全文获取类型
收费全文 | 59068篇 |
免费 | 4666篇 |
国内免费 | 4138篇 |
专业分类
化学 | 20719篇 |
晶体学 | 416篇 |
力学 | 5077篇 |
综合类 | 738篇 |
数学 | 18570篇 |
物理学 | 22352篇 |
出版年
2024年 | 76篇 |
2023年 | 508篇 |
2022年 | 1069篇 |
2021年 | 1230篇 |
2020年 | 1350篇 |
2019年 | 1363篇 |
2018年 | 1268篇 |
2017年 | 1548篇 |
2016年 | 1667篇 |
2015年 | 1265篇 |
2014年 | 2085篇 |
2013年 | 3847篇 |
2012年 | 2438篇 |
2011年 | 2649篇 |
2010年 | 2160篇 |
2009年 | 3321篇 |
2008年 | 3603篇 |
2007年 | 4096篇 |
2006年 | 3484篇 |
2005年 | 2855篇 |
2004年 | 2383篇 |
2003年 | 2661篇 |
2002年 | 3085篇 |
2001年 | 2377篇 |
2000年 | 2221篇 |
1999年 | 1897篇 |
1998年 | 1816篇 |
1997年 | 1150篇 |
1996年 | 1043篇 |
1995年 | 896篇 |
1994年 | 895篇 |
1993年 | 664篇 |
1992年 | 740篇 |
1991年 | 496篇 |
1990年 | 465篇 |
1989年 | 349篇 |
1988年 | 309篇 |
1987年 | 313篇 |
1986年 | 276篇 |
1985年 | 269篇 |
1984年 | 259篇 |
1983年 | 151篇 |
1982年 | 210篇 |
1981年 | 194篇 |
1980年 | 148篇 |
1979年 | 169篇 |
1978年 | 141篇 |
1977年 | 128篇 |
1976年 | 80篇 |
1973年 | 59篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
We discuss an error estimation procedure for the global error of collocation schemes applied to solve singular boundary value problems with a singularity of the first kind. This a posteriori estimate of the global error was proposed by Stetter in 1978 and is based on the idea of Defect Correction, originally due to Zadunaisky. Here, we present a new, carefully designed modification of this error estimate which not only results in less computational work but also appears to perform satisfactorily for singular problems. We give a full analytical justification for the asymptotical correctness of the error estimate when it is applied to a general nonlinear regular problem. For the singular case, we are presently only able to provide computational evidence for the full convergence order, the related analysis is still work in progress. This global estimate is the basis for a grid selection routine in which the grid is modified with the aim to equidistribute the global error. This procedure yields meshes suitable for an efficient numerical solution. Most importantly, we observe that the grid is refined in a way reflecting only the behavior of the solution and remains unaffected by the unsmooth direction field close to the singular point. 相似文献
42.
对由一类非线性抛物型变分不等方程所描述的无穷维动力系统,给出了存在全局吸引子及弱近似惯性流形的充分条件. 相似文献
43.
XU Jun YAN Ai-Zhen XU Qin-Hua Department of Chemistry Nanjing University Nanjing Jiangsu China 《中国化学》1995,(2)
The strength of basic sites has been measured by pyrrole-IR on alkali metal cation exchanged β and X zeolites, as well as NaOH loaded Naβ. The influence of cation type and the structure of zeolites on their basicity has been studied. The acidic and basic properties of the samples were investigated by NH3-TPD and isopropanol reaction. It was shown that the strength of basic sites on samples could be characterized by the shift of vNH band in the pyrrole-IR spectra. The framework oxygen charges were calculated from the Sanderson electronegativity. The changes in basic properties with various alkali metal cation are consistent with the changes of local oxygen charges of the zeolite framework. 相似文献
44.
Summary This paper is concerned with the problem of developing numerical integration algorithms for differential equations that, when
viewed as equations in some Euclidean space, naturally evolve on some embedded submanifold. It is desired to construct algorithms
whose iterates also evolve on the same manifold. These algorithms can therefore be viewed as integrating ordinary differential
equations on manifolds. The basic method “decouples” the computation of flows on the submanifold from the numerical integration
process. It is shown that two classes of single-step and multistep algorithms can be posed and analyzed theoretically, using
the concept of “freezing” the coefficients of differential operators obtained from the defining vector field. Explicit third-order
algorithms are derived, with additional equations augmenting those of their classical counterparts, obtained from “obstructions”
defined by nonvanishing Lie brackets. 相似文献
45.
Jinchao Xu 《应用数学学报(英文版)》2002,18(2):185-200
Abstract Some new local and parallel finite element algorithms are proposed and analyzed in this paper foreigenvalue problems.With these algorithms, the solution of an eigenvalue problem on a fine grid is reduced tothe solution of an eigenvalue problem on a relatively coarse grid together with solutions of some linear algebraicsystems on fine grid by using some local and parallel procedure.A theoretical tool for analyzing these algorithmsis some local error estimate that is also obtained in this paper for finite element approximations of eigenvectorson general shape-regular grids. 相似文献
46.
47.
A likelihood-based method for haplotype association studies of case-control data with genotyping uncertainty 总被引:1,自引:0,他引:1
ZHU Wensheng & GUO Jianhua School of Mathematics Statistics Northeast Normal University Changchun China 《中国科学A辑(英文版)》2006,49(1):130-144
This paper discusses the associations between traits and haplotypes based on Fl (fluorescent intensity) data sets. We consider a clustering algorithm based on mixtures of t distributions to obtain all possible genotypes of each individual (i.e. "GenoSpec-trum"). We then propose a likelihood-based approach that incorporates the genotyping uncertainty to assessing the associations between traits and haplotypes through a haplotype-based logistic regression model. Simulation studies show that our likelihood-based method can reduce the impact induced by genotyping errors. 相似文献
48.
49.
S. Sabchevski T. Idehara I. Ogawa M. Glyavin K. Ohashi 《International Journal of Infrared and Millimeter Waves》2002,23(5):675-692
A computer code for simulation of beam-field interaction in a resonant cavity of a gyrotron has been developed. It is based on a self-consistent, time-independent, single-mode physical model. The code has been applied to the analysis of the electrodynamical system of a novel high-harmonic gyrotron with axis-encircling beam and a permanent magnet. In this paper both the physical model and numerical techniques used are outlined. Some results of the numerical experiments are presented and discussed. 相似文献
50.
使用SAC/SAC-CI方法,利用D95(d),6-311g**以及cc-PVTZ等基组,对B2分子的基态(X3Σg-)和第一激发态(A3Σu-)的平衡结构和谐振频率进行了优化计算.通过对3个基组的计算结果的比较,得出了D95(d)基组为3个基组中的最优基组的结论;使用D95(d)基组,利用SAC的GSUM(Group Sum of Operators)方法对基态(X3Σg-),SAC-CI的GSUM方法对激发态(A3Σu-)进行单点能扫描计算,用正规方程组拟合Murrell-Sorbie函数,得到了相应电子态的完整势能函数;从得到的势能函数计算了与基态(X3Σg-)和第一激发态(A3Σu-)相对应的光谱常数(Be,αe,ωe和ωeχe),结果与实验数据吻合. 相似文献