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351.
352.
Improvement in forecasting accuracy is a difficult task but critical for business success. This paper investigates the potential of neural networks for short- to long-term prediction of monthly tanker freight rates. Procedures are outlined for the development of the neural networks. The problem of under-training and over-training is addressed by controlling the number of iterations during the training process of neural networks. A comparative study of predictive performance between neural networks and ARMA time series models is conducted. Our evience shows that neural networks can significantly outperform time series models, especially for longer-term forecasting. 相似文献
353.
Manolis G. Kavussanos 《Maritime Policy and Management》1996,23(1):27-43
The aim of this paper is to estimate an empirical model of bilateral dry-cargo seaborne import flows in the international economy. Seaborne trade elasticities are estimated for the first time, utilizing the Constant Ratio of Elasticities of Substitution Homogeneous/Homothetic (CRESH) function, a function very rarely used in the past. Highly disaggregated data on volumes of seaborne trade, published by the UN, distinguish between five types of cargo according to the type of ship used for its transportation, and 30 trading regions according to the major sea-lancs used by ships internationally. Multistage budgeting is employed to make the problem of estimation tractable. An empirical model for dry-bulk cargo is estimated based on the CRESH function. Estimation of bilateral export price elasticities enables comparison of the degree of competition in each import market over export regions, and amongst import markets themselves. Risk-averse ship owners may utilize such a comparison to operate in world shiplanes with low degree of competition. 相似文献
354.
The structure and nature of Norske Ø Ice Barrier, the southern boundary of the Northeast Water summer polynya, is deduced from Landsat 5 TM images. Within the barrier, three main areas are identified: glaciers to the west, a permanent central ice region made up of fast ice and a variable zone of attached pack ice. 相似文献
355.
L. M. Clough W. G. Ambrose Jr. C. J. Ashjian D. Piepenburg P. E. Renaud S. L. Smith 《Journal of Marine Systems》1997,10(1-4)
We investigated meroplankton (planktonic larvae of benthic organisms) abundance and distribution in the Northeast Water (NEW) Polynya, located on the northeast coast of Greenland, from July 15 to August 15, 1992. Meroplankton was present at all sites visited (0.03–84.83 individuals per m3); at one station meroplankton comprised 8.28% of total zooplankton. Total meroplankton abundance was correlated with total zooplankton abundance and total benthic infaunal abundance but was not correlated with either microscopic carbon concentration or primary productivity. Examination of distribution data for barnacle nauplii and adults indicated that both adults and larvae were concentrated at the same locations. Patterns of distribution were also examined for stelleroid plutei, polychaete larvae and trochophores. There were distinct geographic patterns in total and class-specific meroplankton distributions, with maximal abundances occurring over the Belgica Bank and in the eastern regions of the Westwind Trough and minimal abundances in the Belgica Trough. The apparent control of meroplankton distribution by the hydrography of the region, coupled with the correlation between meroplankton, zooplankton and adult infaunal abundance, reinforces the hypothesis that hydrography plays a major role in controlling the distribution of biota in the NEW polynya (Ambrose and Renaud, 1995; Ashjian et al., 1995, 1997-this volume; Smith et al., 1995; Piepenburg et al., 1997-this volume). 相似文献
356.
G. Belmonte 《Journal of Marine Systems》1998,15(1-4)
This is the first contribution to acartiid taxonomy based on egg morphology. Eggs laid under laboratory conditions by maintaining female specimens of Acartia adriatica, A. clausi, A. italica, A. margalefi, A. tonsa, Paracartia latisetosa and Pteriacartia josephinae were studied by S.E.M. Most species produced eggs with intraspecific variation in their external morphology. In spite of such variation, however, it was possible to separate three species groups. 相似文献
357.
This paper introduces a Multiobjective Hierarchical Model (MOHLM) for locating public facilities on a transportation network. The proposed model combines the multiobjective nature of the location-allocation problem with the hierarchical character of some public service systems, such as health care delivery. The model examines both maximum and total weighted travel time, facility utilization, and total travel time from the master facility to the attached subordinate facilities. An iterative goal programing algorithm is used to solve the problem. An example related to the location of health care facilities in a rural area of Greece is used to illustrate the application of the proposed model. 相似文献
358.
R.G.M. Huisman F.E. Veldpaus H.J.M. Voets J.J. Kok 《Vehicle System Dynamics: International Journal of Vehicle Mechanics and Mobility》1993,22(1):43-55
A continuous time control strategy for an active suspension with preview, based on optimal control theory, is presented. No approximation is needed to model the time delay between the excitation of the front and the rear wheels. The suspension is applied to a two DOF model of the rear side of the tractor of a tractor-semitrailer. The purpose of the suspension is to reduce either the required suspension working space or the maximum absolute acceleration of the sprung mass, without an increase of the dynamic tire force variation. For a step function as road input, reductions of 65% and 55%, respectively, are possible compared with a passive suspension. 相似文献
359.
S. Carabelli C. Delprete G. Genta M. Muzzarelli S. Zanolli 《Vehicle System Dynamics: International Journal of Vehicle Mechanics and Mobility》2000,34(5):333-356
A simple demonstrator for a magnetic levitation vehicle (MagLev) based on repulsive suspension forces generated by permanent magnets is presented. The lateral and yaw motions are stabilized using controlled electromagnets. A mathematical model, aimed mainly at defining the control strategy, has been developed and then experimentally validated using the demonstrator. Two different control strategies have been tested, namely a zero position error and a zero average current control strategy. Both proved to grant a stable behaviour and to effectively counteract external force disturbances. This demonstrator will be used in future research on MagLev vehicles but also on magnetic suspensions in general. 相似文献
360.
A Comparison of Adaptive LQG and Nonlinear Controllers for Vehicle Suspension Systems 总被引:2,自引:0,他引:2
T.J. Gordon C. Marsh M.G. Milsted 《Vehicle System Dynamics: International Journal of Vehicle Mechanics and Mobility》1991,20(6):321-340
A design feature of many computer-controlled suspension systems, is their ability to adapt control law parameters to suit prevailing road conditions. Here, for systems employing high bandwidth actuators and state variable feedback control, the benefits of such adaptation are shown to be at best marginal. An optimal adaptive LQG system is compared with a fixed structure nonlinear feedback controller in the context of a simple quarter-vehicle suspension model. Performance comparisons are made, and trends considered under more realistic conditions. In consequence the overall usefulness of this type of adaptation is called into question. 相似文献