全文获取类型
收费全文 | 2705篇 |
免费 | 43篇 |
国内免费 | 9篇 |
专业分类
化学 | 1691篇 |
晶体学 | 18篇 |
力学 | 46篇 |
数学 | 426篇 |
物理学 | 576篇 |
出版年
2022年 | 19篇 |
2021年 | 35篇 |
2020年 | 25篇 |
2019年 | 29篇 |
2018年 | 22篇 |
2017年 | 22篇 |
2016年 | 41篇 |
2015年 | 39篇 |
2014年 | 38篇 |
2013年 | 132篇 |
2012年 | 106篇 |
2011年 | 129篇 |
2010年 | 75篇 |
2009年 | 57篇 |
2008年 | 125篇 |
2007年 | 133篇 |
2006年 | 146篇 |
2005年 | 158篇 |
2004年 | 131篇 |
2003年 | 103篇 |
2002年 | 84篇 |
2001年 | 40篇 |
2000年 | 39篇 |
1999年 | 31篇 |
1998年 | 25篇 |
1997年 | 29篇 |
1996年 | 37篇 |
1995年 | 33篇 |
1994年 | 29篇 |
1993年 | 33篇 |
1992年 | 37篇 |
1991年 | 31篇 |
1990年 | 29篇 |
1989年 | 22篇 |
1988年 | 29篇 |
1986年 | 21篇 |
1985年 | 53篇 |
1984年 | 44篇 |
1983年 | 34篇 |
1982年 | 46篇 |
1981年 | 46篇 |
1980年 | 48篇 |
1979年 | 34篇 |
1978年 | 38篇 |
1977年 | 32篇 |
1976年 | 27篇 |
1975年 | 30篇 |
1974年 | 28篇 |
1973年 | 19篇 |
1972年 | 17篇 |
排序方式: 共有2757条查询结果,搜索用时 15 毫秒
71.
Let {C
i}
0 be a sequence of independent and identically distributed random variables with vales in [0, 4]. Let {X
n}
0 be a sequence of random variables with values in [0, 1] defined recursively by X
n+1=C
n+1
X
n(1–X
n). It is shown here that: (i) E ln C
1<0X
n0 w.p.1. (ii) E ln C
1=0X
n0 in probability (iii) E ln C
1>0, E |ln(4–C
1)| such that (0, 1)=1 and is invariant for {X
n}. (iv) If there exits an invariant probability measure such that {0}=0, then E ln C
1>0 and – ln(1–x) (dx)=E ln C
1. (v) E ln C
1>0, E |ln(4–C
1)|< and {X
n} is Harris irreducible implies that the probability distribution of X
n converges in the Cesaro sense to a unique probability distribution on (0, 1) for all X
00. 相似文献
72.
Molality as a unit of measure for expressing 1H MRS brain metabolite concentrations in vivo 总被引:1,自引:0,他引:1
Knight-Scott J Haley AP Rossmiller SR Farace E Mai VM Christopher JM Manning CA Simnad VI Siragy HM 《Magnetic resonance imaging》2003,21(7):787-797
Absolute concentrations of cerebral metabolite in in vivo 1H magnetic resonance spectroscopy studies (1H-MRS) are widely reported in molar units as moles per liter of tissue, or in molal units as moles per kilogram of tissue. Such measurements require external referencing or assumptions as to local water content. To reduce the scan time, avoid assumptions that may be invalid under specific pathologies, and provide a universally accessible referencing procedure, we suggest that metabolite concentrations from 1H-MRS measurements in vivo be reported in molal units as moles per kilogram of tissue water. Using internal water referencing, a two-compartment water model, a simulated brain spectrum for peak identification, and a spectroscopic bi-exponential spin-spin relaxation segmentation technique, we measured the absolute concentrations for the four common 1H brain metabolites: choline (Cho), myo-inositol (mIno), phosphocreatine + creatine (Cr), and N-acetyl-aspartate (NAA), in the hippocampal region (n = 26) and along the Sylvian fissure (n = 61) of 35 healthy adults. A stimulated echo localization method (20 ms echo time, 10 ms mixing time, 4 s repetition time) yielded metabolite concentrations, uncorrected for metabolite relaxation or contributions from macromolecule resonances, that were expectantly higher than with molar literature values. Along the Sylvian fissure the average concentrations (coefficient of variation (CV)) in mmoles/kg of tissue water were 17.6 (12%) for NAA, 14.2 (9%) for Cr, 3.6 (13%) for Cho, and 13.2 (15%) for mIno. Respective values for the hippocampal region were 15.7 (20%), 14.7 (16%), 4.6 (19%), and 17.7 (26%). The concentrations of the two regions were significantly different (p = 0.001) for NAA, mIno, and Cho, a trend in agreement with previous studies. All gray matter Sylvian fissure CV values, except for NAA, were also in agreement with previous 1H-MRS gray matter studies. The reduced precision of the NAA concentration was attributed to overlapping signal contributions from glutamate and glutamine (Glx), suggesting that a detailed Glx model is critical for accurate quantitation of the NAA 2.02 ppm resonance. The reduced precision of the measurements in the hippocampal region was attributed to poor spectral resolution. 相似文献
73.
