全文获取类型
收费全文 | 23277篇 |
免费 | 3797篇 |
国内免费 | 2670篇 |
专业分类
化学 | 16547篇 |
晶体学 | 227篇 |
力学 | 1641篇 |
综合类 | 168篇 |
数学 | 2764篇 |
物理学 | 8397篇 |
出版年
2024年 | 68篇 |
2023年 | 447篇 |
2022年 | 727篇 |
2021年 | 768篇 |
2020年 | 831篇 |
2019年 | 899篇 |
2018年 | 703篇 |
2017年 | 721篇 |
2016年 | 1020篇 |
2015年 | 1055篇 |
2014年 | 1240篇 |
2013年 | 1520篇 |
2012年 | 2041篇 |
2011年 | 2112篇 |
2010年 | 1432篇 |
2009年 | 1241篇 |
2008年 | 1495篇 |
2007年 | 1344篇 |
2006年 | 1326篇 |
2005年 | 1108篇 |
2004年 | 873篇 |
2003年 | 727篇 |
2002年 | 710篇 |
2001年 | 598篇 |
2000年 | 449篇 |
1999年 | 501篇 |
1998年 | 393篇 |
1997年 | 385篇 |
1996年 | 420篇 |
1995年 | 350篇 |
1994年 | 284篇 |
1993年 | 225篇 |
1992年 | 226篇 |
1991年 | 238篇 |
1990年 | 194篇 |
1989年 | 174篇 |
1988年 | 121篇 |
1987年 | 97篇 |
1986年 | 114篇 |
1985年 | 86篇 |
1984年 | 68篇 |
1983年 | 53篇 |
1982年 | 43篇 |
1981年 | 36篇 |
1980年 | 34篇 |
1978年 | 24篇 |
1977年 | 25篇 |
1976年 | 25篇 |
1974年 | 21篇 |
1973年 | 26篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
61.
关于可达矩阵的求法探讨 总被引:6,自引:0,他引:6
在《离散数学》、《图论》课程中 ,用矩阵表示图时 ,涉及到一类重要的矩阵——可达矩阵 ,它是判别图中任意两点是否有通路的重要手段 ,也是求强分图的重要方法 ,但是可达矩阵的求法比较复杂 .本文针对这一问题 ,对可达矩阵的求法进行了改进 ,提出了一种简单可行的算法 . 相似文献
62.
In this research, a simple, sensitive chemiluminescence (CL) method for the determination of humic acid (HA) in water samples was first developed based on the redox reaction between humic acid and cerium(IV) in the acidic condition. Different with the former redox CL reaction which occurred in alkaline solution, no enhancers were needed and neither precipitation nor a second contamination would occur in the present CL system. Comparing with other spectrometric methods, we find that the proposed analysis system had better applicability and accuracy. Under the optimal experiment conditions, the CL peak height was linear with the concentration of HA in the range of 0.03 to 10.0 microg mL(-1). The detection limit is 0.01 microg mL(-1) (S/N = 3), and the relative standard deviation was 2.3% for 0.5 microg mL(-1) HA solution with eleven repeated measurements. The present CL method was successfully applied to the determination of HA in tap water, spring water and river water samples with good recovery from 90.0 to 110.0%. A possible CL mechanism was proposed based on the results of UV and fluorescence spectrometry and the CL spectrum of HA. It was speculated that the semi-quinone radicals in the excited state were the emitters. 相似文献
63.
64.
Xu Cheng 《Mathematische Annalen》2003,325(2):229-248
Let M be a 2m-dimensional compact Riemannian manifold with Anosov geodesic flow. We prove that every closed bounded k form, k≥2, on the universal covering of M is d(bounded). Further, if M is homotopy equivalent to a compact K?hler manifold, then its Euler number χ(M) satisfies (−1)
m
χ(M)>0.
Received: 25 September 2001 / Published Online: 16 October 2002 相似文献
65.
The kinetic behaviour of aluminum naphthenate/benzene system in the process of gelation and degradation was systematically studied. The results obtained from the kinetic experiments indicated that the gelation stage was in accordance with a kinetic model of an auto-catalytic first-order reaction, and the degradation stage was a simple first-order reaction. The rate constants in these two stages at various temperaturs were measured. And the determined apparent activation energies for the gelation stage and for the degradation stage are 27.5±2.0 and 37.7±2.3 kJ/mol, respectively. 相似文献
66.
Jing Ping YANG Shi Hong CHENG Xiao Qian WANG 《数学学报(英文版)》2007,23(3):467-478
This paper investigates bivariate recursive equations on excess-of-loss reinsurance. For an insurance portfolio, under the assumptions that the individual claim severity distribution has bounded continuous density and the number of claims belongs to R1 (a, b) family, bivariate recursive equations for the joint distribution of the cedent's aggregate claims and the reinsurer's aggregate claims are obtained. 相似文献
67.
用Langer变换和Olver变换求得一类具有转向点问题的n阶近似解:y(x)=v(x)ψ(x),其中ψ=λ12-14×(x2-1)14,2332=-λx∫11-τ2dτ,v(z)=A(z,λ)ξ(λ23z)+B(z,λ)'ζ(λ23z).并探讨了其特征值问题,得到λn=4n+1112,n=0,1,2….由此给出了该类问题的解的一般性结论. 相似文献
68.
Wen-Rong Dai 《Journal of Differential Equations》2007,235(1):127-165
In this paper, we study the global existence and the asymptotic behavior of classical solution of the Cauchy problem for quasilinear hyperbolic system with constant multiple and linearly degenerate characteristic fields. We prove that the global C1 solution exists uniquely if the BV norm of the initial data is sufficiently small. Based on the existence result on the global classical solution, we show that, when the time t tends to the infinity, the solution approaches a combination of C1 traveling wave solutions. Finally, we give an application to the equation for time-like extremal surfaces in the Minkowski space-time R1+n. 相似文献
69.
70.
Novel oxyfluoride glasses are developed with the composition of 30SiO2-15Al2O3-28PbF2-22CdF2-0.1TmF3 - xYbF3 - (4.9 - x) AlF3(x=0, 0.5, 1.0, 1.5, 2.0) in tool fraction, Furthermore, the upconversion luminescence characteristics under a 970nm excitation are investigated. Intense blue, red and near infrared luminescences peaked at 453nm, 476nm, 647nm and 789nm, which correspond to the transitions of Tm^3+: ^1D2 →^3F4, ^1G4 →^3H6, ^1G4 →^3F4, and ^3H4 →^3H6, respectively, are observed. Due to the sensitization of Yb^3+ ions, all the upconversion luminescence intensities are enhanced considerably with Yb^3+ concentration increasing. The upconversion mechanisms are discussed based on the energy matching rule and quadratic dependence on excitation power. The results indicate that the dominant mechanism is the excited state absorption for those upconversion emissions. 相似文献