全文获取类型
收费全文 | 13326篇 |
免费 | 1472篇 |
国内免费 | 1113篇 |
专业分类
化学 | 9485篇 |
晶体学 | 88篇 |
力学 | 52篇 |
综合类 | 53篇 |
数学 | 4889篇 |
物理学 | 1344篇 |
出版年
2024年 | 19篇 |
2023年 | 290篇 |
2022年 | 181篇 |
2021年 | 227篇 |
2020年 | 452篇 |
2019年 | 451篇 |
2018年 | 452篇 |
2017年 | 305篇 |
2016年 | 504篇 |
2015年 | 597篇 |
2014年 | 821篇 |
2013年 | 1217篇 |
2012年 | 766篇 |
2011年 | 902篇 |
2010年 | 779篇 |
2009年 | 848篇 |
2008年 | 911篇 |
2007年 | 867篇 |
2006年 | 706篇 |
2005年 | 514篇 |
2004年 | 481篇 |
2003年 | 476篇 |
2002年 | 409篇 |
2001年 | 285篇 |
2000年 | 265篇 |
1999年 | 267篇 |
1998年 | 241篇 |
1997年 | 157篇 |
1996年 | 168篇 |
1995年 | 211篇 |
1994年 | 170篇 |
1993年 | 123篇 |
1992年 | 129篇 |
1991年 | 64篇 |
1990年 | 62篇 |
1989年 | 54篇 |
1988年 | 53篇 |
1987年 | 31篇 |
1986年 | 32篇 |
1985年 | 56篇 |
1984年 | 46篇 |
1982年 | 53篇 |
1981年 | 40篇 |
1980年 | 30篇 |
1979年 | 36篇 |
1978年 | 34篇 |
1977年 | 25篇 |
1976年 | 29篇 |
1975年 | 19篇 |
1973年 | 19篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
151.
Yevhen Zelenyuk 《Topology and its Applications》2007,154(2):339-357
A topological space X is called almost maximal if it is without isolated points and for every x∈X, there are only finitely many ultrafilters on X converging to x. We associate with every countable regular homogeneous almost maximal space X a finite semigroup Ult(X) so that if X and Y are homeomorphic, Ult(X) and Ult(Y) are isomorphic. Semigroups Ult(X) are projectives in the category F of finite semigroups. These are bands decomposing into a certain chain of rectangular components. Under MA, for each projective S in F, there is a countable almost maximal topological group G with Ult(G) isomorphic to S. The existence of a countable almost maximal topological group cannot be established in ZFC. However, there are in ZFC countable regular homogeneous almost maximal spaces X with Ult(X) being a chain of idempotents. 相似文献
152.
Data in social and behavioral sciences are often hierarchically organized. Multilevel statistical methodology was developed to analyze such data. Most of the procedures for analyzing multilevel data are derived from maximum likelihood based on the normal distribution assumption. Standard errors for parameter estimates in these procedures are obtained from the corresponding information matrix. Because practical data typically contain heterogeneous marginal skewnesses and kurtoses, this paper studies how nonnormally distributed data affect the standard errors of parameter estimates in a two-level structural equation model. Specifically, we study how skewness and kurtosis in one level affect standard errors of parameter estimates within its level and outside its level. We also show that, parallel to asymptotic robustness theory in conventional factor analysis, conditions exist for asymptotic robustness of standard errors in a multilevel factor analysis model. 相似文献
153.
I. P. Romanova S. G. Fattakhov A. A. Nafikova I. I. Vandyukova R. R. Shagidullin N. M. Azancheev V. S. Reznik O. G. Sinyáshin 《Russian Chemical Bulletin》1998,47(9):1812-1819
The reactions of phosphine derivatives of diallyl isocyanurates with palladium(ii) dichloride lead to the formation of complexes, whose structure, composition, and stability depend on the length of the methylene
chain between the isocyanurate and diphenylphosphine fragments in the ligand. 1,3-Diallyl-5-[5′-(diphenylphosphino)pentyl
and 10′-(diphenyl-phosphino)decyl] isocyanurates with PdCl2 form monomeric L2PdCl2
trans-complexes in which P atoms of the ligands participate in coordination with the metal. 1,3-Diallyl-5-[2′-(diphenylphosphino)ethyl]
isocyanurate with PdCl2 forms a dimeric (LPdCl2)2 complex, which decomposes in a solution to the monomer including solvent molecule into the coordination sphere of the metal.
The reactions of 1,3-diallyl-5-[4′-(diphenylphosphino)butyl] isocyanurate and 1,3-diallyl-5-[6′-(diphenylphosphino)hexyl]
isocyanurate with PdCl2 give monomeric chelate LPdCl2 complexes in which one of the allyl groups of the isocyanurate cycle participates in coordination with the central ion along
with the phosphorus atom.
Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1859–1865, September, 1998. 相似文献
154.
