2,812 research outputs found
An Empirical Study on E-commerce Logistics Service Quality and Customer Satisfaction
Based on consumerâs perspective, the influences of logistics service quality on customer satisfaction were empirically studied in order to clarify the key logistical factors affecting consumerâs satisfaction. An empirical method is used to investigate the factors in E-commerce logistics service quality that influence customer satisfaction. The results indicate that delivery service quality, after-sales service quality and staff service quality have positive impacts on customer satisfaction; the influence of communication service quality on customer satisfaction is not significant
A multi-wavelength observation and investigation of six infrared dark clouds
Context. Infrared dark clouds (IRDCs) are ubiquitous in the Milky Way, yet
they play a crucial role in breeding newly-formed stars.
Aims. With the aim of further understanding the dynamics, chemistry, and
evolution of IRDCs, we carried out multi-wavelength observations on a small
sample.
Methods. We performed new observations with the IRAM 30 m and CSO 10.4 m
telescopes, with tracers , HCN, , ,
DCO, SiO, and DCN toward six IRDCs G031.97+00.07, G033.69-00.01,
G034.43+00.24, G035.39-00.33, G038.95-00.47, and G053.11+00.05.
Results. We investigated 44 cores including 37 cores reported in previous
work and seven newly-identified cores. Toward the dense cores, we detected 6
DCO, and 5 DCN lines. Using pixel-by-pixel spectral energy distribution
(SED) fits of the 70 to 500 m, we obtained dust
temperature and column density distributions of the IRDCs. We found that emission has a strong correlation with the dust temperature and column
density distributions, while showed the weakest correlation. It
is suggested that is indeed a good tracer in very dense
conditions, but is an unreliable one, as it has a relatively
low critical density and is vulnerable to freezing-out onto the surface of cold
dust grains. The dynamics within IRDCs are active, with infall, outflow, and
collapse; the spectra are abundant especially in deuterium species.
Conclusions. We observe many blueshifted and redshifted profiles,
respectively, with and toward the same core. This
case can be well explained by model "envelope expansion with core collapse
(EECC)".Comment: 24 pages, 11 figures, 4 tables. To be published in A&A. The
resolutions of the pictures are cut dow
Editorial: Distribution patterns, driving mechanisms and ecological service functions of urban plant biodiversity
Peer Reviewe
Exact Variance Component Tests for Longitudinal Microbiome Studies
In metagenomic studies, testing the association of microbiome composition and clinical outcomes translates to testing the nullity of variance components. Motivated by a lung HIV (human immunodeficiency virus) microbiome project, we study longitudinal microbiome data by variance component models with more than two variance components. Current testing strategies only apply to the models with exactly two variance components and when sample sizes are large. Therefore, they are not applicable to longitudinal microbiome studies. In this paper, we propose exact tests (score test, likelihood ratio test, and restricted likelihood ratio test) to (1) test the association of the overall microbiome composition in a longitudinal design and (2) detect the association of one specific microbiome cluster while adjusting for the effects from related clusters. Our approach combines the exact tests for null hypothesis with a single variance component with a strategy of reducing multiple variance components to a single one. Simulation studies demonstrate that our method has correct type I error rate and superior power compared to existing methods at small sample sizes and weak signals. Finally, we apply our method to a longitudinal pulmonary microbiome study of human immunodeficiency virus (HIV) infected patients and reveal two interesting genera Prevotella and Veillonella associated with forced vital capacity. Our findings shed lights on the impact of lung microbiome to HIV complexities. The method is implemented in the open source, high-performance computing language Julia and is freely available at https://github.com/JingZhai63/VCmicrobiome
Comparison of four phaC genes from Haloferax mediterranei and their function in different PHBV copolymer biosyntheses in Haloarcula hispanica
The Third Order Scalar Induced Gravitational Waves
Since the gravitational waves were detected by LIGO and Virgo, it has been
promising that lots of information about the primordial Universe could be
learned by further observations on stochastic gravitational waves background.
The studies on gravitational waves induced by primordial curvature
perturbations are of great interest. The aim of this paper is to investigate
the third order induced gravitational waves. Based on the theory of
cosmological perturbations, the first order scalar induces the second order
scalar, vector and tensor perturbations. At the next iteration, the first order
scalar, the second order scalar, vector and tensor perturbations all induce the
third order tensor perturbations. We present the energy density spectrum of the
third order gravitational waves for a monochromatic primordial power spectrum.
The shape of the energy density spectrum of the third order gravitational waves
is different from that of the second order scalar induced gravitational waves.
