全文获取类型
收费全文 | 445486篇 |
免费 | 7824篇 |
国内免费 | 3832篇 |
专业分类
化学 | 246039篇 |
晶体学 | 6330篇 |
力学 | 19319篇 |
综合类 | 201篇 |
数学 | 52070篇 |
物理学 | 133183篇 |
出版年
2021年 | 2991篇 |
2020年 | 3433篇 |
2019年 | 3417篇 |
2018年 | 3302篇 |
2017年 | 3234篇 |
2016年 | 6371篇 |
2015年 | 5422篇 |
2014年 | 7140篇 |
2013年 | 20678篇 |
2012年 | 16485篇 |
2011年 | 20217篇 |
2010年 | 12754篇 |
2009年 | 12698篇 |
2008年 | 18303篇 |
2007年 | 18454篇 |
2006年 | 17818篇 |
2005年 | 16312篇 |
2004年 | 14574篇 |
2003年 | 12829篇 |
2002年 | 12658篇 |
2001年 | 14092篇 |
2000年 | 10943篇 |
1999年 | 8618篇 |
1998年 | 6855篇 |
1997年 | 6678篇 |
1996年 | 6691篇 |
1995年 | 6119篇 |
1994年 | 5704篇 |
1993年 | 5424篇 |
1992年 | 6052篇 |
1991年 | 5891篇 |
1990年 | 5422篇 |
1989年 | 5260篇 |
1988年 | 5531篇 |
1987年 | 5115篇 |
1986年 | 4953篇 |
1985年 | 7193篇 |
1984年 | 7196篇 |
1983年 | 5865篇 |
1982年 | 6317篇 |
1981年 | 6254篇 |
1980年 | 6031篇 |
1979年 | 6104篇 |
1978年 | 6152篇 |
1977年 | 6099篇 |
1976年 | 6001篇 |
1975年 | 5880篇 |
1974年 | 5680篇 |
1973年 | 5907篇 |
1972年 | 3449篇 |
排序方式: 共有10000条查询结果,搜索用时 11 毫秒
141.
D. J. Hartfiel 《Linear and Multilinear Algebra》1994,36(4):293-304
In this paper we consider the nonnegative matrix system ciA+uiB=ci+1 where the nonnegative matrix A is allowed to vary, within bounds. The cone control problem is to find a nonnegative matrix B such that if Ci is a nonnegative vector in a specified cone, then there is a nonnegative vector ui such that ci+1 is in that cone. We extend this problem to input control by finding a B such that the cone, generated by the rows of B, is as small as possible. Thus, the percent distribution of ∣uiB∣ through the states of the sustem by uiB is either constant or varies little. 相似文献
142.
The authors study concentration profiles of solutes undergoingequilibrium absorption in the vicinity of a water well. Forthe case of a contamination event, the limit problem of vanishingwell radius, which is of self-similar nature, is analysed indetail. Existence, uniqueness, and qualitative properties ofsolutions of the corresponding ordinary differential equationsare shown. Some numerical examples are also presented. 相似文献
143.
144.
Experiments for the determination of mono-, di and tri-butyltin (MBT, DBT and TBT) by hydride generation/gas chromatography/atomic absorption spectrometry in various matrices (sediment, suspended matter, mussel, algae and water) have revealed that poor butyltin recoveries are obtained in sediments displaying high sulphur and hydrocarbon contents; very poor recoveries were also observed for TBT in sediments with high chlorophyll pigment contents as well as in algal samples. It was however not clear whether the hydride generatin was inhibited by these infering compounds, as was previously assumed in the case of hydrocarbons, or whether interferences affected the atomization rate. Further studies were performed to solve this problem in order to validate this method in the case of analyses of, for example, oil-contaminated sediment and algae. This paper presents the results obtained. It is concluded here that the poor recoveries were due to an inhibition of hydride generation rather than to interference at the atomization stage. 相似文献
145.
146.
C. V. Pao 《Numerical Methods for Partial Differential Equations》1993,9(3):285-311
We investigate the dynamics and methods of computation for some nonlinear finite difference systems that are the discretized equations of a time-dependent and a steady-state reaction–diffusion problem. The formulation of the discrete equations for the time-dependent problem is based on the implicit method for parabolic equations, and the computational algorithm is based on the method of monotone iterations using upper and lower solutions as the initial iterations. The monotone iterative method yields improved upper and lower bounds of the solution in each iteration, and the sequence of iterations converges monotonically to a solution for both the time-dependent and the steady-state problems. An important consequence of this method is that it leads to a bifurcation point that determines the dynamic behavior of the time-dependent problem in relation to the corresponding steady-state problem. This bifurcation point also determines whether the steady-state problem has one or two non-negative solutions, and is explicitly given in terms of the physical parameters of the system and the type of boundary conditions. Numerical results are presented for both the time-dependent and the steady-state problems under various boundary conditions, including a test problem with known analytical solution. These numerical results exhibit the predicted dynamic behavior of the time-dependent solution given by the theoretical analysis. Also discussed are the numerical stability of the computational algorithm and the convergence of the finite difference solution to the corresponding continuous solution of the reaction–diffusion problem. © 1993 John Wiley & Sons, Inc. 相似文献
147.
148.
149.
Electric fields of the anions, cations and neutral forms of 2-aminopurine and 6-thioguanine have been mapped. Certain important
features of the maps are similar to those found earlier in the neutral and ionic forms of adenine and guanine. The computed
electric field patterns satisfactorily explain reactive sites and biological activity of the molecules. 相似文献
150.
P. C. Chen 《中国化学会会志》1995,42(5):755-760
Three ab initio calculations (HF/6-3IG, HF/6-3IG*, and HF/6-3IG**) on 2,4,6-trinitrotoluene were made, The results compare well with xray data, except dihedral angles of NO2 relative to the plane of the benzene ring. The deviations are attributed to packing forces and steric effects in the crystal. The most stable structure was a torsional angle 10° of the methyl top with the benzene ring, unlike toluene. The rotational barriers of the methyl top and the 4-nitro group are small. Hydrogen bonding, dipole moments and total atomic charges arc calculated. 相似文献