4,972 research outputs found
Functorial Semantics for Petri Nets under the Individual Token Philosophy
Although the algebraic semantics of place/transition Petri nets under the collective token philosophy has been fully explained in terms of (strictly) symmetric (strict) monoidal categories, the analogous construction under the individual token philosophy is not completely satisfactory because it lacks universality and also functoriality. We introduce the notion of pre-net to recover these aspects, obtaining a fully satisfactory categorical treatment centered on the notion of adjunction. This allows us to present a purely logical description of net behaviours under the individual token philosophy in terms of theories and theory morphisms in partial membership equational logic, yielding a complete match with the theory developed by the authors for the collective token view of net
The Speciality Index as invariant indicator in the BKL Mixmaster Dynamics
The speciality index, which has been mainly used in Numerical Relativity for
studying gravitational waves phenomena as an indicator of the special or
non-special Petrov type character of a spacetime, is applied here in the
context of Mixmaster cosmology, using the Belinski-Khalatnikov-Lifshitz map.
Possible applications for the associated chaotic dynamics are discussed
Two-parameter non-linear spacetime perturbations: gauge transformations and gauge invariance
An implicit fundamental assumption in relativistic perturbation theory is
that there exists a parametric family of spacetimes that can be Taylor expanded
around a background. The choice of the latter is crucial to obtain a manageable
theory, so that it is sometime convenient to construct a perturbative formalism
based on two (or more) parameters. The study of perturbations of rotating stars
is a good example: in this case one can treat the stationary axisymmetric star
using a slow rotation approximation (expansion in the angular velocity Omega),
so that the background is spherical. Generic perturbations of the rotating star
(say parametrized by lambda) are then built on top of the axisymmetric
perturbations in Omega. Clearly, any interesting physics requires non-linear
perturbations, as at least terms lambda Omega need to be considered. In this
paper we analyse the gauge dependence of non-linear perturbations depending on
two parameters, derive explicit higher order gauge transformation rules, and
define gauge invariance. The formalism is completely general and can be used in
different applications of general relativity or any other spacetime theory.Comment: 22 pages, 3 figures. Minor changes to match the version appeared in
Classical and Quantum Gravit
Simultaneous follow-up of planetary transits: revised physical properties for the planetary systems HAT-P-16 and WASP-21
Context. By now more than 300 planets transiting their host star have been
found, and much effort is being put into measuring the properties of each
system. Light curves of planetary transits often contain deviations from a
simple transit shape, and it is generally difficult to differentiate between
anomalies of astrophysical nature (e.g. starspots) and correlated noise due to
instrumental or atmospheric effects. Our solution is to observe transit events
simultaneously with two telescopes located at different observatories. Aims.
Using this observational strategy, we look for anomalies in the light curves of
two transiting planetary systems and accurately estimate their physical
parameters. Methods. We present the first photometric follow-up of the
transiting planet HAT-P-16 b, and new photometric observations of WASP-21 b,
obtained simultaneously with two medium-class telescopes located in different
countries, using the telescope defocussing technique. We modeled these and
other published data in order to estimate the physical parameters of the two
planetary systems. Results. The simultaneous observations did not highlight
particular features in the light curves, which is consistent with the low
activity levels of the two stars. For HAT-P-16, we calculated a new ephemeris
and found that the planet is 1.3 \sigma colder and smaller (Rb = 1.190 \pm
0.037 RJup) than the initial estimates, suggesting the presence of a massive
core. Our physical parameters for this system point towards a younger age than
previously thought. The results obtained for WASP-21 reveal lower values for
the mass and the density of the planet (by 1.0 \sigma and 1.4 \sigma
respectively) with respect to those found in the discovery paper, in agreement
with a subsequent study. We found no evidence of any transit timing variations
in either system.Comment: 8 pages, 6 figures, accepted for publication in A&
Open Transactions on Shared Memory
Transactional memory has arisen as a good way for solving many of the issues
of lock-based programming. However, most implementations admit isolated
transactions only, which are not adequate when we have to coordinate
communicating processes. To this end, in this paper we present OCTM, an
Haskell-like language with open transactions over shared transactional memory:
processes can join transactions at runtime just by accessing to shared
variables. Thus a transaction can co-operate with the environment through
shared variables, but if it is rolled-back, also all its effects on the
environment are retracted. For proving the expressive power of TCCS we give an
implementation of TCCS, a CCS-like calculus with open transactions
Chemotherapy accelerates immune-senescence and functional impairments of Vδ2pos T cells in elderly patients affected by liver metastatic colorectal cancer.
