全文获取类型
收费全文 | 27086篇 |
免费 | 610篇 |
国内免费 | 159篇 |
专业分类
化学 | 17956篇 |
晶体学 | 220篇 |
力学 | 690篇 |
综合类 | 1篇 |
数学 | 4569篇 |
物理学 | 4419篇 |
出版年
2021年 | 180篇 |
2020年 | 218篇 |
2019年 | 292篇 |
2018年 | 237篇 |
2017年 | 216篇 |
2016年 | 462篇 |
2015年 | 351篇 |
2014年 | 477篇 |
2013年 | 1374篇 |
2012年 | 1225篇 |
2011年 | 1552篇 |
2010年 | 810篇 |
2009年 | 650篇 |
2008年 | 1401篇 |
2007年 | 1458篇 |
2006年 | 1449篇 |
2005年 | 1449篇 |
2004年 | 1274篇 |
2003年 | 1078篇 |
2002年 | 1049篇 |
2001年 | 403篇 |
2000年 | 322篇 |
1999年 | 256篇 |
1998年 | 302篇 |
1997年 | 325篇 |
1996年 | 416篇 |
1995年 | 312篇 |
1994年 | 301篇 |
1993年 | 305篇 |
1992年 | 294篇 |
1991年 | 249篇 |
1990年 | 236篇 |
1989年 | 253篇 |
1988年 | 278篇 |
1987年 | 256篇 |
1986年 | 252篇 |
1985年 | 404篇 |
1984年 | 450篇 |
1983年 | 340篇 |
1982年 | 443篇 |
1981年 | 426篇 |
1980年 | 362篇 |
1979年 | 339篇 |
1978年 | 365篇 |
1977年 | 351篇 |
1976年 | 288篇 |
1975年 | 281篇 |
1974年 | 275篇 |
1973年 | 250篇 |
1972年 | 150篇 |
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
61.
If a Horn set I has a single satisfying truth assignment or model then that model is said to be unique for I. The question of determining whether a unique model exists for a given Horn set I is shown to be solved in O((L)*L) time, where L is the sum of the lengths of the clauses in I and is the inverse Ackermann function. It is also shown that if LA*log (A) where A is the number of distinct proposition letters then unique satisfiability can be determined in O(L) time. 相似文献
62.
Vattoly J. MajoJaya Prabhakaran J.John MannJ.S. Dileep Kumar 《Tetrahedron letters》2003,44(47):8535-8537
A novel and convergent palladium catalyzed synthesis of 2-arylbenzothiazoles has been investigated. The key step in the synthesis is a Suzuki biaryl coupling of 2-bromobenzothiazole with aryl boronic acids to provide a variety of 2-arylbenzothiazole derivatives in good yield. The synthetic utility of this methodology is demonstrated by the synthesis of 2-(4-aminophenyl)-6-methoxybenzothiazole, a PET probe precursor for the in vivo imaging of Alzheimer's disease. 相似文献
63.
Shyam K. Singh Oliver S. Fetzer John B. Hynes Thomas C. Williams 《Journal of heterocyclic chemistry》1991,28(5):1459-1461
Nitration of 2-amino-4-oxo-(3H)-5-trifluoromethylquinazoline is shown to occur exclusively at C6 as determined from an analysis of long range 1H and 19F scalar couplings to ring carbons. Nitration of 2-amino-4-oxo-(3H)-5-fluoroquinazoline is found to occur both at C6 and C8 as evident from an analysis of the 19F and 1H couplings of the ring protons. 相似文献
64.
65.
66.
There are essentially two schools of thought regarding the function of decision support systems. One is that the main purpose is to support the decision-maker in whatever style of decision-making he or she wishes. The other is that the support, while suiting the decision-maker's individual style, must also be based on an appropriate theoretical paradigm. The work with students of decision support systems reported here suggests that the latter, normative, view is not one which comes naturally to many of the students, even when working on an example where it seemed relatively easy to incorporate. Other conclusions are also presented, relating to the choice of software for decision support system development, the process of teaching about decision support systems, and group formation. 相似文献
67.
Emulsions of perfluorotributylamine (FTBA) and perflubron were evaluated for their utility in 19F echo planar imaging. Fluorine images of the emulsions were obtained in a phantom and two mice that had been predosed. Both agents, but particularly perflubron, show potential for fluorine echo planar studies because of the long spin-spin relaxation times of the CF3 resonances. High resolution thin slice images obtained in as little as 26.6 ms are presented. 相似文献
68.
Francis J. Vasko John A. McNamara Dennis D. Newhart Floyd E. Wolf 《The Journal of the Operational Research Society》1994,45(11):1285-1292
The optimum assignment of structural steel shapes to rail cars is an important logistical problem in the steel industry. In this paper, we discuss an application at Bethlehem Steel that not only involves weight and dimensional constraints, but also customer unloading constraints. The formulation is a generalized bin packing problem which is solved by modifying and extending the first fit decreasing algorithm. The solution algorithm, SOLID (for Structural Optimal Loading IDentification), has been used extensively for one of Bethlehem's high tonnage customers providing very good practical (implementable) results that achieve the desired goals. Bethlehem has enhanced this approach for use with other customers. 相似文献
69.
Alec B. Scranton John Klier Nikolaos A. Peppas 《Journal of Polymer Science.Polymer Physics》1991,29(2):211-224
The dilute solution complexation equilibrium between linear macromolecules and smaller complementary oligomers is considered when: (1) the oligomers are free in solution; and (2) the oligomers are covalently attached at one end to the polymer. A general statistical mechanical framework is developed and is illustrated using a simple random walk model for polymer conformation. The statistical mechanical partition functions are formulated using a generating function technique, allowing thermodynamic averages in the complexed state to be calculated. Loops, trains, and tails of all possible length are allowed in the conformation of a complexed oligomer. Simulation results for the free oligomer case are compared with those obtained for oligomers covalently attached to the polymeric molecular. The model provides a theoretical explanation for the experimentally observed enhancement of complexation of oligomers grafted to the complementary polymers. 相似文献
70.