1,038 research outputs found
A Model-Based methodology to support the Space System Engineering (MBSSE)
International audienceThis paper presents a model based methodology that relies on the sound basis of the most recent and widespread applicable system engineering standards and model based practices, The methodology has been defined to support domain specific space system engineering standards and practices and assessed through the application on industrial case studies. A complementary formal verification approach has also been experimented
An Optimistic Planning Approach for the Aircraft Landing Problem
International audienceThe Aircraft Landing Problem consists in sequencing aircraft on the available runways and scheduling their landing times taking into consideration several operational constraints, in order to increase the runway capacity and/or to reduce delays.In this work we propose a new Mixed Integer Programming (MIP) model for sequencing and scheduling aircraft landings on a single or multiple independent runways incorporating safety constraints by means of separation between aircraft at runways threshold. Due to the NP-hardness of the problem, solving directly the MIP model for large realistic instances yields redhibitory computation times. Therefore, we introduce a novel heuristic search methodology based on Optimistic Planning that significantly improve the FCFS (First-Come First-Served) solution, and provides good-quality solutions inreasonable computational time. The solution approach is then tested on medium and large realistic instances generated from real-world traffic on Paris-Orly airport to show the benefit of our approach
Very Extended and at low levels, Gravity and Supergravity
We define a level for a large class of Lorentzian Kac-Moody algebras. Using
this we find the representation content of very extended and
(i.e. ) at low levels in terms of and
representations respectively. The results are consistent with the conjectured
very extended and symmetries of gravity and maximal supergravity
theories given respectively in hep-th/0104081 and hep-th/0107209. We explain
how these results provided further evidence for these conjectures.Comment: 16 pages, plain tex (equation 3.3 modified and one reference
expanded
Formal Verification and Validation of AADL Models
International audienceSafety-critical systems are increasingly difficult to com- prehend due to their rising complexity. Methodologies, tools and modeling formalisms have been developed to overcome this. Component-based design is an im- portant paradigm that is shared by many of them
Detailed study of BBN implications of neutrino oscillation generated neutrino asymmetries in some four neutrino models
We re-examine the evolution of neutrino asymmetries in several four neutrino
models. The first case involves the direct creation of by oscillations. In the second case, we consider the mass
hierarchy where oscillations generate a large and some of
this asymmetry is converted into by oscillations. We estimate the implications for BBN for a range of
cosmologically interesting values. The present paper improves on
previous published work by taking into account the finite repopulation rate and
the time dependence of the distortions to the neutrino momentum distributions.
The treatment of chemical decoupling is also improved.Comment: Expanded discussion on the sign of the neutrino asymmetr
Neutrino-Mixing-Generated Lepton Asymmetry and the Primordial He Abundance
It has been proposed that an asymmetry in the electron neutrino sector may be
generated by resonant active-sterile neutrino transformations during Big Bang
Nucleosynthesis (BBN). We calculate the change in the primordial He yield
resulting from this asymmetry, taking into account both the time evolution
of the and distribution function and the spectral
distortions in these. We calculate this change in two schemes: (1) a lepton
asymmetry directly generated by mixing with a lighter right-handed
sterile neutrino ; and (2) a lepton asymmetry generated by a
or transformation
which is subsequently partially converted to an asymmetry in the
sector by a matter-enhanced active-active neutrino
transformation. In the first scheme, we find that the percentage change in
is between -1% and 9% (with the sign depending on the sign of the asymmetry),
bounded by the Majorana mass limit m_{\nu_e}\la 1 eV. In the second scheme,
the maximal percentage reduction in is 2%, if the lepton number asymmetry
in neutrinos is positive; Otherwise, the percentage increase in is \la 5%
for m^2_{\nu_\mu,\nu_\tau}-m^2_{\nu_s}\la 10^4 eV. We conclude that the
change in the primordial He yield induced by a neutrino-mixing-generated
lepton number asymmetry can be substantial in the upward direction, but limited
in the downward direction.Comment: 15 pages, 7 figures, submitted to PR
Dual Linearised Gravity in Arbitrary Dimensions
We construct dual formulation of linearised gravity in first order tetrad
formalism in arbitrary dimensions within the path integral framework following
the standard duality algorithm making use of the global shift symmetry of the
tetrad field. The dual partition function is in terms of the (mixed symmetric)
tensor field in {\it frame-like}
formulation. We obtain in d-dimensions the dual Lagrangian in a closed form in
terms of field strength of the dual frame-like field. Next by coupling a source
with the (linear) Riemann tensor in d-dimensions, dual generating functional is
obtained. Using this an operator mapping between (linear) Riemann tensor and
Riemann tensor corresponding to the dual field is derived and we also discuss
the exchange of equations of motion and Bianchi identity.Comment: 14 pages, typos corrected, Published version: Class. Quantum Grav.
