- 外文数据库
Given a group G, we let Z(G) denote its center, G' its commutator subgroup, and Phi (G) its Frattini subgroup (the intersection of all maximal proper subgroups of G). Given U leq G, we let N_G (U) stand for the normalizer of U in G. A group G is FC iff every element g in G has finitely many conjugates. A p-group E is called extraspecial iff Phi (E) = E' = Z(E) cong Z_p, the cyclic group with p elements.
When generalizing a characterization of centre-by-finite groups due to B. H. Neumann, M. J. Tomkinson asked the following question. Is there an FC-group G with vert G / Z(G) vert = kappa but [G:N_G(U)] < kappa for all (abelian) subgroups U of G, where kappa is an uncountable cardinal. We consider this question for kappa = omega_1 and kappa = omega_2. It turns out that the answer is largely independent of ZFC, and that it differs greatly in the two cases.
More explicitly, for kappa = omega_1, it is consistent with, and independent of, ZFC that there is a group G with vert G / Z(G) vert = omega_1 and [G:N_G (A)] leq omega for all abelian A leq G. We do not know whether the same statement is still consistent if we drop abelian. On the other hand, for kappa = omega_2, the non-existence of groups G with vert G / Z(G) vert = omega_2 and [G : N_G (A) ] leq omega_1 for all (abelian) A leq G is equiconsistent with the existence of an inaccessible cardinal. In particular, there is an extraspecial p-group with this property if there is a Kurepa tree.
- 外文数据库
- 外文数据库
- 外文数据库
- 外文数据库
- 外文数据库
- 外文数据库
- 外文数据库
- 外文数据库
- 外文数据库
- 外文数据库
- 外文数据库
- 外文数据库
- 外文数据库
- 外文数据库
- 外文数据库
Gamma ray burst (GRB) objects are now widely thought to be at cosmological distances, and thus represent enormous energy emission. Gamma ray spectra extending to $GeV$ energies suggest the possiblity of accompanying neutrino emission, and there are several models proposed suggesting the potential detectability of such coincident neutrino bursts. With this in view, we examine possible measurements that might be conducted to give experimental data useful for astronomy, for cosmology and also neutrino properties. Of interest to astronomy and cosmology, we show how measurement of neutrino flavor ratios yields information on the nature and relative distance of the source. We point out that cosmological time dilation might be measured for these sources using neutrinos, as has been done for photons, and that neutrino oscillation lengths in the range of $1$ to $10^5~Mpc$ can be probed with GRB neutrinos. We thus note that these sources may make possible the first non-electromagnetic measurements of the scale size of the universe. We discuss tests of the weak equivalence principle, tests for flavor dependent gravitational couplings, and tests for long time scale variation of physical constants. We also show that a number of new bounds on neutrino properties (charge, mass, speed, lifetime) could be facilitated to levels well beyond those already inferred from the neutrino observation of SN1987A. We also examine the implications of these physics opportunities for designers of neutrino telescopes. We conclude that detection may be possible in planned instruments if the spectra are power law extending to the $TeV$ energy region, and if the neutrino fluxes are equal to or greater than the gamma ray fluxes. We emphasize the importance of low energy detection in future experiments
Inthiswork,Na2Li2Ti6O14@PEDOTcomposites(PEDOT?=?0,4.5,7.5,and10.5?wt%)aresynthesizedbyasolutiondispersionprocessfollowingbyanoxidativepolymerization.BothpristineNa2Li2Ti6O14andNa2Li2Ti6O14@PEDOTcompositesconsistofirregularparticlesandshowsimilarporoussphericalshapeswithasizeof3–5?μm.AppropriateamountofPEDOTcoatingefficientlyenhancestheelectrochemicalperformanceofNa2Li2Ti6O14.Na2Li2Ti6O14@PEDOTcompositesdeliverthedischarge/chargecapacitiesof140.7/140.1,152.3/151.4,179.9/177.7,and122.3/121.8mAhg?1at2?Cover100cycles,respectively.Na2Li2Ti6O14@PEDOTcomposites(PEDOT?=?7.5?wt%)showsthelargestspecificcapacitiesamongallsamples.ThePEDOTcoatinglayerhasaprominentelectronicconductivity,whichcanweakenthepolarization,reducethechargetransferresistance,andimprovethelithium-ionmobilityofNa2Li2Ti6O14.TheseresultsdemonstratethatPEDOTcoatingisanefficientwaytoenhancethelithiumstoragecapacityofNa2Li2Ti6O14.
