81,641 research outputs found
A Fixed Parameter Tractable Approximation Scheme for the Optimal Cut Graph of a Surface
Given a graph cellularly embedded on a surface of genus , a
cut graph is a subgraph of such that cutting along yields a
topological disk. We provide a fixed parameter tractable approximation scheme
for the problem of computing the shortest cut graph, that is, for any
, we show how to compute a approximation of
the shortest cut graph in time .
Our techniques first rely on the computation of a spanner for the problem
using the technique of brick decompositions, to reduce the problem to the case
of bounded tree-width. Then, to solve the bounded tree-width case, we introduce
a variant of the surface-cut decomposition of Ru\'e, Sau and Thilikos, which
may be of independent interest
The Large N_c Baryon-Meson I_t = J_t Rule Holds for Three Flavors
It has long been known that nonstrange baryon-meson scattering in the 1/N_c
expansion of QCD greatly simplifies when expressed in terms of t-channel
exchanges: The leading-order amplitudes satisfy the selection rule I_t = J_t.
We show that I_t = J_t, as well as Y_t = 0, also hold for the leading
amplitudes when the baryon and/or meson contain strange quarks, and also
characterize their 1/N_c corrections, thus opening a new front in the
phenomenological study of baryon-meson scattering and baryon resonances.Comment: 12 pages, 0 figures, ReVTe
Scars on the CBR?
We ask whether the universe can be a patchwork consisting of distinct regions
of matter and antimatter. In previous work we demonstrated that
post-recombination matter-antimatter contact near regional boundaries leads to
an observable (but unobserved) gamma-ray flux for domain sizes of less than a
few thousand Mpc, thereby excluding such domains. In this paper we consider the
pre-recombination signal from domains of larger size.Comment: 6 pages, late
Generation of Rabi frequency radiation using exciton-polaritons
We study the use of exciton-polaritons in semiconductor microcavities to
generate radiation spanning the infrared to terahertz regions of the spectrum
by exploiting transitions between upper and lower polariton branches. The
process, which is analogous to difference-frequency generation (DFG), relies on
the use of semiconductors with a nonvanishing second-order susceptibility. For
an organic microcavity composed of a nonlinear optical polymer, we predict a
DFG irradiance enhancement of , as compared to a bare nonlinear
polymer film, when triple resonance with the fundamental cavity mode is
satisfied. In the case of an inorganic microcavity composed of (111) GaAs, an
enhancement of is found, as compared to a bare GaAs slab. Both
structures show high wavelength tunability and relaxed design constraints due
to the high modal overlap of polariton modes.Comment: 8 pages, 7 figure
Non-equilibrium Thermodynamics and Fluctuations
In the last ten years, a number of ``Conventional Fluctuation Theorems'' have
been derived for systems with deterministic or stochastic dynamics, in a
transient or in a non-equilibrium stationary state. These theorems gave
explicit expressions for the ratio of the probability to find the system with a
certain value of entropy (or heat) production to that of finding the opposite
value. A similar theorem for the fluctuations of the work done on a system has
recently been demonstrated experimentally for a simple system in a transient
state, consisting of a Brownian particle in water, confined by a moving
harmonic potential. In this paper we show that because of the interaction
between the stochastic motion of the particle in water and its deterministic
motion in the potential, very different new heat theorems are found than in the
conventional case. One of the consequences of these new heat Fluctuation
Theorems is that the ratio of the probability for the Brownian particle to
absorb heat from rather than supply heat to the water is much larger than in
the Conventional Fluctuation Theorem. This could be of relevance for
micro/nano-technology.Comment: 10 pages, 6 figures. Some corrections in the text were made.
