from the Ionian Zone, western continental Greece
Integrated biostratigraphic (planktonic foraminifera, calcareous nannofossils), chemostratigraphic (bulk C and O isotopes) and compound-specific organic geochemical studies of a mid-Cretaceous pelagic carbonateâblack shale succession of the Ionian Zone (western Greece), provide the first evidence for the CenomanianâTuronian oceanic anoxic event (OAE2, âBonarelliâ event) in mainland Greece. The event is manifested by the occurrence of a relatively thin (35 cm), yet exceptionally organic carbon-rich (44.5 wt% TOC), carbonate-free black shale, near the CenomanianâTuronian boundary within the Vigla limestone formation (BerriasianâTuronian). Compared to the âBonarelliâ black-shale interval from the type locality of OAE2 in MarcheâUmbria, Italy, this black shale exhibits greatly reduced stratigraphic thickness, coupled with a considerable relative enrichment in TOC. Isotopically, enriched ÎŽ[superscript 13]C values for both bulk organic matter (â22.2â°) and specific organic compounds are up to 5â° higher than those of underlying organic-rich strata of the Aptian-lower Albian Vigla Shale member, and thus compare very well with similar values of CenomanianâTuronian black shale occurrences elsewhere. The relative predominance of bacterial hopanoids in the saturated, apolar lipid fraction of the OAE2 black shale of the Ionian Zone supports recent findings suggesting the abundance of N[subscript 2]-fixing cyanobacteria in Cretaceous oceans during the CenomanianâTuronian and early Aptian oceanic anoxic events
A Sample of Ultra Steep Spectrum Sources Selected from the Westerbork In the Southern Hemisphere (WISH) survey
The 352 MHz Westerbork In the Southern Hemisphere (WISH) survey is the
southern extension of the WENSS, covering 1.60 sr between -9 < DEC < -26 to a
limiting flux density of ~18 mJy (5sigma). Due to the very low elevation of the
observations, the survey has a much lower resolution in declination than in
right ascension (54" x 54"cosec(DEC)). A correlation with the 1.4 GHz NVSS
shows that the positional accuracy is less constrained in declination than in
right ascension, but there is no significant systematic error. We present a
source list containing 73570 sources.
We correlate this WISH catalogue with the NVSS to construct a sample of faint
Ultra Steep Spectrum (USS) sources, which is accessible for follow-up studies
with large optical telescopes in the southern hemisphere. This sample is aimed
at increasing the number of known high redshift radio galaxies to allow
detailed follow-up studies of these massive galaxies and their environments in
the early Universe.Comment: 12 Pages, including 5 PostScript figures. Accepted for publication in
Astronomy & Astrophysics. The full WISH catalog with 73570 sources is
available from http://www.strw.leidenuniv.nl/wenss
Adaptive Optics Imaging Survey of Luminous Infrared Galaxies
We present high resolution imaging observations of a sample of previously
unidentified far-infrared galaxies at z < 0.3. The objects were selected by
cross-correlating the IRAS Faint Source Catalog with the VLA FIRST catalog and
the HST Guide Star Catalog to allow for adaptive optics observations. We found
two new ULIGs (with L_FIR equal to or greater than 10^{12} L_sun) and 19 new
LIGs (with L_FIR equal to or greater than 10^{11} L_sun). Twenty of the
galaxies in the sample were imaged with either the Lick or Keck adaptive optics
systems in H or K'. Galaxy morphologies were determined using the two
dimensional fitting program GALFIT and the residuals examined to look for
interesting structure. The morphologies reveal that at least 30% are involved
in tidal interactions, with 20% being clear mergers. An additional 50% show
signs of possible interaction. Line ratios were used to determine powering
mechanism; of the 17 objects in the sample showing clear emission lines - four
are active galactic nuclei and seven are starburst galaxies. The rest exhibit a
combination of both phenomena.Comment: 29 pages, 8 figures, accepted for publication in A
Distribution-based bisimulation for labelled Markov processes
In this paper we propose a (sub)distribution-based bisimulation for labelled
Markov processes and compare it with earlier definitions of state and event
bisimulation, which both only compare states. In contrast to those state-based
bisimulations, our distribution bisimulation is weaker, but corresponds more
closely to linear properties. We construct a logic and a metric to describe our
distribution bisimulation and discuss linearity, continuity and compositional
properties.Comment: Accepted by FORMATS 201
The radio counter-jet of the QSO 3C~48
We present multi--frequency radio observational results of the quasar 3C~48.
The observations were carried out with the Very Large Array (VLA) at five
frequencies of 0.33, 1.5, 4.8, 8.4, and 22.5 GHz, and with the Multi--Element
Radio Linked Interferometer Network (MERLIN) at the two frequencies of 1.6 and
5 GHz. The source shows a one--sided jet to the north within 1\arcsec, which
then extends to the northeast and becomes diffuse. Two bright components (N2
and N3), containing most of the flux density are present in the northern jet.
The spectral index of the two components is and
(). Our images show the
presence of an extended structure surrounding component N2, suggestive of
strong interaction between the jet and the interstellar medium (ISM) of the
host galaxy. A steep--spectrum component, labelled as S, located 0.25 ardsec
southwest to the flat--spectrum component which could be the core of 3C 48, is
detected at a significance of . Both the location and the steepness
of the spectrum of component S suggest the presence of a counter--jet in 3C 48.Comment: 7 pages, 6 figures, accepted by A&
Giant Lya nebulae associated with high redshift radio galaxies
We report deep Keck narrow-band Lya images of the luminous z > 3 radio
galaxies 4C 41.17, 4C 60.07, and B2 0902+34. The images show giant, 100-200 kpc
scale emission line nebulae, centered on these galaxies, which exhibit a wealth
of morphological structure, including extended low surface brightness emission
in the outer regions, radially directed filaments, cone-shaped structures and
(indirect) evidence for extended Lya absorption. We discuss these features
within a general scenario where the nebular gas cools gravitationally in large
Cold Dark Matter (CDM) halos, forming stars and multiple stellar systems.
Merging of these ``building'' blocks triggers large scale starbursts, forming
the stellar bulges of massive radio galaxy hosts, and feeds super-massive black
holes which produce the powerful radio jets and lobes. The radio sources,
starburst superwinds and AGN radiation then disrupt the accretion process
limiting galaxy and black hole growth, and imprint the observed filamentary and
cone-shaped structures of the Lya nebulae.Comment: 36 Pages, including 8 Postscript figures. Accepted for publication in
the Astrophysical Journa
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