全文获取类型
收费全文 | 86660篇 |
免费 | 15526篇 |
国内免费 | 5232篇 |
专业分类
化学 | 84140篇 |
晶体学 | 1061篇 |
力学 | 3028篇 |
综合类 | 177篇 |
数学 | 6562篇 |
物理学 | 12450篇 |
出版年
2024年 | 65篇 |
2023年 | 383篇 |
2022年 | 1007篇 |
2021年 | 1262篇 |
2020年 | 2296篇 |
2019年 | 3600篇 |
2018年 | 3262篇 |
2017年 | 3651篇 |
2016年 | 4587篇 |
2015年 | 6501篇 |
2014年 | 6376篇 |
2013年 | 8783篇 |
2012年 | 6879篇 |
2011年 | 6731篇 |
2010年 | 5550篇 |
2009年 | 5470篇 |
2008年 | 5822篇 |
2007年 | 5269篇 |
2006年 | 4806篇 |
2005年 | 4559篇 |
2004年 | 3830篇 |
2003年 | 3488篇 |
2002年 | 4545篇 |
2001年 | 2190篇 |
2000年 | 2010篇 |
1999年 | 1192篇 |
1998年 | 562篇 |
1997年 | 473篇 |
1996年 | 378篇 |
1995年 | 313篇 |
1994年 | 257篇 |
1993年 | 180篇 |
1992年 | 175篇 |
1991年 | 143篇 |
1990年 | 102篇 |
1989年 | 106篇 |
1988年 | 82篇 |
1987年 | 68篇 |
1986年 | 93篇 |
1985年 | 69篇 |
1984年 | 39篇 |
1983年 | 26篇 |
1982年 | 37篇 |
1981年 | 25篇 |
1980年 | 22篇 |
1979年 | 23篇 |
1977年 | 13篇 |
1976年 | 18篇 |
1973年 | 18篇 |
1972年 | 13篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
121.
Dalila B. M. M. Fontes Eleni Hadjiconstantinou Nicos Christofides 《Journal of Global Optimization》2006,34(1):97-125
In this paper we obtain Lower Bounds (LBs) to concave cost network flow problems. The LBs are derived from state space relaxations
of a dynamic programming formulation, which involve the use of non-injective mapping functions guaranteing a reduction on
the cardinality of the state space. The general state space relaxation procedure is extended to address problems involving
transitions that go across several stages, as is the case of network flow problems. Applications for these LBs include: estimation
of the quality of heuristic solutions; local search methods that use information of the LB solution structure to find initial
solutions to restart the search (Fontes et al., 2003, Networks, 41, 221–228); and branch-and-bound (BB) methods having as
a bounding procedure a modified version of the LB algorithm developed here, (see Fontes et al., 2005a). These LBs are iteratively
improved by penalizing, in a Lagrangian fashion, customers not exactly satisfied or by performing state space modifications.
Both the penalties and the state space are updated by using the subgradient method. Additional constraints are developed to
improve further the LBs by reducing the searchable space. The computational results provided show that very good bounds can
be obtained for concave cost network flow problems, particularly for fixed-charge problems. 相似文献
122.
We investigate the conjecture that every circulant graph X admits a k‐isofactorization for every k dividing |E(X)|. We obtain partial results with an emphasis on small values of k. © 2006 Wiley Periodicals, Inc. J Combin Designs 14: 406–414, 2006 相似文献
123.
124.
D.H. Delphenich 《Annalen der Physik》2006,15(3):216-246
Some concepts of real and complex projective geometry are applied to the fundamental physical notions that relate to Minkowski space and the Lorentz group. In particular, it is shown that the transition from an infinite speed of propagation for light waves to a finite one entails the replacement of a hyperplane at infinity with a light cone and the replacement of an affine hyperplane – or rest space – with a proper time hyperboloid. The transition from the metric theory of electromagnetism to the pre‐metric theory is discussed in the context of complex projective geometry, and ultimately, it is proposed that the geometrical issues are more general than electromagnetism, namely, they pertain to the transition from point mechanics to wave mechanics. 相似文献
125.
Mehdi Dehghan 《Numerical Methods for Partial Differential Equations》2002,18(2):193-202
Developement of numerical methods for obtaining approximate solutions to the three dimensional diffusion equation with an integral condition will be carried out. The numerical techniques discussed are based on the fully explicit (1,7) finite difference technique and the fully implicit (7,1) finite difference method and the (7,7) Crank‐Nicolson type finite difference formula. The new developed methods are tested on a problem. Truncation error analysis and numerical examples are used to illustrate the accuracy of the new algorithms. The results of numerical testing show that the numerical methods based on the finite difference techniques discussed in the present article produce good results. © 2002 Wiley Periodicals, Inc. Numer Methods Partial Differential Eq 18: 193–202, 2002; DOI 10.1002/num.1040 相似文献
126.
A simple model consisting of two electronic levels and one vibrational mode (phonon) was theoretically studied. The electronic-vibrational interaction was linear in the vibrational displacement. The vibrational mode was taken in the harmonic approximation and was attached to the thermal bath formed by the ambient environment. The kinetic constants of the vibrational dissipation were of the second order in the vibrational-bath coupling and were taken in the Markovian limit. Although, depending on the parameters of the model, different curves of the non-radiative vibration-induced excited state decay were obtained, in general, three time intervals, corresponding to different physical behaviour, were found. In the short-time interval, small oscillations superimposed on the excited state decay were observed. They were determined by the vibrational frequency and influenced by electronic-vibrational coupling. In the middle-time interval, almost quasi-exponential decay was detected; its rate constant increased with stronger electronic-vibrational interaction and speed of vibrational relaxation. In the long-time interval, the decay was very slow and, under special conditions, even an asymptotic non-zero excited state population was observed. Its value increased with the strength of the off-diagonal electron-vibrational coupling. Links of the parameters of the model with quantum chemical terms were estimated. 相似文献
127.
Existence and Uniqueness of Endemic States for the Age-structured MSEIR Epidemic Model 总被引:1,自引:0,他引:1
Geni Gupur 《应用数学学报(英文版)》2002,18(3):441-454
The existence and uniqueness of positive steady states for the age-structured MSEIR epidemic model with age-dependent transmission coefficient is considered. Threshold results for the existence of endemic states are established; under certain conditions, uniqueness is also shown. 相似文献
128.
In this note we show how coverings induced by voltage assignments can be used to produce packings of disjoint copies of the Hoffman‐Singleton graph into K50. © 2003 Wiley Periodicals, Inc. J Combin Designs 11: 408–412, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/jcd.10049 相似文献
129.
Let {Xk}k?1 be a strictly stationary time series. For a strictly increasing sampling function g:?→? define Yk=Xg(k) as the deterministic sub‐sampled time series. In this paper, the extreme value theory of {Yk} is studied when Xk has representation as a moving average driven by heavy‐tailed innovations. Under mild conditions, convergence results for a sequence of point processes based on {Yk} are proved and extremal properties of the deterministic sub‐sampled time series are derived. In particular, we obtain the limiting distribution of the maximum and the corresponding extremal index. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
130.
A. Khrennikov 《Annalen der Physik》2003,12(10):575-585
We study the probabilistic consequences of the choice of the basic number field in the quantum formalism. We demonstrate that by choosing a number field for a linear space representation of quantum model it is possible to describe various interference phenomena. We analyse interference of probabilistic alternatives induced by real, complex, hyperbolic (Clifford) and p‐adic representations. 相似文献