全文获取类型
收费全文 | 48785篇 |
免费 | 1658篇 |
国内免费 | 18篇 |
专业分类
化学 | 31333篇 |
晶体学 | 232篇 |
力学 | 1564篇 |
数学 | 7473篇 |
物理学 | 9859篇 |
出版年
2023年 | 375篇 |
2022年 | 294篇 |
2021年 | 496篇 |
2020年 | 670篇 |
2019年 | 632篇 |
2018年 | 1000篇 |
2017年 | 812篇 |
2016年 | 1519篇 |
2015年 | 1394篇 |
2014年 | 1394篇 |
2013年 | 2879篇 |
2012年 | 3376篇 |
2011年 | 3437篇 |
2010年 | 2060篇 |
2009年 | 1686篇 |
2008年 | 2952篇 |
2007年 | 2968篇 |
2006年 | 2614篇 |
2005年 | 2538篇 |
2004年 | 2182篇 |
2003年 | 1762篇 |
2002年 | 1534篇 |
2001年 | 905篇 |
2000年 | 759篇 |
1999年 | 538篇 |
1998年 | 405篇 |
1997年 | 364篇 |
1996年 | 466篇 |
1995年 | 294篇 |
1994年 | 347篇 |
1993年 | 351篇 |
1992年 | 377篇 |
1991年 | 326篇 |
1990年 | 312篇 |
1989年 | 307篇 |
1988年 | 248篇 |
1987年 | 250篇 |
1986年 | 219篇 |
1985年 | 378篇 |
1984年 | 346篇 |
1983年 | 236篇 |
1982年 | 326篇 |
1981年 | 312篇 |
1980年 | 273篇 |
1979年 | 267篇 |
1978年 | 249篇 |
1976年 | 263篇 |
1975年 | 239篇 |
1974年 | 247篇 |
1973年 | 238篇 |
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
941.
A b-colouring of a graph is a colouring of its vertices such that every colour class contains a vertex that has a neighbour in all other classes. The b-chromatic number of a graph is the largest integer such that the graph has a -colouring with colours. We show here how to compute in polynomial time the -chromatic number of an outerplanar graph of girth at least . This generalizes the seminal result of Irving and Manlove on trees. 相似文献
942.
Shaul K. Bar-Lev Daoud Bshouty Peter Enis Gérard Letac I-Li Lu Donald Richards 《Journal of Theoretical Probability》1994,7(4):883-929
A natural exponential family (NEF)F in ? n ,n>1, is said to be diagonal if there existn functions,a 1,...,a n , on some intervals of ?, such that the covariance matrixV F (m) ofF has diagonal (a 1(m 1),...,a n (m n )), for allm=(m 1,...,m n ) in the mean domain ofF. The familyF is also said to be irreducible if it is not the product of two independent NEFs in ? k and ? n-k , for somek=1,...,n?1. This paper shows that there are only six types of irreducible diagonal NEFs in ? n , that we call normal, Poisson, multinomial, negative multinomial, gamma, and hybrid. These types, with the exception of the latter two, correspond to distributions well established in the literature. This study is motivated by the following question: IfF is an NEF in ? n , under what conditions is its projectionp(F) in ? k , underp(x 1,...,x n )∶=(x 1,...,x k ),k=1,...,n?1, still an NEF in ? k ? The answer turns out to be rather predictable. It is the case if, and only if, the principalk×k submatrix ofV F (m 1,...,m n ) does not depend on (m k+1,...,m n ). 相似文献
943.
Iulian Preda Leonardo Soriano Daniel Díaz‐Fernández Guillermo Domínguez‐Cañizares Alejandro Gutiérrez Germán R. Castro Jesús Chaboy 《Journal of synchrotron radiation》2013,20(4):635-640
This work reports an X‐ray absorption near‐edge structure (XANES) spectroscopy study at the Ni K‐edge in the early stages of growth of NiO on non‐ordered SiO2, Al2O3 and MgO thin films substrates. Two different coverages of NiO on the substrates have been studied. The analysis of the XANES region shows that for high coverages (80 Eq‐ML) the spectra are similar to that of bulk NiO, being identical for all substrates. In contrast, for low coverages (1 Eq‐ML) the spectra differ from that of large coverages indicating that the local order around Ni is limited to the first two coordination shells. In addition, the results also suggest the formation of cross‐linking bonds Ni—O—M (M = Si, Al, Mg) at the interface. 相似文献
944.