- 外文数据库
- 外文数据库
- 外文数据库
- 外文数据库
- 外文数据库
- 外文数据库
ForLi–Sbatteries,theinterlayerbetweentheseparatorandsulfurcathodepreventinglithiumpolysulfide(LiPS)travelacrossthemembraneisaresearchhotspot.ThegoodblockingabilityforLiPSsindicatesthattheseinterlayerscanpromotetheelectrochemistryperformancewithhighSloading.However,mostoftheseinterlayersarejustusedasasimpleblockingwall.Suchablockingwall,forexample,thelowerLi+ionconductivity,wouldoftenreducetheelectrochemicalperformance,especiallyunderlargecurrentdensity.Here,wereportamultifunctionalion-sievemadebythreetwo-dimensional(2D)sheets,graphiticcarbonnitride(g-C3N4),boronnitride(BN),andgraphene.Ag-C3N4sheetwhichpossessesorderlychannelswithasizeof3?inthecrystallinestructurecaneffectivelypreventpolysulfidesfrompassingthroughbutallowlithiumionstopassfreely,whereasaBNsheetactsasanexcellentcatalystforsulfurredox,andgrapheneactsasanextendedcollector,whichcanpromotetheconductivityofthesulfurelectroderegion.Benefitingfromthesynergisticeffectamongthese2Dmaterials,theion-sieveinterlayermakestheLi–Sbatteryshowexcellentperformanceatalargeratewithbothhighsulfurloadingsandhighsulfurcontent.Inaddition,thehostmaterialsarenotnecessaryinthesecells.Theion-sieveliberatedadischargecapacityofabout600mAhg–1after500cyclesat1C,andthecapacityattenuationwaslessthan0.01%percyclewitha6mgcm–2arealS-loading(pureSastheactivematerial).Thereversiblecapacitycouldbemaintainedatmorethan400mAhg–1at2C,whichamountstoanareacurrentdensityof26.88mAcm–2.
- 外文数据库
- 外文数据库
- 外文数据库
In recent years, UAVs have received much attention in both the military and civilian fields for monitoring, emergency relief and searching tasks. UAVs are considered a new technology to obtain data at high altitudes when equipped with sensors. This technology is vital to the success of next-generation monitoring systems, which are expected to be reliable, real-time, efficient and secure. However, due to the bandwidth limitations in UAV-aided networks, the size of the transmitted data is a crucial factor for real-time media data transmission requirements, especially for national defense. To address this issue, in this article, we propose a realtime end-to-end media data transmission mechanism with an unsupervised deep neural network. The proposed mechanism transmutes the media data captured by UAVs into latent codes with a predefined constant size and transmits the codes to the ground console station (GCS) for further reconstruction. We use a real-word dataset containing millions of samples to evaluate the proposed mechanism which achieves a high transmission ratio, low resource usage and good visual quality.
- 外文数据库
- 外文数据库
Building an accurate and large digital terrain model (DTM) of the seabed is a key issue in various applications, especially for covert rapid environment assessment using autonomous underwater vehicles (AUVs). New AUV generations are capable of acquiring bathymetry with multiple acoustic sensors: single-beam echosounder, Doppler velocity log, multibeam echosounder (MBES), interferometric sidescan sonar (ISSS), etc. As these sensors acquire the seabed with different geometries, they can be combined to produce a DTM in shorter time. For example, ISSS can reach a wide swath in shallow water but it shows an information gap at nadir, which is usually covered by completing an additional track. Simultaneously using the MBES to acquire the nadir removes the need for this additional track and reduces the energy consumption of the whole survey. This paper focuses on fusion algorithms to extract best information of the two sensors (MBES and ISSS) to feed DTM production software with optimal bathymetric information. This problem may be solved by taking into account the average information of the two sensors. However, the sensors do not always give accurate information and the average information therefore becomes biased. Another way to tackle the problem is to select \na priori\n information given by the better of the two sensors based on a given geometric parameter (e.g., grazing angle). In this case, when the assumed best sensor fails, the information produced by the second sensor cannot be used to compensate for the erroneous information. Our approach consists in using all the available information and fuses it ahead of producing the DTM. Based on the theory of belief functions, this paper presents a framework to fuse the information coming from the two swath bathymetric sensors (MBES and ISSS). The belief theory, applied successfully to other fields, has been extended to handle the bathymetric information. The reliability and the uncertainty of each sonar are introduced in the fusion process to improve the estimation and the accuracy of the final terrain model. First, simulated sonar data, with perfectly known ground truth, are used to quantitatively assess the performance of the fusion process by comparing DTM obtained with and without fusion. Then, the experimental validation is conducted on actual data, acquired simultaneously by the two sonars systems (Klein K5000 ISSS, Reson 8125 MBES) mounted on the DAURADE AUV. Our evaluation of the fusion method shows significant quantitative and qualitative improvement in the production of the final DTM.
- 外文数据库
- 外文数据库
- 外文数据库
- 外文数据库
- 外文数据库
- 外文数据库
- 外文数据库
- 外文数据库
- 外文数据库
- 外文数据库
- 外文数据库
- 外文数据库
- 外文数据库
- 外文数据库
- 外文数据库
- 外文数据库
This paper presents a novel approach to automatically detect and count cars in different aerial images, which can be satellite or unmanned aerial vehicle (UAV) images. Variations in satellite and/or UAV data make it particularly challenging to have a robust method that works properly on a variety of images. A solution based on the bag-of-words (BoW) model is explored in this paper due to its invariance characteristic and highly stable performance in object/scene categorization. Different from categorization tasks, vehicle detection needs to localize the positions of cars in images. To make BoW suitable for this purpose, we extensively improve the methodology in three aspects, namely, by introducing a recently proposed feature representation, i.e., the local steering kernel descriptor, adding spatial structure constraints, and developing an orientation aware scanning mechanism to produce detection with “one-window-one-car” results. Experiments are conducted on various aerial images with large variations, which consist of data from two public databases, e.g., the Overhead Imagery Research Data Set and Vehicle Detection in Aerial Imagery, as well as other satellite and UAV images. The results demonstrate the effectiveness and robustness of the proposed method. Compared with existing techniques, the proposed method is applicable to a wider range of aerial images.
点击上传
只能上传jpg/png文件，且不超过500kb
