全文获取类型
收费全文 | 17868篇 |
免费 | 370篇 |
国内免费 | 430篇 |
专业分类
化学 | 2445篇 |
晶体学 | 82篇 |
力学 | 135篇 |
综合类 | 35篇 |
数学 | 14515篇 |
物理学 | 1456篇 |
出版年
2023年 | 38篇 |
2022年 | 106篇 |
2021年 | 70篇 |
2020年 | 105篇 |
2019年 | 415篇 |
2018年 | 429篇 |
2017年 | 228篇 |
2016年 | 186篇 |
2015年 | 216篇 |
2014年 | 494篇 |
2013年 | 1415篇 |
2012年 | 582篇 |
2011年 | 1312篇 |
2010年 | 1020篇 |
2009年 | 1084篇 |
2008年 | 1328篇 |
2007年 | 1384篇 |
2006年 | 1029篇 |
2005年 | 683篇 |
2004年 | 565篇 |
2003年 | 515篇 |
2002年 | 404篇 |
2001年 | 391篇 |
2000年 | 355篇 |
1999年 | 404篇 |
1998年 | 361篇 |
1997年 | 289篇 |
1996年 | 312篇 |
1995年 | 384篇 |
1994年 | 343篇 |
1993年 | 277篇 |
1992年 | 238篇 |
1991年 | 164篇 |
1990年 | 173篇 |
1989年 | 130篇 |
1988年 | 95篇 |
1987年 | 95篇 |
1986年 | 90篇 |
1985年 | 147篇 |
1984年 | 122篇 |
1983年 | 73篇 |
1982年 | 109篇 |
1981年 | 103篇 |
1980年 | 87篇 |
1979年 | 55篇 |
1978年 | 84篇 |
1977年 | 60篇 |
1976年 | 46篇 |
1975年 | 27篇 |
1974年 | 22篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
The title compound is a bifunctional receptors including a thiourea group and a crown ether ring. Due to many possible potentials as a new class of reagents for membrane transports,ion-selective electrodes as well as reaction catalysts, the design and synthesis of bifunctional receptors for simultaneous binding of cations and anions is of ongoning interest in srprarnolecular chemistry1-5. In bifunctional receptors, the binding sites for anions and cations are covalently linked so as to exhibit allosteric or cooperative complexation where the binding affinity for anions(cations)is modified as a result of the cation(anion) complexation.Literature[6] reported that the ability of the thiourea group to bind anions is significantly enhanced when Na+ is bound to the crown moiety. To date, however only a few receptors of this class have been reported.6-8In this paper, we report an improved procedure under microwave irradiation that gives higher yields of title compound and needs fewer reaction times than traditional method.The structure of this compound was determined by IR ,element analysis and X-ray analysis.Scheme 1 The reaction equationThe crystal belongs to triclinic crystal system, P-1 space group, a=0.9547(0)nm, b=1.3637(3)nm,c=1.6029(3)nm, α =75.33(3) , β =83.62(3) , γ =70.99(3) ,Z=4,Dc=1.335g/cm3,F(000)=816,R1= 0.0557 ,wR2=0.1281. It is assembled into a three-dimensionalsupramolecule by intermolecular hydrogen bonds. 相似文献
42.
We give a criterion to decide if a given w-homogeneous derivation on A?k[X1,X2,X3] is locally nilpotent. We deduce an algorithm which decides if a k-subalgebra of A, which is finitely generated by w-homogeneous elements, is the kernel of some locally nilpotent derivation. 相似文献
43.
Ramesh Sharma 《Journal of Geometry》2003,78(1-2):156-167
Contact hypersurfaces of a Kaehler manifold have been
characterized and classified, assuming the second fundamental form to be
Codazzi (in particular, parallel). We have also discussed the special
cases when the ambient space is a (i) Calabi-Yau manifold and (ii) a
complex space-form. 相似文献
44.
45.
The ρ-T curves in our single phase HgBa2Ca2Cu3O8+δ superconductor were measured as a function of temperature and magnetic field, ρ=ρ0exp(−Ueff/κBT). It can be transformed to another form d(lnρ)/d(1/T)=−Ueff+TdUeff/dT, then this becomes a plot of the activation energy Ueff as a function of temperature. Our data plotted in these ways show a clear crossover from high-temperature two-dimensional vortex-liquid to a critical region associated with the low-temperature three-dimensional vortex-glass phase transition. The critical exponents v(z−1)=3.9±1.9 in this system are little different with previous measurements in BSCCO and YBCO systems. 相似文献
46.
S. Pumplün 《manuscripta mathematica》1998,97(1):93-108
For a Brauer–Severi variety X over a field k of characteristic not two, every symmetric bilinear space over X up to Witt equivalence is defined over k.
Received: 2 February 1998 相似文献
47.
We shall give a direct proof of the independence result of a Buchholz style-Hydra Game on labeled finite trees. We shall
show that Takeuti-Arai's cut-elimination procedure of and of the iterated inductive definition systems can be directly expressed by the reduction rules of Buchholz's Hydra Game.
As a direct corollary the independence result of the Hydra Game follows.
Received: August 23, 1994 / Revised: July 24, 1995 and May 9, 1996 相似文献
48.
We investigate several versions of a cardinal characteristic defined by Frankiewicz. Vojtáš showed , and Blass showed . We show that all the versions coincide and that is greater than or equal to the splitting number. We prove the consistency of and of .
Received: 2 October 1996 / Revised version: 22 May 1997 相似文献
49.
Modifying complex plane rotations, we derive a new Jacobi-type algorithm for the Hermitian eigendecomposition, which uses only real arithmetic. When the fast-scaled rotations are incorporated, the new algorithm brings a substantial reduction in computational costs. The new method has the same convergence properties and parallelism as the symmetric Jacobi algorithm. Computational test results show that it produces accurate eigenvalues and eigenvectors and achieves great reduction in computational time.The work of this author was supported in part by the National Science Foundation grant CCR-8813493 and by the University of Minnesota Army High Performance Computing Research Center contract DAAL 03-89-C-0038.The work of this author was supported in part by the University of Minnesota Army High Performance Computing Research Center contract DAAL 03-89-C-0038. 相似文献
50.
Genkai Zhang 《manuscripta mathematica》1998,97(3):371-388
Let X=G
* be a compact Hermitian symmetric space. We study the Berezin transform on L
2(X) and calculate its spectrum under the decomposition of L
2(X) into the irreducible representations of G
*. As applications we find the expansion of powers of the canonical polynomial (Bergman reproducing kernel for the canonical
line bundle) in terms of the spherical polynomials on X, and we find the irreducible decomposition of tensor products of Bergman spaces on X.
Received: 10 September 1996 / Revised version: 10 September 1997 相似文献