Ultra‐thin thermally grown SiO2 and atomic‐layer‐deposited (ALD) Al2O3 films are trialled as passivating dielectrics for metal–insulator–semiconductor (MIS) type contacts on top of phosphorus diffused regions applicable to high efficiency silicon solar cells. An investigation of the optimum insulator thickness in terms of contact recombination factor J0_cont and contact resistivity ρc is undertaken on 85 Ω/□ and 103 Ω/□ diffusions. An optimum ALD Al2O3 thickness of ~22 Å produces a J0_cont of ~300 fAcm–2 whilst maintaining a ρc lower than 1 mΩ cm2 for the 103 Ω/□ diffusion. This has the potential to improve the open‐circuit voltage by a maximum 15 mV. The thermally grown SiO2 fails to achieve equivalently low J0_cont values but exhibits greater thermal stability, resulting in slight improvements in ρc when annealed for 10 minutes at 300 °C without significant changes in J0_cont. The after‐anneal J0_cont reaches ~600 fAcm–2 with a ρc of ~2.5 mΩ cm2 for the 85 Ω/□ diffusion amounting to a maximum gain in open‐circuit voltage of 6 mV. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
945.
Giuseppe Savaré 《Numerische Mathematik》1993,65(1):319-335
Summary We study the approximation of linear parabolic Cauchy problems by means of Galerkin methods in space andA -stable multistep schemes of arbitrary order in time. The error is evaluated in the norm ofL
t
2
(H
x
1
) L
t
(L
x
2
). 相似文献
946.
Validation and generalization of DEA and its uses 总被引:3,自引:0,他引:3
R. D. Banker W. W. Cooper E. Grifell-Tajté Jesús T. Pastor Paul W. Wilson Eduardo Ley C. A. K. Lovell 《TOP》1994,2(2):249-314
Summary Rapidly expanding uses of DEA have been accompanied by developments which have enhanced its power and enlarged its utility
for additional applications. Developments covered in the present paper include simulation studies comparing DEA with competing
forms of statistical regressions. Other studies covered show how these two approaches can be combined in complementary fashion.
Another part of this paper deals with Chance Constrained Programming formulations which incorporate probabilistic elements
into DEA. Included also are discussions of statistical characterizations with accompanying tests of statistical significance
for DEA efficiency evaluations. This paper concludes with uses of DEA in “discovery processes”-processes that need strengthening
(and encouragemnt) in contemporary social science and management science research. Suggestions are made for additional research
on further developments which extend to uses of DEA to provide new approaches in economics (including econometrics), management
and psychology and an Appendix introduces new or recently developed efficiency measures for use in DEA.
Invited by J. T. Pastor 相似文献
947.
948.
François Métayer 《Archive for Mathematical Logic》1994,33(3):169-188
This work defines homology groups for proof-structures in multiplicative linear logic (see [Gir1], [Gir2], [Dan]). We will show that these groups characterize proof-nets among arbitrary proof-structures, thus obtaining a new correctness criterion and of course a new polynomial algorithm for testing correctness. This homology also bears information on sequentialization. An unexpected geometrical interpretation of the linear connectives is given in the last section. This paper exclusively focuses onabstract proof-structures, i.e. paired-graphs. The relation with actual proofs is investigated in [Gir1], [Gir2], [Dan], [Ret] and [Tro]. 相似文献
949.
We give asymptotic upper and lower bounds for the diameter of almost everyr-regular graph onn vertices (n → ∞). 相似文献
950.
Béla Bollobás 《Combinatorica》1982,2(3):223-228
We consider random graphs withn labelled vertices in which edges are chosen independently and with probabilityc/n. We prove that almost every random graph of this kind contains a path of length ≧(1 −α(c))n where α(c) is an exponentially decreasing function ofc.
Dedicated to Tibor Gallai on his seventieth birthday 相似文献