And it is found that the third order gravitational waves sourced by the second
order scalar perturbations dominate the energy density spectrum.Comment: 33 pages, 4 figure
The basic reproductive ratio of Barbour's two-host schistosomiasis model with seasonal fluctuations
Motivated by the first mathematical model for schistosomiasis proposed by Macdonald and Barbour's classical schistosomiasis model tracking the dynamics of infected human population and infected snail hosts in a community, in our previous study, we incorporated seasonal fluctuations into Barbour's model, but ignored the effect of bovine reservoir host in the transmission of schistosomiasis. Inspired by the findings from our previous work, the model was further improved by integrating two definitive hosts (human and bovine) and seasonal fluctuations, so as to understand the transmission dynamics of schistosomiasis japonica and evaluate the ongoing control measures in Liaonan village, Xingzi County, Jiangxi Province.; The basic reproductive ratio R 0 and its computation formulae were derived by using the operator theory in functional analysis and the monodromy matrix theory. The mathematical methods for global dynamics of periodic systems were used in order to show that R 0 serves as a threshold value that determines whether there was disease outbreak or not. The parameter fitting and the ratio calculation were performed with surveillance data obtained from the village of Liaonan using numerical simulation. Sensitivity analysis was carried out in order to understand the impact of R 0 on seasonal fluctuations and snail host control. The modified basic reproductive ratios were compared with known results to illustrate the infection risk.; The Barbour's two-host model with seasonal fluctuations was proposed. The implicit expression of R 0 for the model was given by the spectral radius of next infection operator. The R 0 s for the model ranged between 1.030 and 1.097 from 2003 to 2010 in the village of Liaonan, Xingzi County, China, with 1.097 recorded as the maximum value in 2005 but declined dramatically afterwards. In addition, we proved that the disease goes into extinction when R 0 is less than one and persists when R 0 is greater than one. Comparisons of the different improved models were also made.; Based on the mechanism and characteristics of schistosomiasis transmission, Barbour's model was improved by considering seasonality. The implicit formula of R 0 for the model and its calculation were given. Theoretical results showed that R 0 gave a sharp threshold that determines whether the disease dies out or not. Simulations concluded that: (i) ignoring seasonality would overestimate the transmission risk of schistosomiasis, and (ii) mollusiciding is an effective control measure to curtail schistosomiasis transmission in Xingzi County when the removal rate of infected snails is small
Hypothesis test on a mixture forward-incubation-time epidemic model with application to COVID-19 outbreak
The distribution of the incubation period of the novel coronavirus disease
that emerged in 2019 (COVID-19) has crucial clinical implications for
understanding this disease and devising effective disease-control measures. Qin
et al. (2020) designed a cross-sectional and forward follow-up study to collect
the duration times between a specific observation time and the onset of
COVID-19 symptoms for a number of individuals. They further proposed a mixture
forward-incubation-time epidemic model, which is a mixture of an
incubation-period distribution and a forward time distribution, to model the
collected duration times and to estimate the incubation-period distribution of
COVID-19. In this paper, we provide sufficient conditions for the
identifiability of the unknown parameters in the mixture
forward-incubation-time epidemic model when the incubation period follows a
two-parameter distribution. Under the same setup, we propose a likelihood ratio
test (LRT) for testing the null hypothesis that the mixture
forward-incubation-time epidemic model is a homogeneous exponential
distribution. The testing problem is non-regular because a nuisance parameter
is present only under the alternative. We establish the limiting distribution
of the LRT and identify an explicit representation for it. The limiting
distribution of the LRT under a sequence of local alternatives is also
obtained. Our simulation results indicate that the LRT has desirable type I
errors and powers, and we analyze a COVID-19 outbreak dataset from China to
illustrate the usefulness of the LRT.Comment: 34 pages, 2 figures, 2 table
Colossal switchable photocurrents in topological Janus transition metal dichalcogenides
Nonlinear optical properties, such as bulk photovoltaic effects, possess
great potential in energy harvesting, photodetection, rectification, etc. To
enable efficient light-current conversion, materials with strong
photo-responsivity are highly desirable. In this work, we predict that
monolayer Janus transition metal dichalcogenides (JTMDs) in the 1T' phase
possess colossal nonlinear photoconductivity owing to their topological band
mixing, strong inversion symmetry breaking, and small electronic bandgap. 1T'
JTMDs have inverted bandgaps on the order of 10 meV and are exceptionally
responsive to light in the terahertz (THz) range. By first-principles
calculations, we reveal that 1T' JTMDs possess shift current (SC) conductivity
as large as , equivalent to a
photo-responsivity of . The circular current (CC) conductivity
of 1T' JTMDs is as large as . These remarkable
photo-responsivities indicate that the 1T' JTMDs can serve as efficient
photodetectors in the THz range. We also find that external stimuli such as the
in-plane strain and out-of-plane electric field can induce topological phase
transitions in 1T' JTMDs and that the SC can abruptly flip their directions.
The abrupt change of the nonlinear photocurrent can be used to characterize the
topological transition and has potential applications in 2D optomechanics and
nonlinear optoelectronics
- âŚ