18,411 research outputs found
Reducing variance in univariate smoothing
A variance reduction technique in nonparametric smoothing is proposed: at
each point of estimation, form a linear combination of a preliminary estimator
evaluated at nearby points with the coefficients specified so that the
asymptotic bias remains unchanged. The nearby points are chosen to maximize the
variance reduction. We study in detail the case of univariate local linear
regression. While the new estimator retains many advantages of the local linear
estimator, it has appealing asymptotic relative efficiencies. Bandwidth
selection rules are available by a simple constant factor adjustment of those
for local linear estimation. A simulation study indicates that the finite
sample relative efficiency often matches the asymptotic relative efficiency for
moderate sample sizes. This technique is very general and has a wide range of
applications.Comment: Published at http://dx.doi.org/10.1214/009053606000001398 in the
Annals of Statistics (http://www.imstat.org/aos/) by the Institute of
Mathematical Statistics (http://www.imstat.org
Solving the Traffic Problem by Using A Simulation Model
This paper presents a traffic light simulation model, which is composed of 6 submodels coded in Arena to help analyze the traffic problem. The model adopts average arrival time and average departure time to simulate the arrival and leaving number of cars on roads. In the experiment, each submodel represents a road that has 3 intersections. The simulation results show that different traffic light duration policies will cause different effects on traffic congestion. Therefore, we can use this model to obtain a good traffic light duration policy for solving the traffic problem
The Luminosity - E_p Relation within Gamma--Ray Bursts and Implications for Fireball Models
Using a sample of 2408 time-resolved spectra for 91 BATSE gamma-ray bursts
(GRBs) presented by Preece et al., we show that the relation between the
isotropic-equivalent luminosity (L_iso) and the spectral peak energy (E_p) in
the cosmological rest frame, L_iso \propto E_p^2, not only holds within these
bursts, but also holds among these GRBs, assuming that the burst rate as a
function of redshift is proportional to the star formation rate. The possible
implications of this relation for the emission models of GRBs are discussed. We
suggest that both the kinetic-energy-dominated internal shock model and the
magnetic-dissipation-dominated external shock model can well interpret this
relation. We constrain the parameters for these two models, and find that they
are in a good agreement with the parameters from the fittings to the afterglow
data (abridged).Comment: 3 pages plus 5 figures, emulateapj style, accepted for publication in
ApJ Letter
A General Theorem Relating the Bulk Topological Number to Edge States in Two-dimensional Insulators
We prove a general theorem on the relation between the bulk topological
quantum number and the edge states in two dimensional insulators. It is shown
that whenever there is a topological order in bulk, characterized by a
non-vanishing Chern number, even if it is defined for a non-conserved quantity
such as spin in the case of the spin Hall effect, one can always infer the
existence of gapless edge states under certain twisted boundary conditions that
allow tunneling between edges. This relation is robust against disorder and
interactions, and it provides a unified topological classification of both the
quantum (charge) Hall effect and the quantum spin Hall effect. In addition, it
reconciles the apparent conflict between the stability of bulk topological
order and the instability of gapless edge states in systems with open
boundaries (as known happening in the spin Hall case). The consequences of time
reversal invariance for bulk topological order and edge state dynamics are
further studied in the present framework.Comment: A mistake corrected in reference
Measurement of the topological surface state optical conductance in bulk-insulating Sn-doped BiSbTeS single crystals
Topological surface states have been extensively observed via optics in thin
films of topological insulators. However, in typical thick single crystals of
these materials, bulk states are dominant and it is difficult for optics to
verify the existence of topological surface states definitively. In this work,
we studied the charge dynamics of the newly formulated bulk-insulating Sn-doped
BiSbTeS crystal by using time-domain terahertz
spectroscopy. This compound shows much better insulating behavior than any
other bulk-insulating topological insulators reported previously. The
transmission can be enhanced an amount which is 5 of the zero-field
transmission by applying magnetic field to 7 T, an effect which we believe is
due to the suppression of topological surface states. This suppression is
essentially independent of the thicknesses of the samples, showing the
two-dimensional nature of the transport. The suppression of surface states in
field allows us to use the crystal slab itself as a reference sample to extract
the surface conductance, mobility, charge density and scattering rate. Our
measurements set the stage for the investigation of phenomena out of the
semi-classical regime, such as the topological magneto-electric effect.Comment: 5 pages, 3 figures, submitted in Augus
A new time-frequency method to reveal quantum dynamics of atomic hydrogen in intense laser pulses: Synchrosqueezing Transform
This study introduces a new adaptive time-frequency (TF) analysis technique,
synchrosqueezing transform (SST), to explore the dynamics of a laser-driven
hydrogen atom at an {\it ab initio} level, upon which we have demonstrated its
versatility as a new viable venue for further exploring quantum dynamics. For a
signal composed of oscillatory components which can be characterized by
instantaneous frequency, the SST enables rendering the decomposed signal based
on the phase information inherited in the linear TF representation with
mathematical support. Compared with the classical type TF methods, the SST
clearly depicts several intrinsic quantum dynamical processes such as selection
rules, AC Stark effects, and high harmonic generation
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