15,193 research outputs found
The smallest eigenvalue of Hankel matrices
Let H_N=(s_{n+m}),n,m\le N denote the Hankel matrix of moments of a positive
measure with moments of any order. We study the large N behaviour of the
smallest eigenvalue lambda_N of H_N. It is proved that lambda_N has exponential
decay to zero for any measure with compact support. For general determinate
moment problems the decay to 0 of lambda_N can be arbitrarily slow or
arbitrarily fast. In the indeterminate case, where lambda_N is known to be
bounded below by a positive constant, we prove that the limit of the n'th
smallest eigenvalue of H_N for N tending to infinity tends rapidly to infinity
with n. The special case of the Stieltjes-Wigert polynomials is discussed
Sensory innervation of the guinea pig colon and rectum compared using retrograde tracing and immunohistochemistry.
Embargoed until 2 April 2017 as per publisher's policy
Formal matched asymptotics for degenerate Ricci flow neckpinches
Gu and Zhu have shown that Type-II Ricci flow singularities develop from
nongeneric rotationally symmetric Riemannian metrics on , for all . In this paper, we describe and provide plausibility arguments for a
detailed asymptotic profile and rate of curvature blow-up that we predict such
solutions exhibit
Molecular architecture of Gαo and the structural basis for RGS16-mediated deactivation
Heterotrimeric G proteins relay extracellular cues from heptahelical transmembrane receptors to downstream effector molecules. Composed of an α subunit with intrinsic GTPase activity and a βγ heterodimer, the trimeric complex dissociates upon receptor-mediated nucleotide exchange on the α subunit, enabling each component to engage downstream effector targets for either activation or inhibition as dictated in a particular pathway. To mitigate excessive effector engagement and concomitant signal transmission, the Gα subunit's intrinsic activation timer (the rate of GTP hydrolysis) is regulated spatially and temporally by a class of GTPase accelerating proteins (GAPs) known as the regulator of G protein signaling (RGS) family. The array of G protein-coupled receptors, Gα subunits, RGS proteins and downstream effectors in mammalian systems is vast. Understanding the molecular determinants of specificity is critical for a comprehensive mapping of the G protein system. Here, we present the 2.9 Å crystal structure of the enigmatic, neuronal G protein Gαo in the GTP hydrolytic transition state, complexed with RGS16. Comparison with the 1.89 Å structure of apo-RGS16, also presented here, reveals plasticity upon Gαo binding, the determinants for GAP activity, and the structurally unique features of Gαo that likely distinguish it physiologically from other members of the larger Gαi family, affording insight to receptor, GAP and effector specificity
Overcoming Calibration Problems in Pattern Labeling with Pairwise Ratings: Application to Personality Traits
We address the problem of calibration of workers whose task is to label patterns with continuous variables, which arises for instance in labeling images of videos of humans with continuous traits. Worker bias is particularly difficult to evaluate and correct when many workers contribute just a few labels, a situation arising typically when labeling is crowd-sourced. In the scenario of labeling short videos of people facing a camera with personality traits, we evaluate the feasibility of the pairwise ranking method to alleviate bias problems. Workers are exposed to pairs of videos at a time and must order by preference. The variable levels are reconstructed by fitting a Bradley-Terry-Luce model with maximum likelihood. This method may at first sight, seem prohibitively expensive because for N videos, p=N(N−1)/2 pairs must be potentially processed by workers rather that N videos. However, by performing extensive simulations, we determine an empirical law for the scaling of the number of pairs needed as a function of the number of videos in order to achieve a given accuracy of score reconstruction and show that the pairwise method is affordable. We apply the method to the labeling of a large scale dataset of 10,000 videos used in the ChaLearn Apparent Personality Trait challenge
Hamiltonian analysis of Poincar\'e gauge theory scalar modes
The Hamiltonian constraint formalism is used to obtain the first explicit
complete analysis of non-trivial viable dynamic modes for the Poincar\'e gauge
theory of gravity. Two modes with propagating spin-zero torsion are analyzed.
