159 research outputs found
Modal Logics of Topological Relations
Logical formalisms for reasoning about relations between spatial regions play
a fundamental role in geographical information systems, spatial and constraint
databases, and spatial reasoning in AI. In analogy with Halpern and Shoham's
modal logic of time intervals based on the Allen relations, we introduce a
family of modal logics equipped with eight modal operators that are interpreted
by the Egenhofer-Franzosa (or RCC8) relations between regions in topological
spaces such as the real plane. We investigate the expressive power and
computational complexity of logics obtained in this way. It turns out that our
modal logics have the same expressive power as the two-variable fragment of
first-order logic, but are exponentially less succinct. The complexity ranges
from (undecidable and) recursively enumerable to highly undecidable, where the
recursively enumerable logics are obtained by considering substructures of
structures induced by topological spaces. As our undecidability results also
capture logics based on the real line, they improve upon undecidability results
for interval temporal logics by Halpern and Shoham. We also analyze modal
logics based on the five RCC5 relations, with similar results regarding the
expressive power, but weaker results regarding the complexity
Hybridization of institutions
Extended version including all proofsModal logics are successfully used as specification logics for reactive systems. However, they are not expressive enough to refer to individual states and reason about the local behaviour of such systems. This limitation is overcome in hybrid logics which introduce special symbols for naming states in models. Actually, hybrid logics have recently regained interest, resulting in a number of new results and techniques as well as applications to software specification.
In this context, the first contribution of this paper is an attempt to ‘universalize’ the hybridization idea. Following the lines of [DS07], where a method to modalize arbitrary institutions is presented, the paper introduces a method to hybridize logics at the same institution-independent level. The method extends arbitrary institutions with Kripke semantics (for multi-modalities with arbitrary arities) and hybrid features. This paves the ground for a general result: any encoding (expressed as comorphism) from an arbitrary institution to first order logic (FOL) deter- mines a comorphism from its hybridization to FOL. This second contribution opens the possibility of effective tool support to specification languages based upon logics with hybrid features.Fundação para a Ciência e a Tecnologia (FCT
Structure of the hDmc1-ssDNA filament reveals the principles of its architecture
In eukaryotes, meiotic recombination is a major source of genetic diversity, but its defects in humans lead to abnormalities such as Down's, Klinefelter's and other syndromes. Human Dmc1 (hDmc1), a RecA/Rad51 homologue, is a recombinase that plays a crucial role in faithful chromosome segregation during meiosis. The initial step of homologous recombination occurs when hDmc1 forms a filament on single-stranded (ss) DNA. However the structure of this presynaptic complex filament for hDmc1 remains unknown. To compare hDmc1-ssDNA complexes to those known for the RecA/Rad51 family we have obtained electron microscopy (EM) structures of hDmc1-ssDNA nucleoprotein filaments using single particle approach. The EM maps were analysed by docking crystal structures of Dmc1, Rad51, RadA, RecA and DNA. To fully characterise hDmc1-DNA complexes we have analysed their organisation in the presence of Ca2+, Mg2+, ATP, AMP-PNP, ssDNA and dsDNA. The 3D EM structures of the hDmc1-ssDNA filaments allowed us to elucidate the principles of their internal architecture. Similar to the RecA/Rad51 family, hDmc1 forms helical filaments on ssDNA in two states: extended (active) and compressed (inactive). However, in contrast to the RecA/Rad51 family, and the recently reported structure of hDmc1-double stranded (ds) DNA nucleoprotein filaments, the extended (active) state of the hDmc1 filament formed on ssDNA has nine protomers per helical turn, instead of the conventional six, resulting in one protomer covering two nucleotides instead of three. The control reconstruction of the hDmc1-dsDNA filament revealed 6.4 protein subunits per helical turn indicating that the filament organisation varies depending on the DNA templates. Our structural analysis has also revealed that the N-terminal domain of hDmc1 accomplishes its important role in complex formation through domain swapping between adjacent protomers, thus providing a mechanistic basis for coordinated action of hDmc1 protomers during meiotic recombination
The chemical evolution of the solar neighbourhood for planet-hosting stars
Theoretical physical-chemical models for the formation of planetary systems
depend on data quality for the Sun's composition, that of stars in the solar
neighbourhood, and of the estimated "pristine" compositions for stellar
systems. The effective scatter and the observational uncertainties of elements
within a few hundred parsecs from the Sun, even for the most abundant metals
like carbon, oxygen and silicon, are still controversial. Here we analyse the
stellar production and the chemical evolution of key elements that underpin the
formation of rocky (C, O, Mg, Si) and gas/ice giant planets (C, N, O, S). We
calculate 198 galactic chemical evolution (GCE) models of the solar
neighbourhood to analyse the impact of different sets of stellar yields, of the
upper mass limit for massive stars contributing to GCE () and of
supernovae from massive-star progenitors which do not eject the bulk of the
iron-peak elements (faint supernovae). Even considering the GCE variation
produced via different sets of stellar yields, the observed dispersion of
elements reported for stars in the Milky Way disk is not reproduced. Among
others, the observed range of super-solar [Mg/Si] ratios, sub-solar [S/N], and
the dispersion of up to 0.5 dex for [S/Si] challenge our models. The impact of
varying depends on the adopted supernova yields. Thus,
observations do not provide a constraint on the M parametrization.
