全文获取类型
收费全文 | 142328篇 |
免费 | 10293篇 |
国内免费 | 6936篇 |
专业分类
化学 | 68244篇 |
晶体学 | 1394篇 |
力学 | 10883篇 |
综合类 | 515篇 |
数学 | 39609篇 |
物理学 | 38912篇 |
出版年
2023年 | 1356篇 |
2022年 | 1371篇 |
2021年 | 2121篇 |
2020年 | 2492篇 |
2019年 | 2385篇 |
2018年 | 11995篇 |
2017年 | 11612篇 |
2016年 | 8619篇 |
2015年 | 3737篇 |
2014年 | 3780篇 |
2013年 | 4990篇 |
2012年 | 9395篇 |
2011年 | 16058篇 |
2010年 | 9439篇 |
2009年 | 9753篇 |
2008年 | 10502篇 |
2007年 | 12239篇 |
2006年 | 3757篇 |
2005年 | 4337篇 |
2004年 | 3946篇 |
2003年 | 3793篇 |
2002年 | 2658篇 |
2001年 | 1757篇 |
2000年 | 1701篇 |
1999年 | 1715篇 |
1998年 | 1520篇 |
1997年 | 1298篇 |
1996年 | 1361篇 |
1995年 | 1174篇 |
1994年 | 1074篇 |
1993年 | 951篇 |
1992年 | 809篇 |
1991年 | 723篇 |
1990年 | 597篇 |
1989年 | 538篇 |
1988年 | 422篇 |
1987年 | 404篇 |
1986年 | 352篇 |
1985年 | 348篇 |
1984年 | 244篇 |
1983年 | 211篇 |
1982年 | 186篇 |
1981年 | 150篇 |
1980年 | 129篇 |
1979年 | 91篇 |
1978年 | 90篇 |
1977年 | 70篇 |
1976年 | 66篇 |
1975年 | 73篇 |
1973年 | 82篇 |
排序方式: 共有10000条查询结果,搜索用时 312 毫秒
991.
I T Vieira P R Harper A K Shahani V De Senna 《The Journal of the Operational Research Society》2003,54(7):713-722
Mother-to-child transmission (MTCT) of HIV is the most significant source of HIV infection in children below the age of 15 years. In 2000 alone, about 600 000 new infections occurred, the vast majority from mothers living in developing countries who were not aware of their HIV-positive status. To date, at least 4.3 million children have died from AIDS. In this paper, we discuss the development of an operational model that can be used to evaluate intervention options for the prevention of MTCT of HIV. The problem was approached by defining suitable pregnancy risk groups, prevention options and the required model outputs to evaluate different intervention strategies. The method chosen to approach the problem was a discrete-event, three-phase simulation, built in Visual Basic, with a stochastic semi-Markov structure. The developed model takes individual pregnant women through each stage of their pregnancy, labour and birth. Different intervention strategies may be modelled at any time, including short-course antiretroviral drugs and cessation of breastfeeding. The model is demonstrated with data from Botswana, which has one of the highest HIV-infection rates in the world. 相似文献
992.
993.
994.
Stephen P. Humphries 《Israel Journal of Mathematics》2004,143(1):189-222
We prove that (with two possible exceptions) the Hurwitz braid group action on the sequence of standard generators of an irreducible
Artin group has a finite orbit if and only if the Artin group is of finite type (i.e., the corresponding Coxeter group is
finite). 相似文献
995.
Oleg Safronov 《Arkiv f?r Matematik》2004,42(2):363-377
We study the spectral properties of Jacobi matrices. By combining Killip's technique [12] with the technique of Killip and
Simon [13] we obtain a result relating the properties of the elements of Jacobi matrices and the corresponding spectral measures.
This theorem is a natural extension of a recent result of Laptev-Naboko-Safronov [17].
The author thanks Sergei Naboko for useful discussions and Barry Simon for pointing out the conjecture. 相似文献
996.
Effects of barium on the nonlinear electrical characteristics and dielectric properties of SnO2-based varistors 下载免费PDF全文
The effects of barium on electrical and dielectric properties of the SnO_2·Co_2O_3·Ta_2O_5 varistor system sintered at 1250℃ for 60min were investigated. It is found that barium significantly improves the nonlinear properties. The breakdown electrical field increases from 378.0 to 2834.5V/mm, relative dielectric constant (at 1kHz) falls from 1206 to 161 and the resistivity (at 1kHz) rises from 60.3 to 1146.5kΩ·cm with an increase of BaCO_3 concentration from 0mol% to 1.00mol%. The sample with 1.00mol% barium has the best nonlinear electrical property and the highest nonlinear coefficient (α=29.2). A modified defect barrier model is introduced to illustrate the grain-boundary barrier formation of barium-doped SnO_{2}-based varistors. 相似文献
997.
On the characterisation of paired monotone metrics 总被引:1,自引:1,他引:0
Paolo Gibilisco Tommaso Isola 《Annals of the Institute of Statistical Mathematics》2004,56(2):369-381
Hasegawa and Petz introduced the notion of paired monotone metrics. They also gave a characterisation theorem showing that
Wigner-Yanase-Dyson metrics are the only members of the paired family. In this paper we show that the characterisation theorem
holds true under hypotheses that are more general than those used in the above quoted references. 相似文献
998.
999.
Let A be a Weil algebra. The bijection between all natural operators lifting vector fields from m-manifolds to the bundle functor K
A
of Weil contact elements and the subalgebra of fixed elements SA of the Weil algebra A is determined and the bijection between all natural affinors on K
A
and SA is deduced. Furthermore, the rigidity of the functor K
A
is proved. Requisite results about the structure of SA are obtained by a purely algebraic approach, namely the existence of nontrivial SA is discussed. 相似文献
1000.
Richard P. Stanley 《Israel Journal of Mathematics》2004,143(1):317-339
We survey three recent developments in algebraic combinatorics. The first is the theory of cluster algebras and the Laurent
phenomenon of Sergey Fomin and Andrei Zelevinsky. The second is the construction of toric Schur functions and their application
to computing three-point Gromov-Witten invariants, by Alexander Postnikov. The third development is the construction of intersection
cohomology for nonrational fans by Paul Bressler and Valery Lunts and its application by Kalle Karu to the torich-vector of a nonrational polytope. We also briefly discuss the “half hard Lefschetz theorem” of Ed Swartz and its application
to matroid complexes.
Partially supported by NSF grant #DMS-9988459. 相似文献