全文获取类型
收费全文 | 3483篇 |
免费 | 85篇 |
国内免费 | 27篇 |
专业分类
化学 | 2167篇 |
晶体学 | 35篇 |
力学 | 64篇 |
数学 | 625篇 |
物理学 | 704篇 |
出版年
2023年 | 17篇 |
2022年 | 69篇 |
2021年 | 74篇 |
2020年 | 72篇 |
2019年 | 84篇 |
2018年 | 76篇 |
2017年 | 61篇 |
2016年 | 123篇 |
2015年 | 111篇 |
2014年 | 139篇 |
2013年 | 239篇 |
2012年 | 202篇 |
2011年 | 226篇 |
2010年 | 159篇 |
2009年 | 157篇 |
2008年 | 197篇 |
2007年 | 196篇 |
2006年 | 157篇 |
2005年 | 138篇 |
2004年 | 107篇 |
2003年 | 78篇 |
2002年 | 87篇 |
2001年 | 44篇 |
2000年 | 47篇 |
1999年 | 35篇 |
1998年 | 21篇 |
1997年 | 29篇 |
1996年 | 40篇 |
1995年 | 29篇 |
1994年 | 29篇 |
1993年 | 32篇 |
1992年 | 35篇 |
1991年 | 24篇 |
1990年 | 27篇 |
1989年 | 20篇 |
1988年 | 24篇 |
1987年 | 24篇 |
1986年 | 12篇 |
1985年 | 24篇 |
1984年 | 36篇 |
1983年 | 20篇 |
1982年 | 24篇 |
1981年 | 22篇 |
1980年 | 24篇 |
1979年 | 15篇 |
1978年 | 32篇 |
1977年 | 23篇 |
1976年 | 19篇 |
1975年 | 12篇 |
1968年 | 10篇 |
排序方式: 共有3595条查询结果,搜索用时 0 毫秒
1.
In this work we study nonnegativity and positivity of a discrete quadratic functional with separately varying endpoints. We introduce a notion of an interval coupled with 0, and hence, extend the notion of conjugate interval to 0 from the case of fixed to variable endpoint(s). We show that the nonnegativity of the discrete quadratic functional is equivalent to each of the following conditions: The nonexistence of intervals coupled with 0, the existence of a solution to Riccati matrix equation and its boundary conditions. Natural strengthening of each of these conditions yields a characterization of the positivity of the discrete quadratic functional. Since the quadratic functional under consideration could be a second variation of a discrete calculus of variations problem with varying endpoints, we apply our results to obtain necessary and sufficient optimality conditions for such problems. This paper generalizes our recent work in [R. Hilscher, V. Zeidan, Comput. Math. Appl., to appear], where the right endpoint is fixed. 相似文献
2.
M. Mayr K. -H. Speidel M. Knopp W. Karle T. Faestermann F. Hagelberg H. -J. Simonis P. N. Tandon J. Gerber 《Zeitschrift für Physik A Hadrons and Nuclei》1987,327(2):157-161
[Theg-factor ratio of the first excited 3? and 5? states in40Ca was measured to beg 3/g 5=1.01(10) employing the implantation perturbed angular correlation technique. The static hyperfine fields (SF) in Fe and Gd hosts were used. In addition the lifetime of the 5? state was measured to be τ=426(7)ps. The values of the SF in Gd and Fe hosts were deduced and compared with systematics in this element region. 相似文献
3.
4.
Orotic acid (vitamin B13) is a key intermediate in biosynthesis of the pyrimidine nucleotides in living organisms, moreover, it may serve as the biological carrier for some metal ions. cis-Diammine(orotato)platinum(II), cis-[Pt(C5H2N2O4)(NH3)2] can be considered as a new potential cisplatin analogue. The FT-Raman and FT-IR spectra of the title complex are reported, for the first time. The molecular structure, vibrational frequencies, and the theoretical infrared and Raman intensities have been calculated by the density functional mPW1PW91 method. The detailed vibrational assignment has been made on the basis of the calculated potential energy distribution. The theoretically predicted IR and Raman spectra show very good agreement with experiment. Natural bond orbital (NBO) analyses were performed for cisplatin, carboplatin and the title complex. The results provided new data on the nature of platinum–ligand bonding in these compounds. Strong intramolecular hydrogen bond between the orotate ligand and the coordinated ammonia group stabilizes the structure of the platinum(II) complex. Thus, it is suggested that the orotate ligand in the title complex is more inert to the substitution reactions than the chloride ligands in cisplatin. 相似文献
5.
Marlies Gerber 《Geometriae Dedicata》2003,98(1):123-160
We establish some criteria for the existence or nonexistence of focal points near closed geodesics on surfaces. These criteria are in terms of the curvature of the manifold along the closed geodesic and the average values of the partial derivatives of the curvature in the direction perpendicular to the geodesic. Our criteria lead to a new family of examples of surfaces with no focal points. We also show that if S is a compact surface with no focal points and an inequality relating the curvature of the surface to the curvature of the horocycles holds, then the horocycles (considered as curves in S) are uniformly C
2+Lipschitz. 相似文献
6.
7.
Stanis aw Urban Maria Massalska-Arod
Albert Wü rflinger Roman D browski 《Liquid crystals》2003,30(3):313-318
The pressure-temperature ( p - T ) phase diagrams for four smectogenic members of the 4'-alkyl-4-cyanobiphenyl homologous series ( n CB, n =9, 10, 11 and 12) over the temperature range 320-410 K and pressure range 0.1-300 MPa (3 kbar) were constructed using DTA. At 1 atm 9CB exhibits nematic and smectic A d phases, while the other members show only the smectic A d phase. However, at elevated pressures the clearing line splits in the case of 10CB and 11CB which indicates the induction of a nematic phase. It was found that the triple point, where the isotropic, nematic and smectic phases coexist, is strongly shifted to higher pressures with increasing chain length. This was interpreted as being caused by a loss of the rod-like shape of the molecules containing longer alkyl tails which explore a range of conformations. The slope of the clearing line, d T /d p , depends strongly on the length of the alkyl chain for the n CB series, but does not show a step-wise change between the nematogenic and smectogenic members. 相似文献
8.
Roman N. Karasev 《Discrete and Computational Geometry》2005,34(1):25-45
Three theorems of this paper generalize previous results of the author
on conjectures of A. Bezdek and V.V. Proizvolov. They show the existence of mappings from a given point set to the set of facets of a given polytope that satistfy some special conditions. Developing the same technique, some results on convex polytope partitions are presented, two of them dealing with partitions with prescribed measures of parts. Then we prove a corollary on the existence of a possibly nonconvex polytope with a given set of vertices, containing given points in its interior. We also consider problems of the following type: find an assignment of vectors from a given set to the parts of a given convex partition of ℝn so that the shifts of the parts by their corresponding vectors either do not intersect by interior points or cover ℝn 相似文献
9.
Alexei A. Belik Andrei V. Mironov Roman V. Shpanchenko Eiji Takayama‐Muromachi 《Acta Crystallographica. Section C, Structural Chemistry》2007,63(7):i37-i39
The crystal structure of a new high‐pressure modification of cadmium divanadium hexaoxide, CdV2O6, was refined from X‐ray single‐crystal data. It contains zigzag chains of edge‐sharing VO6 octahedra. Octahedra in adjacent chains share corners and form corrugated layers. Octahedrally coordinated Cd atoms, which lie on twofold axes, are situated between the layers. The columbite‐like structure results in a strong distortion of the CdO6 octahedra which may be stabilized only at high pressure. 相似文献
10.