Human (gamma delta) γδ T cells are unconventional innate-like lymphocytes displaying a broad array of anti-tumor activities with promising perspectives in cancer immunotherapy. In this context, Vδ2pos T cells represent the preferential target of several immunotherapy protocols against solid tumors. However, the impact of both aging and chemotherapy (CHT) on Vδ2pos T cells is still unknown. The present study evaluates with multi-parametric flow cytometry the frequencies, terminal differentiation, senescence and effector-functions of peripheral blood and tumor infiltrating Vδ2pos T cells purified from liver metastases (CLM) of patients affected by colorectal cancer (CRC) compared to those of sex- and age-matched healthy donors. The peripheral blood of CLM patients underwent CHT is characterized by decreased amounts of Vδ2pos T cells showing a relative increase of terminally-differentiated CD27neg/CD45RApos (TEMRA) cells. The enrichment of this latter subset is associated with an increased expression of the senescent marker CD57. The acquisition of CD57 on TEMRA Vδ2pos T cells is also coupled with impairments in cytotoxicity and production of TNF-α and IFN-γ. These features resemble the acquisition of an immune-senescent profile by Vδ2pos T cells from CLM patients that received CHT, a phenomenon that is also associated with the loss of the co-stimulatory marker CD28 and with the induced expression of CD16. The group of CLM patients underwent CHT and older than 60 years old showed higher frequencies of CD57pos and TEMRA Vδ2pos T cells. Similar results were found for tumor infiltrating Vδ2pos T cell subset purified from CLM specimens of patients treated with CHT. The toxicity of CHT regimens also affects the homeostasis of Vδ2pos T cells by inducing higher frequencies of circulating CD57pos TEMRA subset in CLM underwent CHT and younger than 60 years old. Taken together, our data demonstrate that the enrichment of senescent Vδ2pos T cells in CLM patients is not only induced by patients' aging but also by the toxicity of CHT that further accelerates the accumulation of CD57pos TEMRA cells highly dysfunctional in their anti-tumor activities. These results are important to both predict the clinical outcome of CLM and to optimize those protocols of cell cancer immunotherapy employing unconventional Vδ2pos T cells
Are Simple Real Pole Solutions Physical?
We consider exact solutions generated by the inverse scattering technique,
also known as the soliton transformation. In particular, we study the class of
simple real pole solutions. For quite some time, those solutions have been
considered interesting as models of cosmological shock waves. A coordinate
singularity on the wave fronts was removed by a transformation which induces a
null fluid with negative energy density on the wave front. This null fluid is
usually seen as another coordinate artifact, since there seems to be a general
belief that that this kind of solution can be seen as the real pole limit of
the smooth solution generated with a pair of complex conjugate poles in the
transformation. We perform this limit explicitly, and find that the belief is
unfounded: two coalescing complex conjugate poles cannot yield a solution with
one real pole. Instead, the two complex conjugate poles go to a different
limit, what we call a ``pole on a pole''. The limiting procedure is not unique;
it is sensitive to how quickly some parameters approach zero. We also show that
there exists no improved coordinate transformation which would remove the
negative energy density. We conclude that negative energy is an intrinsic part
of this class of solutions.Comment: 13 pages, 3 figure
Revisiting causality, coalgebraically
In this paper we recast the classical Darondeau–Degano’s causal semantics of concurrency in a coalgebraic setting, where we derive a compact model. Our construction is inspired by the one of Montanari and Pistore yielding causal automata, but we show that it is instance of an existing categorical framework for modeling the semantics of nominal calculi, whose relevance is further demonstrated. The key idea is to represent events as names, and
the occurrence of a new event as name generation. We model causal semantics as a coalgebra
over a presheaf, along the lines of the Fiore–Turi approach to the semantics of nominal
calculi. More specifically, we take a suitable category of finite posets, representing causal
relations over events, and we equip it with an endofunctor that allocates new events and
relates them to their causes. Presheaves over this category express the relationship between
processes and causal relations among the processes’ events. We use the allocation operator to
define a category of well-behaved coalgebras: it models the occurrence of a new event along
each transition. Then we turn the causal transition relation into a coalgebra in this category,
where labels only exhibit maximal events with respect to the source states’ poset, and we
show that its bisimilarity is essentially Darondeau–Degano’s strong causal bisimilarity. This
coalgebra is still infinite-state, but we exploit the equivalence between coalgebras over a
class of presheaves and History Dependent automata to derive a compact representation,
where states only retain the poset of the most recent events for each atomic subprocess, and
are isomorphic up to order-preserving permutations. Remarkably, this reduction of states is
automatically performed along the equivalence
RadioAstron space VLBI imaging of polarized radio emission in the high-redshift quasar 0642+449 at 1.6 GHz
Polarization of radio emission in extragalactic jets at a sub-milliarcsecond
angular resolution holds important clues for understanding the structure of the
magnetic field in the inner regions of the jets and in close vicinity of the
supermassive black holes in the centers of active galaxies. Space VLBI
observations provide a unique tool for polarimetric imaging at a
sub-milliarcsecond angular resolution and studying the properties of magnetic
field in active galactic nuclei on scales of less than 10^4 gravitational
radii. A space VLBI observation of high-redshift quasar TXS 0642+449 (OH 471),
made at a wavelength of 18 cm (frequency of 1.6 GHz) as part of the Early
Science Programme (ESP) of the RadioAstron} mission, is used here to test the
polarimetric performance of the orbiting Space Radio Telescope (SRT) employed
by the mission, to establish a methodology for making full Stokes polarimetry
with space VLBI at 1.6 GHz, and to study the polarized emission in the target
object on sub-milliarcsecond scales. Polarization leakage of the SRT at 18 cm
is found to be within 9 percents in amplitude, demonstrating the feasibility of
high fidelity polarization imaging with RadioAstron at this wavelength. A
polarimetric image of 0642+449 with a resolution of 0.8 mas (signifying an ~4
times improvement over ground VLBI observations at the same wavelength) is
obtained. The image shows a compact core-jet structure with low (~2%)
polarization and predominantly transverse magnetic field in the nuclear region.
The VLBI data also uncover a complex structure of the nuclear region, with two
prominent features possibly corresponding to the jet base and a strong
recollimation shock. The maximum brightness temperature at the jet base can be
as high as 4*10^13 K.Comment: Accepted for publication in A&A, 10 pages, 6 figure
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