全文获取类型
收费全文 | 54003篇 |
免费 | 668篇 |
国内免费 | 426篇 |
专业分类
化学 | 27388篇 |
晶体学 | 450篇 |
力学 | 2006篇 |
综合类 | 13篇 |
数学 | 13234篇 |
物理学 | 12006篇 |
出版年
2021年 | 180篇 |
2020年 | 228篇 |
2019年 | 293篇 |
2018年 | 1409篇 |
2017年 | 1640篇 |
2016年 | 1128篇 |
2015年 | 880篇 |
2014年 | 903篇 |
2013年 | 1934篇 |
2012年 | 4251篇 |
2011年 | 3754篇 |
2010年 | 2544篇 |
2009年 | 2134篇 |
2008年 | 1882篇 |
2007年 | 1986篇 |
2006年 | 1972篇 |
2005年 | 5747篇 |
2004年 | 5067篇 |
2003年 | 3286篇 |
2002年 | 1477篇 |
2001年 | 646篇 |
2000年 | 380篇 |
1999年 | 411篇 |
1998年 | 390篇 |
1997年 | 377篇 |
1996年 | 435篇 |
1995年 | 337篇 |
1994年 | 323篇 |
1993年 | 307篇 |
1992年 | 433篇 |
1991年 | 384篇 |
1990年 | 357篇 |
1989年 | 341篇 |
1988年 | 357篇 |
1987年 | 302篇 |
1986年 | 281篇 |
1985年 | 419篇 |
1984年 | 464篇 |
1983年 | 347篇 |
1982年 | 444篇 |
1981年 | 420篇 |
1980年 | 349篇 |
1979年 | 353篇 |
1978年 | 370篇 |
1977年 | 348篇 |
1976年 | 353篇 |
1975年 | 307篇 |
1974年 | 300篇 |
1973年 | 287篇 |
1972年 | 182篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
Arthur G. Maki John C. Grecu Brenda Winnewisser Manfred Winnewisser 《Journal of Molecular Spectroscopy》2003,222(2):198-212
High-resolution spectra of 15N12C12C15N and 14N13C13C14N have been measured and analyzed from 200 to 3600 cm−1. All the vibrational levels below 900 cm−1 have been observed and characterized. The Fermi resonance between ν2 and 2ν4 has been studied and the resonance constant has been determined for several cases. Several Σ− states have been directly observed for the first time for each isotopomer, the (0001111)0f, (0011111)0f, and (0002222)0f states. The pattern of the energy levels for clusters of l-type resonance coupled levels, such as 0001131,3, has been determined for cyanogen for the first time. Among other things this involved the determination of the vibrational l-type resonance constant, r45. Many of the power series constants, αi and xij, and higher order constants have been determined. 相似文献
92.
John M. Brown 《Journal of organometallic chemistry》2004,689(24):4006-4015
An overview of the contribution of NMR to the development of our understanding of homogeneous catalysis is presented, with an emphasis on work from the author’s research group. 相似文献
93.
94.
Parameters of Gaussian multivariate models are often estimated using the maximum likelihood approach. In spite of its merits, this methodology is not practical when the sample size is very large, as, for example, in the case of massive georeferenced data sets. In this paper, we study the asymptotic properties of the estimators that minimize three alternatives to the likelihood function, designed to increase the computational efficiency. This is achieved by applying the information sandwich technique to expansions of the pseudo-likelihood functions as quadratic forms of independent normal random variables. Theoretical calculations are given for a first-order autoregressive time series and then extended to a two-dimensional autoregressive process on a lattice. We compare the efficiency of the three estimators to that of the maximum likelihood estimator as well as among themselves, using numerical calculations of the theoretical results and simulations. 相似文献
95.
The selection of the branching variable can greatly affect the speed of the branch and bound solution of a mixed-integer or
integer linear program. Traditional approaches to branching variable selection rely on estimating the effect of the candidate
variables on the objective function. We present a new approach that relies on estimating the impact of the candidate variables
on the active constraints in the current LP relaxation. We apply this method to the problem of finding the first feasible
solution as quickly as possible. Empirical experiments demonstrate a significant improvement compared to a state-of-the art
commercial MIP solver. 相似文献
96.
In this paper, by using elementary analysis, we establish some new Lyapunov-type inequalities for nonlinear systems of differential equations, special cases of which contain the well-known equations such as Emden-Fowler-type and half-linear equations. The inequalities obtained here can be used as handy tools in the study of qualitative behaviour of solutions of the associated equations. 相似文献
97.
98.
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
99.
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
100.
E. Beinrohr M. Németh P. Tsch?pel G. T?lg 《Fresenius' Journal of Analytical Chemistry》1992,344(3):93-99
Summary Flow-through electrochemical cells with porous working electrodes made of crushed reticulated vitreous carbon and plated with mercury were used for absolute analysis of trace amounts of lead by anodic stripping coulometry with collection (ASCWC) in a flow system. The role of mercury coating, flow rate and pH were investigated. The coulombic content of the collection peak corresponded to the theoretical values calculated by Faraday's law in a concentration range from about 10–9 to 10–6 mol/l. The relative error and the relative standard deviation was +0.15% and 0.8%, respectively for 2×10–6 mol/l analyte concentration. The absolute detection limit (3 s) was 0.1 ng of Pb, the linear response range 7×104.
One leave from: Department of Analytical Chemistry, Slovak Technical University, CS-812 37 Bratislava, Czechoslovakia 相似文献