Cross ratios constitute an important tool in classical projective geometry. Using the theory of Tutte groups as discussed in [6] it will be shown in this note that the concept of cross ratios extends naturally to combinatorial geometries or matroids. From a thorough study of these cross ratios which, among other observations, includes a new matroid theoretic version and proof of the Pappos theorem, it will be deduced that for any projective space M=
n
(K) of dimension n2 of M over some skewfield K the inner Tutte group is isomorphic to the commutator factor group K
*/[K
*, K
*] of K
*K{0}. This shows not only that in case M=
n
(K) our matroidal cross ratios are nothing but the classical ones. It can also be used to correlate orientations of the matroid M=
n
(K) with the orderings of K. And it implies that Dieudonné's (non-commutative) determinants which, by Dieudonné's definition, take their values in K
*/[K
*, K
*] as well, can be viewed as a special case of a determinant construction which works for just every combinatorial geometry.Research supported by the DFG (Deutsche Forschungsgemeinschaft). 相似文献
74.
Aubert B Barate R Boutigny D Gaillard JM Hicheur A Karyotakis Y Lees JP Robbe P Tisserand V Zghiche A Palano A Pompili A Chen JC Qi ND Rong G Wang P Zhu YS Eigen G Ofte I Stugu B Abrams GS Borgland AW Breon AB Brown DN Button-Shafer J Cahn RN Charles E Gill MS Gritsan AV Groysman Y Jacobsen RG Kadel RW Kadyk J Kerth LT Kolomensky YG Kral JF Kukartsev G LeClerc C Levi ME Lynch G Mir LM Oddone PJ Orimoto TJ Pripstein M Roe NA Romosan A Ronan MT Shelkov VG Telnov AV Wenzel WA Harrison TJ Hawkes CM 《Physical review letters》2003,91(2):021801
We present results for the branching fractions and charge asymmetries in B+/--->h(+/-)pi(0) (where h(+/-)=pi(+/-),K+/-) and a search for the decay B0-->pi(0)pi(0) using a sample of approximately 88 x 10(6) BBmacr; pairs collected by the BABAR detector at the PEP-II asymmetric-energy B Factory at SLAC. We measure B(B+/--->pi(+/-)pi(0))=(5.5(+1.0)(-0.9)+/-0.6)x10(-6), where the first error is statistical and the second is systematic. The B+/--->pi(+/-)pi(0) signal has a significance of 7.7sigma including systematic uncertainties. We simultaneously measure the K+/-pi(0) branching fraction to be B(B+/--->K+/-pi(0))=(12.8(+1.2)(-1.1)+/-1.0)x10(-6). The charge asymmetries are Api(+/-)(pi(0))=-0.03(+0.18)(-0.17)+/-0.02 and AK+/-(pi(0))=-0.09+/-0.09+/-0.01. We place a 90% confidence-level upper limit on the branching fraction B(B0-->pi(0)pi(0)) of 3.6 x 10(-6). 相似文献
75.
76.
The problem is to find the best location in the plane of a minisum annulus with fixed width using a partial coverage distance model. Using the concept of partial coverage distance, those demand points within the area of the annulus are served at no cost, while for ‘uncovered’ demand points there will be additional costs proportional to their distances to the annulus. The objective of the problem is to locate the annulus such that the sum of distances from the uncovered demand points to the annulus (covering area) is minimized. The distance is measured by the Euclidean norm. We discuss the case where the radius of the inner circle of the annulus is variable, and prove that at least two demand points must be on the boundary of any optimal annulus. An algorithm to solve the problem is derived based on this result. 相似文献
77.