Spin-lattice relaxation mechanisms in kaolinite have been reinvestigated by magic-angle spinning (MAS) of the sample. MAS is useful to distinguish between relaxation mechanisms: the direct relaxation rate caused by the dipole-dipole interaction with electron spins is not affected by spinning while the spin diffusion-assisted relaxation rate is. Spin diffusion plays a dominant role in 1H relaxation. MAS causes only a slight change in the relaxation behavior, because the dipolar coupling between 1H spins is strong. 29Si relaxes directly through the dipole-dipole interaction with electron spins under spinning conditions higher than 2 kHz. A spin diffusion effect has been clearly observed in the 29Si relaxation of relatively pure samples under static and slow-spinning conditions. 27Al relaxes through three mechanisms: phonon-coupled quadrupole interaction, spin diffusion and dipole-dipole interaction with electron spins. The first mechanism is dominant, while the last is negligibly small. Spin diffusion between 27Al spins is suppressed completely at a spinning rate of 2.5 kHz. We have analyzed the relaxation behavior theoretically and discussed quantitatively. Concentrations of paramagnetic impurities, electron spin-lattice relaxation times and spin diffusion rates have been estimated. 相似文献
155.
We study the large-time behavior and rate of convergence to the invariant measures of the processes dX
(t)=b(X)
(t)) dt + (X
(t)) dB(t). A crucial constant appears naturally in our study. Heuristically, when the time is of the order exp( – )/2 , the transition density has a good lower bound and when the process has run for about exp( – )/2, it is very close to the invariant measure. LetL
=(2/2) – U · be a second-order differential operator on d. Under suitable conditions,L
z has the discrete spectrum
- \lambda _2^\varepsilon ...and lim \varepsilon ^2 log \lambda _2^\varepsilon = - \Lambda \hfill \\ \varepsilon \to 0 \hfill \\ \end{gathered} $$
" align="middle" vspace="20%" border="0"> 相似文献
156.
Martin Schlichenmaier 《Letters in Mathematical Physics》1990,19(2):151-165
Krichever-Novikov algebras of meromorphic vector fields with more than two poles on higher genus Riemann surfaces are introduced. The structure of these algebras and their induced modules of forms of weight is studied. 相似文献
157.
D. Ugolini M. -H. Tuilier J. Eitle S. Schelz J. Q. Wang P. Oelhafen 《Applied Physics A: Materials Science & Processing》1990,51(6):526-536
Polymeric styrene films with thicknesses ranging from about one nm up to a few m have been deposited by means of a direct ion beam deposition (IBD) technique. The deposition energy, which can be chosen independently of the parameters which govern the plasma conditions, has been varied between few eV and 1000 eV. The correlation between the deposition parameters and the resulting film properties in terms of the electronic structure is discussed. The in situ characterization by electron spectroscopy has proved to be a very useful characterization method and ultraviolet photoelectron spectroscopy in particular revealed an extremely high sensitivity to structural differences in the deposited films. The polymeric films have also been characterized by scanning electron microscopy and optical spectroscopy. These techniques have also served to compare films prepared by the direct IBD technique with films obtained by standard rf plasma polymerization (RFPP) in a tubular reactor. Significant differences have been found which are dependent on the deposition parameters; these are discussed in detail.Department of Chemical Engineering, Beijing Institute of Technology, P.O. Box 327 Beijing, 100081 Beijing, P.R. China 相似文献
158.
J. Vukman 《Aequationes Mathematicae》1988,36(2-3):165-175
Summary In the first section of this paper we consider some functional equations which are closely connected to derivations (i.e. additive mappings with the propertyD(ab) = aD(b) + D(a)b) on Banach algebras. IfD is a derivation on some algebraA, then the equationD(a) = – aD(a
–1
)a holds for all invertible elementsa A. It seems natural to ask whether this functional equation characterizes derivations among all additive mappings. It is too much to expect an affirmative answer to this question in arbitrary algebras, since it may happen that even in normed algebras the group of all invertible elements contains only scalar multiples of the identity. We try to answer the question above in Banach algebras, since in Banach algebras invertible elements exist in abundance. In the second section of the paper we prove some results concerning representability of quadratic forms by bilinear forms. 相似文献
159.
Christos Faloutsos 《BIT Numerical Mathematics》1988,28(4):736-754
We design and analyze integrated ways of applying the signature file approach for text and attributes simultaneously. In traditional signature file methods, the records are stored sequentially in the main file; for every record, a hash-coded abstraction of it (record signature) is created and stored in the signature file (usually, sequentially). To resolve a query, the signature file is scanned; the signatures retrieved correspond to all the qualifying records, plus some false drops.Here, we extend some signature file methods, namely superimposed coding and disjoint coding, to handle text and attributes. We develop a mathematical model and derive formulas for the optimal choice of parameters. The proposed methods achieve significant performance improvements, because they can take advantage of the skewed distribution of the queries. Depending on the query frequencies, the false drop probability can be reduced 40–45 times ( 97% savings), for the same overhead.Also with the University of Maryland Institute for Advanced Computer Studies (U.M.I.A.C.S.). This research was sponsored partially by the National Science Foundation under the grant DCR-86-16833. 相似文献
160.
Summary As shown in preceding papers of the authors, the verification of anR-convergence order for sequences coupled by a system (1.1) of basic inequalities can be reduced to the positive solvability of system (3.3) of linear inequalities. Further, the bestR-order
implied by (1.1) is equal to the minimal spectral radius of certain matrices composed from the exponents occuring in (1.1). Now, these results are proven in a unified and essentially simpler way. Moreover, they are somewhat extended in order to facilitate applications to concrete methods. 相似文献
|