全文获取类型
收费全文 | 130865篇 |
免费 | 3730篇 |
国内免费 | 1220篇 |
专业分类
化学 | 59716篇 |
晶体学 | 1080篇 |
力学 | 8476篇 |
综合类 | 47篇 |
数学 | 39158篇 |
物理学 | 27338篇 |
出版年
2023年 | 524篇 |
2022年 | 490篇 |
2021年 | 748篇 |
2020年 | 991篇 |
2019年 | 943篇 |
2018年 | 11243篇 |
2017年 | 10898篇 |
2016年 | 7684篇 |
2015年 | 2571篇 |
2014年 | 2131篇 |
2013年 | 3802篇 |
2012年 | 7704篇 |
2011年 | 14102篇 |
2010年 | 7933篇 |
2009年 | 7944篇 |
2008年 | 9743篇 |
2007年 | 11730篇 |
2006年 | 3246篇 |
2005年 | 4147篇 |
2004年 | 3962篇 |
2003年 | 3894篇 |
2002年 | 2734篇 |
2001年 | 1345篇 |
2000年 | 1175篇 |
1999年 | 839篇 |
1998年 | 735篇 |
1997年 | 656篇 |
1996年 | 802篇 |
1995年 | 515篇 |
1994年 | 532篇 |
1993年 | 531篇 |
1992年 | 482篇 |
1991年 | 437篇 |
1990年 | 419篇 |
1989年 | 403篇 |
1988年 | 333篇 |
1987年 | 329篇 |
1986年 | 306篇 |
1985年 | 446篇 |
1984年 | 406篇 |
1983年 | 288篇 |
1982年 | 375篇 |
1981年 | 359篇 |
1980年 | 331篇 |
1979年 | 320篇 |
1978年 | 303篇 |
1976年 | 290篇 |
1975年 | 263篇 |
1974年 | 268篇 |
1973年 | 267篇 |
排序方式: 共有10000条查询结果,搜索用时 875 毫秒
991.
This paper addresses a kind of risk decision-making problem existing widely in public administration and business management, which is characterized by (1) occurrence probabilities of states of nature can be estimated by analysing historical observations, but historical observations of different objects are unhomogeneous, (2) the relation between observations and occurrence probabilities of states of nature are affected by some qualitative and quantitative indicators, (3) it is a real-time decision-making problem, that is, there are many decisions for different objects to be made in a limited time, (4) considering decision's execution, impact of resource constrains is an important issue in decision-making process. In this paper, we develop a rule-based approach to address the problem. In the proposed approach, a two-step clustering method is employed to classify objects into categories, and observations in each category can be approximately viewed as homogeneous. For objects in each category, occurrence probabilities of states of nature are estimated by logistic regression, and the decision rule is obtained through solving an optimization model, which is to minimize the total decision risks while satisfying resource constrains. Effect and efficacy of our approach are illustrated through its application to China's customs inspection decision. 相似文献
992.
Abraham Mehrez Zilla Sinuany-Stern Alan Stulman 《The Journal of the Operational Research Society》1986,37(10):971-977
In this note, we examine the algorithm of Drezner and Wesolowsky for locating a facility with maximin rectilinear distance to a given set of demand points. The concept of the closest-point constraint is introduced as a technique for increasing the efficiency of the algorithm. This potential increase in efficiency is demonstrated by an example. 相似文献
993.
J D Griffiths Z F Lawson J E Williams 《The Journal of the Operational Research Society》2006,57(12):1413-1424
Advances in recent treatments for HIV/AIDS patients have shown dramatic outcomes in extending the incubation period and AIDS survival time, while also providing significant improvements in the quality of patients' lives. A compartmental model is proposed to analyse the effects of the various treatment regimens which have been introduced. The results produced are in good agreement with routinely collected data relating to levels of HIV/AIDS incidence and prevalence in the UK homosexual population. Some parameter values within the model are obtained from surveys, census results, etc, but others are derived using a maximum likelihood estimation procedure. Finally, the model is used to project levels of incidence and prevalence over the next few years, and to investigate several possible scenarios. 相似文献
994.
J. Balogh D. Kaptás L. F. Kiss T. Kemény L. Bujdosó I. Vincze 《Hyperfine Interactions》2006,169(1-3):1343-1347
Magnetic multilayers of 57Fe with nominal thickness, T
nom, between 0.4 and 1.0 nm separated by 3.0 nm Al spacer layers were prepared by alternate deposition of the constituents in
high vacuum. The samples were investigated at 4.2 K in external magnetic field. A fraction of Fe atoms corresponding to about
0.3 nm equivalent Fe-thickness was found to mix into the Al spacer. The extremely strong magnetic anisotropy observed for
T
nom < 0.8 nm is attributed to Fe layers of approximately two atomic planes thick. The anisotropy decreases considerably after
the building up of the third Fe atomic layer starts at T
nom = 0.8 nm, but full saturation was not achieved even for T
nom = 1 nm and 3 T magnetic field applied perpendicularly to the sample plane. 相似文献
995.
996.
It has been described earlier that imaging measurements of laser-induced fluorescence (LIF) in flames can be calibrated to number densities with an integrated absorption measurement provided the integrated absorption is small. In this paper a method is presented that extends the technique to flames with substantial absorption, improves the number density determination and allows the experimental parameters to be chosen more freely. The method is based on an iterative computer procedure that reconstructs the 1-D spatially resolved absorption profile from laser measurements of the 1-D spatially resolved LIF and the integrated absorption of the laser beam. The technique is experimentally demonstrated by measurements of OH number densities in atmospheric flames. It is potentially a single-pulse method. Other applications of the iterative procedure are mentioned. 相似文献
997.
In this article, local optimality in multiobjective combinatorial optimization is used as a baseline for the design and analysis
of two iterative improvement algorithms. Both algorithms search in a neighborhood that is defined on a collection of sets
of feasible solutions and their acceptance criterion is based on outperformance relations. Proofs of the soundness and completeness
of these algorithms are given. 相似文献
998.
We prove the vanishing of the Koszul homology group H
μ(Kos(M)μ), where μ is the minimal number of generators of M. We give a counterexample that the Koszul complex of a module is not always
acyclic and show its relationship with the homology of commutative rings. 相似文献
999.
For about thirty years, time series models with time-dependent coefficients have sometimes been considered as an alternative
to models with constant coefficients or non-linear models. Analysis based on models with time-dependent models has long suffered
from the absence of an asymptotic theory except in very special cases. The purpose of this paper is to provide such a theory
without using a locally stationary spectral representation and time rescaling. We consider autoregressive-moving average (ARMA)
models with time-dependent coefficients and a heteroscedastic innovation process. The coefficients and the innovation variance
are deterministic functions of time which depend on a finite number of parameters. These parameters are estimated by maximising
the Gaussian likelihood function. Deriving conditions for consistency and asymptotic normality and obtaining the asymptotic
covariance matrix are done using some assumptions on the functions of time in order to attenuate non-stationarity, mild assumptions
for the distribution of the innovations, and also a kind of mixing condition. Theorems from the theory of martingales and
mixtingales are used. Some simulation results are given and both theoretical and practical examples are treated.
Received 2004; Final version 23 December 2004 相似文献
1000.
Anil Kumar Karn 《Positivity》2007,11(2):369-374
An error has been detected (and also corrected) in Theorem 2.8 of the paper entitled “Adjoining an Order Unit to a Matrix
Ordered Space” (Positivity, (2005)9: 207–223; DOI ). Accordingly, some of the results of the paper have been modified. Also, a notion of C*-matricially, Riesz normed spaces has been introduced. 相似文献