26,870 research outputs found
Remarks on automorphism and cohomology of finite cyclic coverings of projective spaces
For a smooth finite cyclic covering over a projective space of dimension greater than one, we show that its group of automorphisms faithfully acts on its cohomology except for a few cases. In characteristic zero, we study the equivariant deformation theory and groups of automorphisms for complex cyclic coverings. The proof uses the decomposition of the sheaf of differential forms due to Esnault and Viehweg. In positive characteristic, a lifting criterion of automorphisms reduce the faithfulness problem to characteristic zero. To apply this criterion, we prove the degeneration of the Hodge-de Rham spectral sequences for a family of smooth cfinite yclic coverings, and the infinitesimal Torelli theorem for finite cyclic coverings defined over an arbitrary field
Hurst parameter analysis of radio pulsar timing noise
We present an analysis of timing residual (noise) of 54 pulsars obtained from
25-m radio telescope at Urumqi Observatory with a time span of 5~8 years,
dealing with statistics of the Hurst parameter. The majority of these pulsars
were selected to have timing noise that look like white noise rather than
smooth curves. The results are compared with artificial series of different
constant pairwise covariances. Despite the noise like appearance, many timing
residual series showed Hurst parameters significantly deviated from that of
independent series. We concluded that Hurst parameter may be capable of
detecting dependence in timing residual and of distinguishing chaotic behavior
from random processes.Comment: 7 pages, 3 figures, 2 tables, Submitted to MNRA
Properties of superconducting MgB_2 wires: "in-situ" versus "ex-situ" reaction technique
We have fabricated a series of iron-sheathed superconducting wires prepared
by the powder-in-tube technique from (MgB_2)_{1-x}:(Mg+2B)_x initial powder
mixtures taken with different proportions, so that x varies from 0 to 1. It
turned out that "ex-situ" prepared wire (x = 0) has considerable disadvantages
compared to all the other wires in which "in-situ" assisted (0 < x < 1) or pure
"in-situ" (x = 1) preparation was used due to weaker inter-grain connectivity.
As a result, higher critical current densities J_c were measured over the
entire range of applied magnetic fields B_a for all the samples with x > 0.
Pinning of vortices in MgB_2 wires is shown to be due to grain boundaries.
J_c(B_a) behavior is governed by an interplay between the transparency of grain
boundaries and the amount of "pinning" grain boundaries. Differences between
thermo-magnetic flux-jump instabilities in the samples and a possible threat to
practical applications are also discussed.Comment: To be published in Supercond. Sci. Technol. (2003), in pres
Superconductivity, critical current density, and flux pinning in MgB_{2-x}(SiC)_{x/2} superconductor after SiC nanoparticle doping
We investigated the effect of SiC nano-particle doping on the crystal lattice
structure, critical temperature T_c, critical current density J_c, and flux
pinning in MgB_2 superconductor. A series of MgB_{2-x}(SiC)_{x/2} samples with
x = 0 to 1.0 were fabricated using in-situ reaction process. The contraction of
the lattice and depression of T_c with increasing SiC doping level remained
rather small due to the counter-balanced effect of Si and C co-doping. The high
level Si and C co-doping allowed the creation of intra-grain defects and highly
dispersed nano-inclusions within the grains which can act as effective pinning
centers for vortices, improving J_c behavior as a function of the applied
magnetic field. The enhanced pinning is mainly attributable to the
substitution-induced defects and a local structure fluctuations within grains.
A pinning mechanism is proposed to account for different contributions of
different defects in MgB_{2-x}(SiC)_{x/2} superconductors.Comment: 7 pages, 8 figure
Optimal Energy Management for Fuel and Emissions Minimization of Series Hybrid Electric Vehicles with Consideration of Engine Preheating
As a result of multiple energy sources, hybrid
electric vehicles (HEVs) provide additional flexibility of the
engine operating point, which enables optimization of fuel
economy and emissions reduction. This paper introduces an
energy management (EM) control strategy by an optimal
control approach that jointly optimizes fuel consumption and
various vehicle pollutant emissions. Engine thermal dynamics
are modeled and integrated into the engine-out emission and
fuel consumption models for enhanced modeling accuracy.
The proposed method is formulated as an optimal control
problem (OCP) that is benchmarked against a baseline EM
strategy for fuel consumption only optimization. The proposed
temperature sensitive emission and fuel consumption models
enable a thorough investigation of engine temperature-emissions
relationships, which provide important insights into the optimal
power split during the engine preheating phase. Simulation
results validate the effectiveness of the proposed approach and
highlight the importance of analyzing the fuel consumptionemissions trade-off, as small compromises in fuel consumption
lead to significant reductions in emissions
Entropy and weak solutions in the thermal model for the compressible Euler equations
Among the existing models for compressible fluids, the one by Kataoka and
Tsutahara (KT model, Phys. Rev. E 69, 056702, 2004) has a simple and rigorous
theoretical background. The drawback of this KT model is that it can cause
numerical instability if the local Mach number exceeds 1. The precise mechanism
of this instability has not yet been clarified. In this paper, we derive
entropy functions whose local equilibria are suitable to recover the Euler-like
equations in the framework of the lattice Boltzmann method for the KT model.
Numerical examples are also given, which are consistent with the above
theoretical arguments, and show that the entropy condition is not fully
guaranteed in KT model. The negative entropy may be the inherent cause for the
non-physical oscillations in the vicinity of the shock. In contrast to these
Karlin's microscopic entropy approach, the corresponding subsidiary entropy
condition in the LBM calculation could also be deduced explicitly from the
macroscopic version, which provides some insights on the numerical instability
of the lattice Boltzmann model for shock calculation.Comment: 27 pages,6 figure
Development of the ALMA-North America Sideband-Separating SIS Mixers
As the Atacama Large Millimeter/submillimeter Array (ALMA) nears completion,
73 dual-polarization receivers have been delivered for each of Bands 3 (84-116
GHz) and 6 (211-275 GHz). The receivers use sideband-separating superconducting
Nb/Al-AlOx/Nb tunnel-junction (SIS) mixers, developed for ALMA to suppress
atmospheric noise in the image band. The mixers were designed taking into
account dynamic range, input return loss, and signal-to-image conversion (which
can be significant in SIS mixers). Typical SSB receiver noise temperatures in
Bands 3 and 6 are 30 K and 60 K, resp., and the image rejection is typically 15
dB.Comment: Submitted to IEEE Trans. Microwave Theory Tech., June 2013. 10 pages,
21 figure
Ecosystem evolution mechanism of manufacturing service system driven by service providers
This is the author accepted manuscript. The final version is available from Taylor & Francis via the DOI in this record.To date, research has mainly discussed the definition and classification of producer services. Yet many companies require an integrated solution of products and producer services. The aim of this paper is to propose a model of the ecological evolution of manufacturing service systems (MSS) driven by service providers. This model adopts modular thinking to integrate dispersive producer services into an entire MSS, the process of which forms the ecological evolution of MSS. This paper applies ecological theory to build the ecological evolution mechanism of MSS. In this mechanism, the predator–prey relationship is used as a metaphor for the relationships between manufacturers and service providers in a MSS. A predator–prey model is built to simulate the ecological evolution of MSS. A predation cellular genetic algorithm is adopted to optimise this evolution. The proposed model and algorithm are demonstrated and validated by a case study of MSS in bicycle industry. This study contributes to theory by proposing and clarifying the mechanism for provision of services between businesses. The future research will focus on the application and empirical analysis of this mechanism derived from big data.This work has been supported by the National Natural Science Foundation of China [grant number 51205353], [grant number 71571161], [grant number 51475434]
- …