全文获取类型
收费全文 | 18285篇 |
免费 | 1918篇 |
国内免费 | 1270篇 |
专业分类
化学 | 7070篇 |
晶体学 | 76篇 |
力学 | 779篇 |
综合类 | 244篇 |
数学 | 4589篇 |
物理学 | 4866篇 |
无线电 | 3849篇 |
出版年
2024年 | 19篇 |
2023年 | 188篇 |
2022年 | 287篇 |
2021年 | 405篇 |
2020年 | 410篇 |
2019年 | 430篇 |
2018年 | 398篇 |
2017年 | 624篇 |
2016年 | 550篇 |
2015年 | 540篇 |
2014年 | 968篇 |
2013年 | 1081篇 |
2012年 | 1061篇 |
2011年 | 1213篇 |
2010年 | 964篇 |
2009年 | 1072篇 |
2008年 | 1264篇 |
2007年 | 1259篇 |
2006年 | 1106篇 |
2005年 | 1069篇 |
2004年 | 911篇 |
2003年 | 728篇 |
2002年 | 645篇 |
2001年 | 486篇 |
2000年 | 494篇 |
1999年 | 448篇 |
1998年 | 444篇 |
1997年 | 345篇 |
1996年 | 302篇 |
1995年 | 236篇 |
1994年 | 243篇 |
1993年 | 160篇 |
1992年 | 151篇 |
1991年 | 119篇 |
1990年 | 100篇 |
1989年 | 105篇 |
1988年 | 88篇 |
1987年 | 72篇 |
1986年 | 51篇 |
1985年 | 76篇 |
1984年 | 49篇 |
1983年 | 38篇 |
1982年 | 49篇 |
1981年 | 24篇 |
1980年 | 27篇 |
1979年 | 31篇 |
1978年 | 34篇 |
1977年 | 23篇 |
1976年 | 26篇 |
1973年 | 15篇 |
排序方式: 共有10000条查询结果,搜索用时 664 毫秒
21.
普遍认为分布函数f在容器壁上的取值为零,如果情况确实如此,则平衡分布函数在容器壁上将不连续,从而会产生不合理的结果.分析指出了在证明H定理时不应假定f在容器壁上为零. 相似文献
22.
Susana Conde Ceide 《Tetrahedron letters》2006,47(26):4415-4418
We herein report that flash heating microwave irradiation is a helpful tool in the formation of arylpyrimidines from the corresponding halopyrimidines. The palladium-catalyzed cross-coupling reactions of 2,4-di- and 2,4,5-trihalopyrimidines with phenylboronic acid under the above conditions are described. By use of the appropriate catalyst and the adequate halopyrimidine, good regioselectivity can be achieved in the 2-, 4-, or 5-positions of the heterocycle. In addition, we show that this methodology is ameneable for the stepwise preparation of mono-, di-, and triphenylpyrimidines. 相似文献
23.
Shufeng Zuo 《Applied Surface Science》2006,253(5):2508-2514
Al-pillared clays supported rare earths (RE/Al-PILC) are prepared and used as supports of palladium catalysts for deep oxidation of low concentrations of benzene (130-160 ppm). The supports and catalysts are characterized by X-ray powder diffraction (XRD), FT-IR, BET, transmission electron microscopy (TEM) and temperature-programmed reduction (H2-TPR). The results show that Al-pillaring results in a strong increase in the basal spacing (d0 0 1) from about 1.2 to 1.8 nm, and an increase in the BET surface area from 63.6 (±3.2) to 238.8 (±11.9) m2/g. Activity tests of deep oxidation of low concentration benzene show catalysts supported on Al-PILC and RE/Al-PILC are obviously more active than that on raw clay. Pd/6% Ce/Al-PILC, in particular, can catalyze the complete oxidation of low concentration benzene at a temperature as low as about 290 °C. 相似文献
24.
根据Pegg-Barnett位相定义, 计算了一种新的奇偶非线性相干态的位相概率分布函数, 利用数值计算方法研究了它们的位相统计性质. 数值计算结果表明:新的奇偶非线性相干态的位相特性与通常奇偶相干态的位相特性截然不同. 相似文献
25.
设E为一个可数集,Q=(qi,j;i,j∈E)为E×E上的矩阵,满足m为E上的概率分布满足何时存在Q过程,使得m是它的不变分布? 这个问题由Williams(1979)作为一个开问题提出.文[15]对全稳定情形,解决了这个问题;本文对单瞬时情形,完整地解决了该问题. 相似文献
26.
A method using third order moments for estimating the regression coefficients as well as the latent state scores of the reduced-rank regression model when the latent variable(s) are non-normally distributed is presented in this paper. It is shown that the factor analysis type indeterminacy of the regression coefficient matrices is eliminated. A real life example of the proposed method is presented. Differences of this solution with the reduced-rank regression eigen solution are discussed. 相似文献
27.
28.
A. D. Rogers 《Acta Mathematica Hungarica》2006,110(1-2):13-21
Summary A lower bound is established for the strip discrepancy of a broad class of point distributions. The bound implies unbounded
strip discrepancy for equally weighted point distributions under favorable conditions. The methods of proof use notions from
integral geometry. 相似文献
29.
30.
Several a priori tests of a systematic stochastic mode reduction procedure recently devised by the authors [Proc. Natl. Acad. Sci. 96 (1999) 14687; Commun. Pure Appl. Math. 54 (2001) 891] are developed here. In this procedure, reduced stochastic equations for a smaller collections of resolved variables are derived systematically for complex nonlinear systems with many degrees of freedom and a large collection of unresolved variables. While the above approach is mathematically rigorous in the limit when the ratio of correlation times between the resolved and the unresolved variables is arbitrary small, it is shown here on a systematic hierarchy of models that this ratio can be surprisingly big. Typically, the systematic reduced stochastic modeling yields quantitatively realistic dynamics for ratios as large as 1/2. The examples studied here vary from instructive stochastic triad models to prototype complex systems with many degrees of freedom utilizing the truncated Burgers–Hopf equations as a nonlinear heat bath. Systematic quantitative tests for the stochastic modeling procedure are developed here which involve the stationary distribution and the two-time correlations for the second and fourth moments including the resolved variables and the energy in the resolved variables. In an important illustrative example presented here, the nonlinear original system involves 102 degrees of freedom and the reduced stochastic model predicted by the theory for two resolved variables involves both nonlinear interaction and multiplicative noises. Even for large value of the correlation time ratio of the order of 1/2, the reduced stochastic model with two degrees of freedom captures the essentially nonlinear and non-Gaussian statistics of the original nonlinear systems with 102 modes extremely well. Furthermore, it is shown here that the standard regression fitting of the second-order correlations alone fails to reproduce the nonlinear stochastic dynamics in this example. 相似文献