An ab initio study of six carbon-to-carbon identity proton transfers is reported. They refer to the benzenium ion/benzene (C6H7(+)/C6H6), the 2,4-cyclopentadiene/cyclopentadienyl anion (C5H6/C5H5(-)), and the cyclobutenyl cation/cyclobutadiene (C4H5(+)/C4H4) systems and their respective noncyclic reference systems, that is, [structure: see text], [structure: see text] and [structure: see text]. For the aromatic C6H7(+)/C6H6 and C5H6/C5H5(-) systems, geometric parameters and aromaticity indices indicate that the transition states are highly aromatic. The proton-transfer barriers in these systems are quite low, which is consistent with a disproportionately high degree of transition-state aromaticity. For the antiaromatic C4H5(+)/C4H4 system, the geometric parameters and aromaticity indices indicate a rather small degree of antiaromaticity of the transition state. However, the proton-transfer barrier is higher than expected for a transition state with a low antiaromaticity. This implies that another factor contributes to the barrier; it is suggested that this factor is angle and torsional strain in the transition state. The question whether charge delocalization at the transition state might correlate with the development of aromaticity was also examined. No such correlation was found, that is, charge delocalization lags behind proton transfer as is commonly observed in nonaromatic systems involving pi-acceptor groups. 相似文献
78.
Ren Amstutz W. Bernd Schweizer Dieter Seebach Jack D. Dunitz 《Helvetica chimica acta》1981,64(8):2617-2621
The structures of the crystalline lithium enolates derived from 3, 3-dimethyl-2-butanone and from cyclopentanone have been determined by X-ray analysis. Both structures are tetrameric aggregates based on a Li4O4-cube, each Li+-ion being additionally bonded to the O-atom of a tetrahydrofuran solvent molecule. 相似文献
79.
A tetra L-prolinylmethyl derivative of a tetra-sulfonated calix[4]resorcarene (1) is an effective chiral NMR solvating agent for water-soluble compounds with phenyl, pyridyl, bicyclic aromatic, or indole rings. These aromatic compounds form host-guest complexes with the calix[4]resorcarene in water. Complexation of substrates with the calix[4]resorcarene is likely promoted by hydrophobic effects, and bicyclic substrates have association constants with the calix[4]resorcarene larger than those of similar phenyl-containing compounds. Aromatic resonances of the substrates show substantial upfield shifts because of shielding from the aromatic rings of the calix[4]resorcarene, and several resonances in the 1H NMR spectra typically exhibit enantiomeric discrimination. The extent of enantiomeric discrimination depends in part on interactions of the substituent groups of the substrates with the prolinylmethyl groups of the calix[4]resorcarene. The effectiveness of a calix[4]resorcarene prepared from N-methyl-L-alanine (2) as a chiral NMR discriminating agent is compared to the L-prolinylmethyl derivative. 相似文献
80.
Triangles and tetrahedra: metal directed self-assembly of metallo-supramolecular structures incorporating bis-beta-diketonato ligands 总被引:1,自引:0,他引:1
Clegg JK Lindoy LF Moubaraki B Murray KS McMurtrie JC 《Dalton transactions (Cambridge, England : 2003)》2004,(16):2417-2423
The interaction of six aryl-linked bis-beta-diketones, including a new naphthylene linked species, with copper(II), iron(III) and, in one instance gallium(III), has been investigated with the aim of obtaining metallo-supramolecular assemblies exhibiting different geometries. New examples of two assembly types incorporating the above bis-beta-diketones (L) were generated. The first type is represented by a range of molecular triangles of formula [Cu(3)(L-H(2))(3)](solvent)(n) while the second is given by a corresponding selection of less-common neutral molecular tetrahedra of formula [Fe(4)(L-H(2))(6)](solvent)(n) as well as [Ga(4)(L-H(2))(6)].8.5THF.0.5H(2)O; an example of each type has been characterised by X-ray crystallography. A magnetochemical investigation of [Fe(4)(-H(2))(6)].6THF is reported. The susceptibility is Curie like and consistent with very weak coupling occurring between the iron(III) d(5)(high spin) centres. The X-ray structures of two trinuclear copper(II) as well as a tetranuclear iron(III) and a tetranuclear gallium(III) assembly confirm their discrete triangular and tetrahedral geometries, respectively. The structure of the gallium(III) species is closely related to that of the corresponding iron(III) species. The tetrahedral structures provide rare examples of such assemblies encapsulating guest solvent molecules--in each case tetrahydrofuran is incorporated in the central cavity. 相似文献