1,117 research outputs found
Lines on projective varieties and applications
The first part of this note contains a review of basic properties of the
variety of lines contained in an embedded projective variety and passing
through a general point. In particular we provide a detailed proof that for
varieties defined by quadratic equations the base locus of the projective
second fundamental form at a general point coincides, as a scheme, with the
variety of lines. The second part concerns the problem of extending embedded
projective manifolds, using the geometry of the variety of lines. Some
applications to the case of homogeneous manifolds are included.Comment: 15 pages. One example removed; one remark and some references added;
typos correcte
The BRST quantization and the no-ghost theorem for AdS_3
In our previous papers, we prove the no-ghost theorem without light-cone
directions (hep-th/0005002, hep-th/0303051). We point out that our results are
valid for more general backgrounds. In particular, we prove the no-ghost
theorem for AdS_3 in the context of the BRST quantization (with the standard
restriction on the spin). We compare our BRST proof with the OCQ proof and
establish the BRST-OCQ equivalence for AdS_3. The key in both approaches lies
in the certain structure of the matter Hilbert space as a product of two Verma
modules. We also present the no-ghost theorem in the most general form.Comment: 22 pages, JHEP and AMS-LaTeX; v2 & 3: minor improvement
Gauge-invariant gravitational wave modes in pre-big bang cosmology
The t<0 branch of pre-big bang cosmological scenarios is subject to a
gravitational wave instability. The unstable behaviour of tensor perturbations
is derived in a very simple way in Hwang's covariant and gauge-invariant
formalism developed for extended theories of gravity. A simple interpretation
of this instability as the effect of an "antifriction" is given, and it is
argued that a universe must eventually enter the expanding phase.Comment: 4 pages, latex, to appear in Eur. Phys. J.
Note on "An efficient approach for solving the lot-sizing problem with time-varying storage capacities"
In a recent paper Gutiérrez et al. (2008) show that the lot-sizing problem with inventory bounds can be solved in O(T log T) time. In this note we show that their algorithm does not lead to an optimal solution in general
Recommended Finite Element Formulations for the Analysis of Off-shore Blast Walls in an Explosion
This study suggests relevant finite element (FE) formulations for the structural analysis of offshore blast walls subjected to blast loadings due to hydrocarbon explosions. The present blast wall model adopted from HSE (2003) consists of a corrugated panel and supporting members, and was modelled with shell, thick-shell, and solid element combinations in LS-DYNA, an explicit finite element analysis (FEA) solver. Stainless and mild steels were employed as materials for the blast wall model, with consideration of strain rate effect throughout ten (10) pulse pressure load regimes. The obtained FEA results were validated by experimental data from HSE (2003) with decent agreement. In the present study, recommended FE formulations with additional hourglass control functions were widely discussed from the perspectives of solution accuracy and computational cost based on a statistical approach. The obtained outcomes could be used for the structural analysis and design of offshore blast walls in the estimations of maximum and permanent deformations under blast loadings.111Ysciescopu
Non-linear corrections to inflationary power spectrum
We study non-linear contributions to the power spectrum of the curvature
perturbation on super-horizon scales, produced during slow-roll inflation
driven by a canonical single scalar field. We find that on large scales the
linear power spectrum completely dominates and leading non-linear corrections
remain totally negligible, indicating that we can safely rely on linear
perturbation theory to study inflationary power spectrum. We also briefly
comment on the infrared and ultraviolet behaviour of the non-linear
corrections.Comment: (v1) 14 pages, 2 figures; (v2) references added and discussions
expanded, including a new version of Figure 2, to appear in Journal of
Cosmology and Astroparticle Physic
BRST Quantization of String Theory in AdS(3)
We study the BRST quantization of bosonic and NSR strings propagating in
AdS(3) x N backgrounds. The no-ghost theorem is proved using the
Frenkel-Garland-Zuckerman method. Regular and spectrally-flowed representations
of affine SL(2,R) appear on an equal footing. Possible generalizations to
related curved backgrounds are discussed.Comment: JHEP style, 23 pages; v2:minor changes and references added; v3:
typos corrected, version to appear in JHEP; v4: one reference adde
Low-scale Quintessential Inflation
In quintessential inflationary model, the same master field that drives
inflation becomes, later on, the dynamical source of the (present) accelerated
expansion. Quintessential inflationary models require a curvature scale at the
end of inflation around in order to explain the large scale
fluctuations observed in the microwave sky. If the curvature scale at the end
of inflation is much smaller than , the large scale adiabatic
mode may be produced thanks to the relaxation of a scalar degree of freedom,
which will be generically denoted, according to the recent terminology, as the
curvaton field. The production of the adiabatic mode is analysed in detail in
the case of the minimal quintessential inflationary model originally proposed
by Peebles and Vilenkin.Comment: 25 pages; 5 figure
Density Perturbations in the Ekpyrotic Scenario
We study the generation of density perturbations in the ekpyrotic scenario
for the early universe, including gravitational backreaction. We expose
interesting subtleties that apply to both inflationary and ekpyrotic models.
Our analysis includes a detailed proposal of how the perturbations generated in
a contracting phase may be matched across a `bounce' to those in an expanding
hot big bang phase. For the physical conditions relevant to the ekpyrotic
scenario, we re-obtain our earlier result of a nearly scale-invariant spectrum
of energy density perturbations. We find that the perturbation amplitude is
typically small, as desired to match observation.Comment: 36 pages, compressed and RevTex file, one postscript figure file.
Minor typographical and numerical errors corrected, discussion added. This
version to appear in Physical Review
Remote oil spill detection and monitoring on ice-covered waters
The spillage of oil in Polar Regions is particularly serious due to the threat to the environment and the difficulties in detecting and tracking the full extent of the oil seepage beneath the sea ice. Development of fast and reliable sensing techniques is highly desirable. In this paper hyperspectral imaging is proposed as a potential tool to detect the presence of oil beneath the sea ice. A feasibility study project was initiated to explore the detectability of the oil under ice layer. Some preliminary results obtained during this project are discussed
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