全文获取类型
收费全文 | 109224篇 |
免费 | 16758篇 |
国内免费 | 1612篇 |
专业分类
化学 | 94529篇 |
晶体学 | 520篇 |
力学 | 3587篇 |
综合类 | 19篇 |
数学 | 12797篇 |
物理学 | 16142篇 |
出版年
2023年 | 4302篇 |
2022年 | 2202篇 |
2021年 | 3375篇 |
2020年 | 5335篇 |
2019年 | 3136篇 |
2018年 | 3359篇 |
2017年 | 1611篇 |
2016年 | 7320篇 |
2015年 | 6856篇 |
2014年 | 6527篇 |
2013年 | 8592篇 |
2012年 | 7058篇 |
2011年 | 5344篇 |
2010年 | 5757篇 |
2009年 | 5792篇 |
2008年 | 4551篇 |
2007年 | 4264篇 |
2006年 | 3226篇 |
2005年 | 3578篇 |
2004年 | 3690篇 |
2003年 | 2735篇 |
2002年 | 2211篇 |
2001年 | 1552篇 |
2000年 | 1353篇 |
1999年 | 966篇 |
1998年 | 788篇 |
1997年 | 796篇 |
1996年 | 938篇 |
1995年 | 812篇 |
1994年 | 751篇 |
1993年 | 928篇 |
1992年 | 753篇 |
1991年 | 602篇 |
1990年 | 536篇 |
1989年 | 507篇 |
1988年 | 501篇 |
1985年 | 632篇 |
1984年 | 632篇 |
1983年 | 493篇 |
1982年 | 521篇 |
1981年 | 570篇 |
1980年 | 551篇 |
1978年 | 511篇 |
1977年 | 604篇 |
1976年 | 625篇 |
1975年 | 693篇 |
1974年 | 697篇 |
1973年 | 503篇 |
1970年 | 626篇 |
1968年 | 502篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
52.
Maximiliano A. Burgos Paci Gustavo A. Argüello Plcido García Helge Willner 《国际化学动力学杂志》2003,35(1):15-19
Thermal decomposition of bis(trifluoromethyl) peroxydicarbonate has been studied. The mechanism of decomposition is a simple bond fission, homogeneous first‐order process when the reaction is carried out in the presence of inert gases such as N2 or CO. An activation energy of 28.5 kcal mol?1 was determined for the temperature range of 50–90°C. Decomposition is accelerated by nitric oxide because of a chemical attack on the peroxide forming substances different from those formed with N2 or CO. An interpretation on the influence of the substituents in different peroxides on the O? O bond is given. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 35: 15–19, 2003 相似文献
53.
54.
D. Barrios Rolanía G. Lpez Lagomasino E. B. Saff 《Journal of Approximation Theory》2003,124(2):263-281
Using a convergence theorem for Fourier–Padé approximants constructed from orthogonal polynomials on the unit circle, we prove an analogue of Hadamard's theorem for determining the radius of m-meromorphy of a function analytic on the unit disk and apply this to the location of poles of the reciprocal of Szeg
functions. 相似文献
55.
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
56.
Pedro lvarez‐Boo Jos Sergio Casas Alfonso Castieiras María Delfina Couce Eduardo Freijanes Eva Novoa Jos Sordo 《应用有机金属化学》2003,17(9):725-729
Reaction of dichloro‐ and dibromodimethyltin(IV) with 2‐(pyrazol‐1‐ylmethyl)pyridine (PMP) afforded [SnMe2Cl2(PMP)] and [SnMe2Br2(PMP)] respectively. The new complexes were characterized by elemental analysis and mass spectrometry and by IR, Raman and NMR (1H, 13C) spectroscopies. Structural studies by X‐ray diffraction techniques show that the compounds consist of discrete units with the tin atom octahedrally coordinated to the carbon atoms of the two methyl groups in a trans disposition (Sn? C = 2.097(5), 2.120(5) Å and 2.110(6), 2.121(6) Å in the chloro and in the bromo compounds respectively), two cis halogen atoms (Sn? Cl = 2.4908(16), 2.5447(17) Å; Sn? Br = 2.6875(11), 2.7464(9) Å) and the two donor atoms of the ligand (Sn? N = 2.407(4), 2.471(4) Å and 2.360(5), 2.455(5) Å). In both cases, the Sn? N(pyridine) bond length is markedly longer than the Sn? N(pyrazole) distance. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
57.
René Meziat Diego Patiño Pablo Pedregal 《Computational Optimization and Applications》2007,38(1):147-171
We propose an alternative method for computing effectively the solution of non-linear, fixed-terminal-time, optimal control
problems when they are given in Lagrange, Bolza or Mayer forms. This method works well when the nonlinearities in the control
variable can be expressed as polynomials. The essential of this proposal is the transformation of a non-linear, non-convex
optimal control problem into an equivalent optimal control problem with linear and convex structure. The method is based on
global optimization of polynomials by the method of moments. With this method we can determine either the existence or lacking
of minimizers. In addition, we can calculate generalized solutions when the original problem lacks of minimizers. We also
present the numerical schemes to solve several examples arising in science and technology. 相似文献
58.
E. de A. Gonçalves V. M. de Oliveira A. Rosas P. R.A. Campos 《The European Physical Journal B - Condensed Matter and Complex Systems》2007,59(1):127-132
Adaptation of populations takes place with the occurrence and subsequent fixation of mutations that confer some selective
advantage
to the individuals which acquire it. For this reason, the study of the process of fixation of advantageous mutations has a
long history
in the population genetics literature. Particularly, the previous investigations aimed to find out the main evolutionary forces
affecting the strength of natural selection in the populations.
In the current work, we investigate the dynamics of fixation of beneficial mutations in a subdivided population. The subpopulations
(demes)
can exchange migrants
among their neighbors, in a migration network which is assumed to have either a random graph or a scale-free topology. We
have observed that the migration rate
drastically affects the dynamics of mutation fixation, despite of the fact that the probability of fixation is invariant on
the
migration rate, accordingly to
Maruyama's conjecture. In addition, we have noticed a topological dependence of the adaptive evolution of the population when
clonal interference
becomes effective. 相似文献
59.
We derive the incompressible and compressible k–ε model for locally homogeneous turbulence. The model is rigorously derived on formal mathematical grounds using the MPP modelling technique. This lets us calculate by either analytical or numerical means the closure constants of the model. To cite this article: T. Chacón Rebollo, D. Franco Coronil, C. R. Acad. Sci. Paris, Ser. I 337 (2003). 相似文献
60.
We consider toy cosmological models in which a classical, homogeneous, spinor field provides a dominant or sub-dominant contribution to the energy-momentum tensor of a flat Friedmann-Robertson-Walker universe. We find that, if such a field were to exist, appropriate choices of the spinor self-interaction would generate a rich variety of behaviors, quite different from their widely studied scalar field counterparts. We first discuss solutions that incorporate a stage of cosmic inflation and estimate the primordial spectrum of density perturbations seeded during such a stage. Inflation driven by a spinor field turns out to be unappealing as it leads to a blue spectrum of perturbations and requires considerable fine-tuning of parameters. We next find that, for simple, quartic spinor self-interactions, non-singular cyclic cosmologies exist with reasonable parameter choices. These solutions might eventually be incorporated into a successful past- and future-eternal cosmological model free of singularities. In an Appendix, we discuss the classical treatment of spinors and argue that certain quantum systems might be approximated in terms of such fields. 相似文献