22(2005)538
Temperature Fluctuations and Abundances in HII Galaxies
There is evidence for temperature fluctuations in Planetary Nebulae and in
Galactic HII regions. If such fluctuations occur in the low-metallicity,
extragalactic HII regions used to probe the primordial helium abundance, the
derived 4He mass fraction, Y_P, could be systematically different from the true
primordial value. For cooler, mainly high-metallicity HII regions the derived
helium abundance may be nearly unchanged but the oxygen abundance could have
been seriously underestimated. For hotter, mainly low-metallicity HII regions
the oxygen abundance is likely accurate but the helium abundance could be
underestimated. The net effect is to tilt the Y vs. Z relation, making it
flatter and resulting in a higher inferred Y_P. Although this effect could be
large, there are no data which allow us to estimate the size of the temperature
fluctuations for the extragalactic HII regions. Therefore, we have explored
this effect via Monte Carlos in which the abundances derived from a fiducial
data set are modified by \Delta-T chosen from a distribution with 0 < \Delta-T
< \Delta-T_max where \Delta-T_max is varied from 500K to 4000K. It is
interesting that although this effect shifts the locations of the HII regions
in Y vs. O/H plane, it does not introduce any significant additional
dispersion.Comment: 11 pages, 9 postscript figures; submitted to the Ap
Rational invariants of even ternary forms under the orthogonal group
In this article we determine a generating set of rational invariants of
minimal cardinality for the action of the orthogonal group on
the space of ternary forms of even degree . The
construction relies on two key ingredients: On one hand, the Slice Lemma allows
us to reduce the problem to dermining the invariants for the action on a
subspace of the finite subgroup of signed permutations. On the
other hand, our construction relies in a fundamental way on specific bases of
harmonic polynomials. These bases provide maps with prescribed
-equivariance properties. Our explicit construction of these
bases should be relevant well beyond the scope of this paper. The expression of
the -invariants can then be given in a compact form as the
composition of two equivariant maps. Instead of providing (cumbersome) explicit
expressions for the -invariants, we provide efficient algorithms
for their evaluation and rewriting. We also use the constructed
-invariants to determine the -orbit locus and
provide an algorithm for the inverse problem of finding an element in
with prescribed values for its invariants. These are
the computational issues relevant in brain imaging.Comment: v3 Changes: Reworked presentation of Neuroimaging application,
refinement of Definition 3.1. To appear in "Foundations of Computational
Mathematics
Time Variations in the Scale of Grand Unification
We study the consequences of time variations in the scale of grand
unification, , when the Planck scale and the value of the unified coupling
at the Planck scale are held fixed. We show that the relation between the
variations of the low energy gauge couplings is highly model dependent. It is
even possible, in principle, that the electromagnetic coupling varies,
but the strong coupling does not (to leading approximation). We
investigate whether the interpretation of recent observations of quasar
absorption lines in terms of time variation in can be accounted for by
time variation in . Our formalism can be applied to any scenario where a
time variation in an intermediate scale induces, through threshold corrections,
time variations in the effective low scale couplings.Comment: 14 pages, revtex4; Updated observational results and improved
statistical analysis (section IV); added reference
- âŠ