Submitted to Physica
Algorithmic Aspects of Switch Cographs
This paper introduces the notion of involution module, the first
generalization of the modular decomposition of 2-structure which has a unique
linear-sized decomposition tree. We derive an O(n^2) decomposition algorithm
and we take advantage of the involution modular decomposition tree to state
several algorithmic results. Cographs are the graphs that are totally
decomposable w.r.t modular decomposition. In a similar way, we introduce the
class of switch cographs, the class of graphs that are totally decomposable
w.r.t involution modular decomposition. This class generalizes the class of
cographs and is exactly the class of (Bull, Gem, Co-Gem, C_5)-free graphs. We
use our new decomposition tool to design three practical algorithms for the
maximum cut, vertex cover and vertex separator problems. The complexity of
these problems was still unknown for this class of graphs. This paper also
improves the complexity of the maximum clique, the maximum independant set, the
chromatic number and the maximum clique cover problems by giving efficient
algorithms, thanks to the decomposition tree. Eventually, we show that this
class of graphs has Clique-Width at most 4 and that a Clique-Width expression
can be computed in linear time
On a nonlinear theory of elastic shells
Nonlinear theory of elastic shells with deformation gradient
A subset solution to the sign problem in random matrix simulations
We present a solution to the sign problem in dynamical random matrix
simulations of a two-matrix model at nonzero chemical potential. The sign
problem, caused by the complex fermion determinants, is solved by gathering the
matrices into subsets, whose sums of determinants are real and positive even
though their cardinality only grows linearly with the matrix size. A detailed
proof of this positivity theorem is given for an arbitrary number of fermion
flavors. We performed importance sampling Monte Carlo simulations to compute
the chiral condensate and the quark number density for varying chemical
potential and volume. The statistical errors on the results only show a mild
dependence on the matrix size and chemical potential, which confirms the
absence of sign problem in the subset method. This strongly contrasts with the
exponential growth of the statistical error in standard reweighting methods,
which was also analyzed quantitatively using the subset method. Finally, we
show how the method elegantly resolves the Silver Blaze puzzle in the
microscopic limit of the matrix model, where it is equivalent to QCD.Comment: 18 pages, 11 figures, as published in Phys. Rev. D; added references;
in Sec. VB: added discussion of model satisfying the Silver Blaze for all N
(proof in Appendix E
Optical Dielectric Functions of III-V Semiconductors in Wurtzite Phase
Optical properties of semiconductors can exhibit strong polarization
dependence due to crystalline anisotropy. A number of recent experiments have
shown that the photoluminescence intensity in free standing nanowires is
polarization dependent. One contribution to this effect is the anisotropy of
the dielectric function due to the fact that most nanowires crystalize in the
wurtzite form. While little is known experimentally about the band structures
wurtzite phase III-V semiconductors, we have previously predicted the bulk band
structure of nine III-V semiconductors in wurtzite phase.Here, we predict the
frequency dependent dielectric functions for nine non-Nitride wurtzite phase
III-V semiconductors (AlP, AlAs, AlSb, GaP, GaAs, GaSb, InP, InAs and InSb).
Their complex dielectric functions are calculated in the dipole approximation
by evaluating the momentum matrix elements on a dense grid of special k-points
using empirical pseudopotential wave functions. Corrections to the momentum
matrix elements accounting for the missing core states are made using a scaling
factor which is determined by using the optical sum rules on the calculated
dielectric functions for the zincblende polytypes. The dielectric function is
calculated for polarizations perpendicular and parallel to the c-axis of the
crystal
Crinoid phylogeny: a preliminary analysis (Echinodermata: Crinoidea)
We describe the first molecular and morphological analysis of extant crinoid high-level inter-relationships. Nuclear and mitochondrial gene sequences and a cladistically coded matrix of 30 morphological characters are presented, and analysed by phylogenetic methods. The molecular data were compiled from concatenated nuclear-encoded 18S rDNA, internal transcribed spacer 1, 5.8S rDNA, and internal transcribed spacer 2, together with part of mitochondrial 16S rDNA, and comprised 3,593 sites, of which 313 were parsimony-informative. The molecular and morphological analyses include data from the bourgueticrinid Bathycrinus; the antedonid comatulids Dorometra and Florometra; the cyrtocrinids Cyathidium, Gymnocrinus, and Holopus; the isocrinids Endoxocrinus, and two species of Metacrinus; as well as from Guillecrinus and Caledonicrinus, whose ordinal relationships are uncertain, together with morphological data from Proisocrinus. Because the molecular data include indel-rich regions, special attention was given to alignment procedure, and it was found that relatively low, gene-specific, gap penalties gave alignments from which congruent phylogenetic information was obtained from both well-aligned, indel-poor and potentially misaligned, indel-rich regions. The different sequence data partitions also gave essentially congruent results. The overall direction of evolution in the gene trees remains uncertain: an asteroid outgroup places the root on the branch adjacent to the slowly evolving isocrinids (consistent with palaeontological order of first appearances), but maximum likelihood analysis with a molecular clock places it elsewhere. Despite lineage-specific rate differences, the clock model was not excluded by a likelihood ratio test. Morphological analyses were unrooted. All analyses identified three clades, two of them generally well-supported. One well-supported clade (BCG) unites Bathycrinus and Guillecrinus with the representative (chimaeric) comatulid in a derived position, suggesting that comatulids originated from a sessile, stalked ancestor. In this connection it is noted that because the comatulid centrodorsal ossicle originates ontogenetically from the column, it is not strictly correct to describe comatulids as unstalked crinoids. A second, uniformly well-supported clade contains members of the Isocrinida, while the third clade contains Gymnocrinus, a well-established member of the Cyrtocrinida, together with the problematic taxon Caledonicrinus, currently classified as a bourgueticrinid. Another cyrtocrinid, Holopus, joins this clade with only weak molecular, but strong morphological support. In one morphological analysis Proisocrinus is weakly attached to the isocrinid clade. Only an unusual, divergent 18S rDNA sequence was obtained from the morphologically strange cyrtocrinid Cyathidium. Although not analysed in detail, features of this sequence suggested that it may be a PCR artefact, so that the apparently basal position of this taxon requires confirmation. If not an artefact, Cyathidium either diverged from the crinoid stem much earlier than has been recognised hitherto (i.e., it may be a Palaeozoic relic), or it has an atypically high rate of molecular evolution
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