When including the impact of faint supernova models in GCE calculations,
elemental ratios vary by up to 0.1-0.2 dex in the Milky Way disk; this
modification better reproduces observations.Comment: 36 pages, 26 figures, 1 Table, 1 Appendix, Accepted for publication
in MNRA
Stream diatom biodiversity in islands and continents—A global perspective on effects of area, isolation and environment
Aim The species-area relationship (SAR) is one of the most distinctive biogeographic patterns, but global comparisons of the SARs between island and mainland are lacking for microbial taxa. Here, we explore whether the form of the SAR and the drivers of species richness, including area, environmental heterogeneity, climate and physico-chemistry, differ between islands and similarly sized areas on mainland, referred to as continental area equivalents (CAEs). Location Global. Taxon Stream benthic diatoms. Methods We generated CAEs on six continental datasets and examined the SARs of CAEs and islands (ISAR). Then, we compared CAEs and islands in terms of total richness and richness of different ecological guilds. We tested the factors contributing to richness in islands and CAEs with regressions. We used structural equation models to determine the effects of area versus environmental heterogeneity, climate and local conditions on species richness. Results We found a non-significant ISAR, but a significant positive SAR in CAEs. Richness in islands was related to productivity. Richness in CAEs was mainly dependent on area and climate, but not directly on environmental heterogeneity. Species richness within guilds exhibited inconsistent relationships with island isolation and area. Main conclusions Ecological and evolutionary processes shaping diatom island biogeography do not depend on area at the worldwide scale probably due to the presence of distinct species pool across islands. Conversely, area was an important driver of diatom richness in continents, and this effect could be attributed to dispersal. Continents had greater richness than islands, but this was a consequence of differences in environmental conditions such as specific island climatic conditions. We stress the need for more island data on benthic diatoms, particularly from archipelagos, to better understand the biogeography of this most speciose group of algae
The Blue Stragglers of the Old Open Cluster NGC 188
The old (7 Gyr) open cluster NGC 188 has yielded a wealth of astrophysical
insight into its rich blue straggler population. Specifically, the NGC 188 blue
stragglers are characterized by: A binary frequency of 80% for orbital periods
less than days;Typical orbital periods around 1000 days;Typical
secondary star masses of 0.5 M; At least some white dwarf companion
stars; Modestly rapid rotation; A bimodal radial spatial distribution;
Dynamical masses greater than standard stellar evolution masses (based on
short-period binaries); Under-luminosity for dynamical masses (short-period
binaries). Extensive -body modeling of NGC 188 with empirical initial
conditions reproduces the properties of the cluster, and in particular the
main-sequence solar-type binary population. The current models also reproduce
well the binary orbital properties of the blue stragglers, but fall well short
of producing the observed number of blue stragglers. This deficit could be
resolved by reducing the frequency of common-envelope evolution during Roche
lobe overflow. Both the observations and the -body models strongly indicate
that the long-period blue-straggler binaries - which dominate the NGC 188 blue
straggler population - are formed by asymptotic-giant (primarily) and red-giant
mass transfer onto main sequence stars. The models suggest that the few
non-velocity-variable blue stragglers formed from mergers or collisions.
Several remarkable short-period double-lined binaries point to the importance
of subsequent dynamical exchange encounters, and provide at least one example
of a likely collisional origin for a blue straggler.Comment: Chapter 3, in Ecology of Blue Straggler Stars, H.M.J. Boffin, G.
Carraro & G. Beccari (Eds), Astrophysics and Space Science Library, Springe
Testing spooky action at a distance
In science, one observes correlations and invents theoretical models that
describe them. In all sciences, besides quantum physics, all correlations are
described by either of two mechanisms. Either a first event influences a second
one by sending some information encoded in bosons or molecules or other
physical carriers, depending on the particular science. Or the correlated
events have some common causes in their common past. Interestingly, quantum
physics predicts an entirely different kind of cause for some correlations,
named entanglement. This new kind of cause reveals itself, e.g., in
correlations that violate Bell inequalities (hence cannot be described by
common causes) between space-like separated events (hence cannot be described
by classical communication). Einstein branded it as spooky action at a
distance. A real spooky action at a distance would require a faster than light
influence defined in some hypothetical universally privileged reference frame.
Here we put stringent experimental bounds on the speed of all such hypothetical
influences. We performed a Bell test during more than 24 hours between two
villages separated by 18 km and approximately east-west oriented, with the
source located precisely in the middle. We continuously observed 2-photon
interferences well above the Bell inequality threshold. Taking advantage of the
Earth's rotation, the configuration of our experiment allowed us to determine,
for any hypothetically privileged frame, a lower bound for the speed of this
spooky influence. For instance, if such a privileged reference frame exists and
is such that the Earth's speed in this frame is less than 10^-3 that of the
speed of light, then the speed of this spooky influence would have to exceed
that of light by at least 4 orders of magnitude.Comment: Preliminary version of Nature 454, 861-864 (14 August 2008). 5 pages
and 5 figure
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