The explicit form of the Hamiltonian is presented. All constraints are obtained
and classified. The Lagrange multipliers are derived. It is shown that a
massive spin- mode has normal dynamical propagation but the associated
massless is pure gauge. The spin- mode investigated here is also
viable in general. Both modes exhibit a simple type of ``constraint
bifurcation'' for certain special field/parameter values.Comment: 28 pages, LaTex, submitted to International Journal of Modern Physics
Chemistry in Infrared Dark Cloud Clumps: a Molecular Line Survey at 3 mm
We have observed 37 Infrared Dark Clouds (IRDCs), containing a total of 159
clumps, in high-density molecular tracers at 3 mm using the 22-meter ATNF Mopra
Telescope located in Australia. After determining kinematic distances, we
eliminated clumps that are not located in IRDCs and clumps with a separation
between them of less than one Mopra beam. Our final sample consists of 92 IRDC
clumps. The most commonly detected molecular lines are (detection rates higher
than 8%): N2H+, HNC, HN13C, HCO+, H13CO+, HCN, C2H, HC3N, HNCO, and SiO. We
investigate the behavior of the different molecular tracers and look for
chemical variations as a function of an evolutionary sequence based on Spitzer
IRAC and MIPS emission. We find that the molecular tracers behave differently
through the evolutionary sequence and some of them can be used to yield useful
relative age information. The presence of HNC and N2H+ lines do not depend on
the star formation activity. On the other hand, HC3N, HNCO, and SiO are
predominantly detected in later stages of evolution. Optical depth calculations
show that in IRDC clumps the N2H+ line is optically thin, the C2H line is
moderately optically thick, and HNC and HCO+ are optically thick. The HCN
hyperfine transitions are blended, and, in addition, show self-absorbed line
profiles and extended wing emission. These factors combined prevent the use of
HCN hyperfine transitions for the calculation of physical parameters. Total
column densities of the different molecules, except C2H, increase with the
evolutionary stage of the clumps. Molecular abundances increase with the
evolutionary stage for N2H+ and HCO+. The N2H+/HCO+ and N2H+/HNC abudance
ratios act as chemical clocks, increasing with the evolution of the clumps.Comment: Accepted to ApJ. 29 page
Experimental demonstration of a BDCZ quantum repeater node
Quantum communication is a method that offers efficient and secure ways for
the exchange of information in a network. Large-scale quantum communication (of
the order of 100 km) has been achieved; however, serious problems occur beyond
this distance scale, mainly due to inevitable photon loss in the transmission
channel. Quantum communication eventually fails when the probability of a dark
count in the photon detectors becomes comparable to the probability that a
photon is correctly detected. To overcome this problem, Briegel, D\"{u}r, Cirac
and Zoller (BDCZ) introduced the concept of quantum repeaters, combining
entanglement swapping and quantum memory to efficiently extend the achievable
distances. Although entanglement swapping has been experimentally demonstrated,
the implementation of BDCZ quantum repeaters has proved challenging owing to
the difficulty of integrating a quantum memory. Here we realize entanglement
swapping with storage and retrieval of light, a building block of the BDCZ
quantum repeater. We follow a scheme that incorporates the strategy of BDCZ
with atomic quantum memories. Two atomic ensembles, each originally entangled
with a single emitted photon, are projected into an entangled state by
performing a joint Bell state measurement on the two single photons after they
have passed through a 300-m fibre-based communication channel. The entanglement
is stored in the atomic ensembles and later verified by converting the atomic
excitations into photons. Our method is intrinsically phase insensitive and
establishes the essential element needed to realize quantum repeaters with
stationary atomic qubits as quantum memories and flying photonic qubits as
quantum messengers.Comment: 5 pages, 4 figure
Influence of gauge-field fluctuations on composite fermions near the half-filled state
Taking into account the transverse gauge field fluctuations, which interact
with composite fermions, we examine the finite temperature compressibility of
the fermions as a function of an effective magnetic field ( is the density of electrons) near the half-filled state. It is
shown that, after including the lowest order gauge field correction, the
compressibility goes as for , where . Here we assume that the interaction between
the fermions is given by , where is a dependent constant. This result can be
interpreted as a divergent correction to the activation energy gap and is
consistent with the divergent renormalization of the effective mass of the
composite fermions.Comment: Plain Tex, 24 pages, 5 figures available upon reques
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