首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   67686篇
  免费   4967篇
  国内免费   6313篇
化学   25802篇
晶体学   285篇
力学   6233篇
综合类   1225篇
数学   24983篇
物理学   20438篇
  2023年   520篇
  2022年   789篇
  2021年   1140篇
  2020年   1242篇
  2019年   1487篇
  2018年   1241篇
  2017年   1492篇
  2016年   1701篇
  2015年   1550篇
  2014年   2222篇
  2013年   4290篇
  2012年   2613篇
  2011年   2971篇
  2010年   2575篇
  2009年   3690篇
  2008年   4132篇
  2007年   4337篇
  2006年   4060篇
  2005年   3446篇
  2004年   3176篇
  2003年   3228篇
  2002年   2971篇
  2001年   2506篇
  2000年   2449篇
  1999年   2208篇
  1998年   2067篇
  1997年   1693篇
  1996年   1509篇
  1995年   1377篇
  1994年   1277篇
  1993年   1092篇
  1992年   1057篇
  1991年   819篇
  1990年   680篇
  1989年   590篇
  1988年   540篇
  1987年   421篇
  1986年   364篇
  1985年   428篇
  1984年   423篇
  1983年   209篇
  1982年   339篇
  1981年   400篇
  1980年   301篇
  1979年   290篇
  1978年   220篇
  1977年   205篇
  1976年   154篇
  1974年   109篇
  1973年   104篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
It is shown that for a large collection of independent martingales, the martingale property is preserved on the empirical processes. Under the assumptions of independence and identical finite-dimensional distributions, it is proved that a large collection of stochastic processes are martingales essentially if and only if the empirical processes are also martingales. These two results have implications on the testability of the martingale property in scientific modeling. Extensions to submartingales and supermartingales are given.

  相似文献   

992.
Summary The oxidative dehydrogenation of crotonaldehyde to furan and maleic anhydride was carried out over K2HPMo12O40catalyst. A positive effect of water vapor on furan formation is explained by ability of the catalyst to isomerize 2E- to 2Z-crotonaldehyde.</o:p>  相似文献   
993.
This paper revisits the historical sequence in which some of the major developments of 20th-century physics occurred, and explores how theories could have turned out differently, if the sequence of developments had been different. It shows how a delay in founding special relativity theory until after (1) at least one puzzling problem in electromagnetic theory could be acknowledged, and (2) sat least some of the experimental observations pertinent to the development of quantum mechanics had become well known, could have resulted in a larger theory that covers both domains in a manner quite different from that of any of the theories we use today. The revised theory dispenses with a separate postulate introducing Planck’s constant h, identifying instead a physical mechanism that implies the constant. Some important aspects of quantum chemistry then follow. Editor, Galilean Electrodynamics, Proceedings of the Natural Philosophy Alliance; Visiting Industry Professor, Tufts University, retired  相似文献   
994.
孙立群  孔志宏 《大学数学》2007,23(1):161-165
主要指出了微分法与参数法的实质及二者的本质区别,以及求奇解的一个注意事项.  相似文献   
995.
研究(QU)型模糊拓扑群族直积,证明了任意(QU)型模糊拓扑群族的直积仍是(QU)型模糊拓扑群;研究了(QU)型模糊拓扑群族直积的模糊单位元之重域基的结构,得到了(QU)型模糊拓扑群族直积的一些重要性质.  相似文献   
996.
Organic nonlinear optics (NLO) is also called mo-lecular nonlinear optics. In recent years organic mate-rials have been intensively studied because of their large NLO coefficients and structural diversities[1―3]. Second harmonic generation (SHG) is a bas…  相似文献   
997.
高密度氩气的原子间相互作用与状态方程   总被引:5,自引:4,他引:1  
本文从高密度气体的原子间相互作用出发进行思考,在修改范德瓦耳方程的基础上,提出了一种新的适用于高密度气体的状态方程,并用以对氩气临界区的等温压缩线进行计算,结果与实验值很符合.  相似文献   
998.
Polypropylene microporous membranes were treated with plasma in a mixture of N2 and H2 (1:2 in volume). Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), X-ray photoelectron spectroscopy (XPS) and ultra-violet (UV) spectra demonstrated the success of grafting amino groups. The density of the polar amino groups on the membrane surface is about 0.59 μmol/cm^2. The as-treated membranes were successively applied to the in situ synthesis of oligonucleotides and an average coupling yield was more than 98%. The surface feature of the treated membrane is suggested to be responsible for its advantage over a glass slide.  相似文献   
999.
An isothermal equation of state (EOS) for solids, recently suggested by the authors in the realistic form, V/V0=f(P), with relative volume as the dependent and the pressure as the independent variable, was shown to have an advantage for some close-packed materials in that it allows B=(∂Bs/∂P)s(P→∞) to be fitted, and this is where the usual standard equations fail. In the present study, our EOS is applied to a number of inorganic as well as organic solids, including alloys, glasses, rubbers and plastics; varying widely in their bonding and structural characteristics, as well as in their bulk modulus values. A very good agreement is observed between the data and fits. The results obtained are compared with those from two well-known equations, expressible in the realistic form, proposed by Murnaghan and Luban. Further, the results are also compared with those from the widely used two- and three-parameter EOSs, expressible in the unrealistic form only, P=f(V/V0), proposed by Birch—and also with those from the EOS model of Keane in which B is explicitly expressed as an equation of state parameter. The results obtained from our model compare well to these EOSs. Our EOS, in general, yields the smallest mean-squared deviations between data and fits. The values of Bcalculated from our EOS are compared with those from Keane's model. Further, we have studied the variation of Bwith temperature using the experimental isotherms of Mo and W at 10 different temperatures ranging from 100 to 1000 K, and observed that the values of B yielded by our model and that of Keane vary, as expected, within a narrow range. Furthermore, our EOS is applied to study the stability of the fit parameters with variation in the pressure ranges with reference to the isothermal compression data on Mo and W—and also to study the variation of isothermal bulk modulus with pressure, with reference to the ultrasonic data on NaCl and noted a very good agreement with experiment. In addition, our model is applied, with B0 and B0 constrained to the theoretical values, to the five theoretical isotherms of MgO at 300, 500, 1000, 1500 and 2000 K obtained on the basis of a first principles approach—a good agreement is observed with the predictions, and the values of B inferred at different temperatures tend to converge to a constant value.  相似文献   
1000.
Starting with the unification hypothesis of mixings of quarks and leptons and small quark-like mixings at the see-saw scale, we find that two large mixings for νe —νx03BC; andv μv τ at the weak scale are obtained as a result of renormalization group evolution and radiative magnification if the three neutrinos are quasi degenerate in masses and possess the same CP parity. We also find thatU e3 remains small and well within the CHOOZ-Palo Verde bound since the correspondingV ub for CKM mixing is very small. Several testable pedictions are pointed out.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号