全文获取类型
收费全文 | 54094篇 |
免费 | 1090篇 |
国内免费 | 416篇 |
专业分类
化学 | 28643篇 |
晶体学 | 469篇 |
力学 | 1857篇 |
综合类 | 12篇 |
数学 | 12249篇 |
物理学 | 12370篇 |
出版年
2023年 | 180篇 |
2021年 | 382篇 |
2020年 | 514篇 |
2019年 | 487篇 |
2018年 | 1495篇 |
2017年 | 1708篇 |
2016年 | 1428篇 |
2015年 | 1161篇 |
2014年 | 1201篇 |
2013年 | 1858篇 |
2012年 | 4685篇 |
2011年 | 4052篇 |
2010年 | 2791篇 |
2009年 | 2373篇 |
2008年 | 2029篇 |
2007年 | 2000篇 |
2006年 | 1966篇 |
2005年 | 5665篇 |
2004年 | 4982篇 |
2003年 | 3125篇 |
2002年 | 1368篇 |
2001年 | 722篇 |
2000年 | 460篇 |
1999年 | 545篇 |
1998年 | 473篇 |
1997年 | 408篇 |
1996年 | 396篇 |
1995年 | 335篇 |
1994年 | 369篇 |
1993年 | 327篇 |
1992年 | 419篇 |
1991年 | 332篇 |
1990年 | 350篇 |
1989年 | 288篇 |
1988年 | 292篇 |
1987年 | 232篇 |
1986年 | 206篇 |
1985年 | 310篇 |
1984年 | 292篇 |
1983年 | 208篇 |
1982年 | 220篇 |
1981年 | 204篇 |
1980年 | 194篇 |
1979年 | 196篇 |
1978年 | 224篇 |
1977年 | 204篇 |
1976年 | 220篇 |
1975年 | 177篇 |
1974年 | 202篇 |
1973年 | 176篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
31.
Let (\gnm)n,m ? \Zst(\gnm)_{n,m\in\Zst} be a Gabor frame for \LtR\LtR for given window gg. We show that the window \ho = \SQI g\ho=\SQI g that generates the canonically associated tight Gabor frame minimizes ||g-h||\|g-h\| among all windows hh generating a normalized tight Gabor frame. We present and prove versions of this result in the time domain, the frequency domain, the time-frequency domain, and the Zak transform domain, where in each domain the canonical \ho\ho is expressed using functional calculus for Gabor frame operators. Furthermore, we derive a Wiener--Levy type theorem for rationally oversampled Gabor frames. Finally, a Newton-type method for a fast numerical calculation of \ho\ho is presented. We analyze the convergence behavior of this method and demonstrate the efficiency of the proposed algorithm by some numerical examples. 相似文献
32.
33.
J.-C. Thomas L. Achouri J. Äystö R. Béraud B. Blank G. Canchel S. Czajkowski P. Dendooven A. Ensallem J. Giovinazzo N. Guillet J. Honkanen A. Jokinen A. Laird M. Lewitowicz C. Longour F. de Oliveira Santos K. Peräjärvi M. Stanoiu 《The European Physical Journal A - Hadrons and Nuclei》2004,21(3):419-435
34.
G.P. Romanelli D. Bennardi D.M. Ruiz G. Baronetti H.J. Thomas 《Tetrahedron letters》2004,45(48):8935-8939
Substituted coumarins are synthesized from phenols and β-ketoesters by the Pechmann reaction, using a Wells-Dawson heteropolyacid (H6P2W18O62·24H2O) as catalyst by a solvent-free procedure. This one requires low reaction times, 130 °C temperature and as little as 1 mol % of Wells-Dawson acid, obtaining good to excellent yields of coumarins. The catalyst showed to be reusable with no differences in the yields. The results are compared with those of the reactions performed in toluene solution. The presented synthetic procedure is a convenient, clean and fast alternative for synthesizing 4-substituted coumarins (17 examples). 相似文献
35.
We consider the one-dimensional steady-state semiconductor deviceequations modelling a pnpn device. There are two relevant scalingsof the equations corresponding to small and large applied voltages.In both scalings, the semiconductor equations can be consideredas singularly perturbed. It turns out that the small-voltagescaling breaks down for current values between two saturationcurrents. In that interval, the large-voltage scaling has tobe employed. For both scalings, we derive the first-order termsof an asymptotic expansion and show that the reduced problemhas a solution. An example verifies that the current-voltagecurves obtained have the expected qualitative structure. 相似文献
36.
K. H. Geib J. Reitstötter R. E. Liesegang Thomas M. Richter Thierbach P. Krais 《Colloid and polymer science》1938,84(1):119-122
Ohne Zusammenfassung 相似文献
37.
Thomas Moore 《Fresenius' Journal of Analytical Chemistry》1883,22(1):247-248
Ohne Zusammenfassung 相似文献
38.
Could simple intraannular-arm macrocyclic systems exist in enantiopure stable forms? The effective synthesis of two representative compounds of such a class, their resolution into enantiomers, and experiments justifying their stability toward racemization are presented. 相似文献
39.
Micha? Chmielewski 《Tetrahedron letters》2004,45(31):6007-6010
Comparison of the anion binding properties of a series of uncharged macrocyclic tetraamides reveal significant effects of the receptor's size on the strength of its anion complexes. This study allowed for estimation of the optimal size of a macroring for complexation of common anions. 相似文献
40.
Jocelyn Sabatier Serge Poullain Patrick Latteux Jean Luc Thomas Alain Oustaloup 《Nonlinear dynamics》2004,38(1-2):383-400
Robust speed control of a low damped electromechanical system with backlash is studied, controlled load angular speed being not measured. The proposed control strategy combines a Luenberger observer (load angular speed and load torque disturbance estimations) and a robust CRONE controller. The observer provides estimation of the load angular speed and of the disturbance torque applied on the load. Through the computation of only three independent parameters (as many as a PID controller), the CRONE controller permits to ensure the robust speed control of the load in spite of plant parametric variations and speed observation errors. The proposed control strategy is applied to a four mass experimental test bench. 相似文献