全文获取类型
收费全文 | 32999篇 |
免费 | 810篇 |
国内免费 | 259篇 |
专业分类
化学 | 22275篇 |
晶体学 | 143篇 |
力学 | 862篇 |
综合类 | 1篇 |
数学 | 6165篇 |
物理学 | 4622篇 |
出版年
2022年 | 256篇 |
2021年 | 349篇 |
2020年 | 408篇 |
2019年 | 383篇 |
2018年 | 331篇 |
2017年 | 350篇 |
2016年 | 721篇 |
2015年 | 731篇 |
2014年 | 803篇 |
2013年 | 1657篇 |
2012年 | 1730篇 |
2011年 | 2087篇 |
2010年 | 1152篇 |
2009年 | 1024篇 |
2008年 | 1729篇 |
2007年 | 1737篇 |
2006年 | 1846篇 |
2005年 | 1672篇 |
2004年 | 1548篇 |
2003年 | 1309篇 |
2002年 | 1244篇 |
2001年 | 518篇 |
2000年 | 475篇 |
1999年 | 440篇 |
1998年 | 445篇 |
1997年 | 496篇 |
1996年 | 460篇 |
1995年 | 404篇 |
1994年 | 443篇 |
1993年 | 376篇 |
1992年 | 347篇 |
1991年 | 344篇 |
1990年 | 300篇 |
1989年 | 297篇 |
1988年 | 280篇 |
1987年 | 234篇 |
1986年 | 256篇 |
1985年 | 386篇 |
1984年 | 388篇 |
1983年 | 327篇 |
1982年 | 362篇 |
1981年 | 328篇 |
1980年 | 356篇 |
1979年 | 292篇 |
1978年 | 317篇 |
1977年 | 290篇 |
1976年 | 237篇 |
1975年 | 200篇 |
1974年 | 204篇 |
1973年 | 171篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
Peter Francis Guangming Zhang Karen Smilowitz 《European Journal of Operational Research》2007,180(3):1045-1059
This paper presents modeling and solution method improvements for the Multi-Resource Routing Problem (MRRP) with flexible tasks. The MRRP with flexible tasks is used to model routing and scheduling problems for intermodal drayage operations in which two resources (tractors and trailers) perform tasks to transport loaded and empty equipment. Tasks may be either well defined, in which both the origin and the destination of a movement are given, or flexible, in which the origin or the destination is chosen by the model. This paper proposes methods to effectively manage the number of options considered for flexible tasks (either feasible origins for a known destination or feasible destinations for a known origin). This modeling change generates sufficient options to allow for low-cost solutions while maintaining reasonable computational effort. We also propose a new solution method that uses randomized route generation. Computational results from test cases show that these changes improve the quality of solutions by at least 5% in the test cases as compared to methods from previous studies. 相似文献
72.
73.
74.
Peter Enders 《Optical and Quantum Electronics》1989,21(3):167-181
The small-signal analysis of a simple model of three rate equations is performed rigorously by means of both the Routh-Hurwitz theorem and Descartes' sign rule, and the relationship between bistable behaviour and self-sustained pulsations (SSPs) is investigated. Because one eigenvalue of the Jacobian of the model system is always negative, small-amplitude, sinusoidal SSPs can be described in terms of Haken's dressed-mode variables on a two-dimensional centre manifold. Possible simplifications for the switching modelling are also mentioned. 相似文献
75.
Empirical minimization 总被引:3,自引:0,他引:3
We investigate the behavior of the empirical minimization algorithm using various methods. We first analyze it by comparing
the empirical, random, structure and the original one on the class, either in an additive sense, via the uniform law of large
numbers, or in a multiplicative sense, using isomorphic coordinate projections. We then show that a direct analysis of the
empirical minimization algorithm yields a significantly better bound, and that the estimates we obtain are essentially sharp.
The method of proof we use is based on Talagrand's concentration inequality for empirical processes.
Research partially supported by NSF under award DMS-0434393.
Research partially supported by the Australian Research Council Discovery Porject DP0343616. 相似文献
76.
77.
78.
79.
We consider the M(t)/M(t)/m/m queue, where the arrival rate λ(t) and service rate μ(t) are arbitrary (smooth) functions of time. Letting pn(t) be the probability that n servers are occupied at time t (0≤ n≤ m, t > 0), we study this distribution asymptotically, for m→∞ with a comparably large arrival rate λ(t) = O(m) (with μ(t) = O(1)). We use singular perturbation techniques to solve the forward equation for pn(t) asymptotically. Particular attention is paid to computing the mean number of occupied servers and the blocking probability
pm(t). The analysis involves several different space-time ranges, as well as different initial conditions (we assume that at t = 0 exactly n0 servers are occupied, 0≤ n0≤ m). Numerical studies back up the asymptotic analysis.
AMS subject classification: 60K25,34E10
Supported in part by NSF grants DMS-99-71656 and DMS-02-02815 相似文献
80.
The steel ASTM A213 P22 is used for superheater outlet header in power plants. During duty cycles lasting over tens of years the temperature is of the order of 545°C and the pressure 125 atm. The microscopic changes in these steels are hard to analyze under working conditions, but they are believed to be responsible for the appearance of creeps in such devices. Investigation of the microscopic modifications will help to predict future failures due to creeps, increasing the reliability and saving a lot of money.We report the investigation of such changes in ASTM A213 P22 steel in use for 30 years as a Superheater header at Haifa power station. The analysis has been carried out using Mössbauer Spectroscopy, X-ray diffraction and optical microscopy. As a result it seems that the morphology and phase change of the carbide phases M3C/M7C3 and M23C6 are the most prominent changes leading to the material failures. 相似文献