全文获取类型
收费全文 | 204594篇 |
免费 | 2959篇 |
国内免费 | 1350篇 |
专业分类
化学 | 105974篇 |
晶体学 | 2776篇 |
力学 | 9243篇 |
综合类 | 59篇 |
数学 | 27466篇 |
物理学 | 63385篇 |
出版年
2021年 | 1259篇 |
2020年 | 1408篇 |
2019年 | 1364篇 |
2018年 | 2500篇 |
2017年 | 2750篇 |
2016年 | 3285篇 |
2015年 | 2736篇 |
2014年 | 3224篇 |
2013年 | 8762篇 |
2012年 | 9661篇 |
2011年 | 10627篇 |
2010年 | 6841篇 |
2009年 | 6776篇 |
2008年 | 8230篇 |
2007年 | 8228篇 |
2006年 | 7927篇 |
2005年 | 11062篇 |
2004年 | 9817篇 |
2003年 | 7433篇 |
2002年 | 5582篇 |
2001年 | 6386篇 |
2000年 | 4734篇 |
1999年 | 3720篇 |
1998年 | 2789篇 |
1997年 | 2745篇 |
1996年 | 2692篇 |
1995年 | 2369篇 |
1994年 | 2273篇 |
1993年 | 2102篇 |
1992年 | 2534篇 |
1991年 | 2475篇 |
1990年 | 2173篇 |
1989年 | 2122篇 |
1988年 | 2131篇 |
1987年 | 2032篇 |
1986年 | 1923篇 |
1985年 | 2831篇 |
1984年 | 2811篇 |
1983年 | 2242篇 |
1982年 | 2435篇 |
1981年 | 2303篇 |
1980年 | 2255篇 |
1979年 | 2234篇 |
1978年 | 2287篇 |
1977年 | 2230篇 |
1976年 | 2228篇 |
1975年 | 2163篇 |
1974年 | 2078篇 |
1973年 | 2163篇 |
1972年 | 1241篇 |
排序方式: 共有10000条查询结果,搜索用时 22 毫秒
991.
992.
Preparation of Papers 总被引:8,自引:0,他引:8
We motivate the study of a vector variational inequality by a practical flow equilibrium problem on a network, namely a generalization of the well-known Wardrop equilibrium principle. Both weak and strong forms of the vector variational inequality are discussed and their relationships to a vector optimization problem are established under various convexity assumptions. 相似文献
993.
We classify smooth subvarieties of codimension twoX⊂P
n
, 4≤n≤5, which are arithmetically Cohen-Macaulay and of non general type. By the way we exhibit some classes of non extendable
subvarieties. Then we give new proofs of the classification of scrolls inP
4; finally we consider smooth surfaces of non general type inP
4 arising from rank three vector bundles.
Sunto Classifichiamo le sottovarietà lisce di codimensione dueX⊂P n , 4≤n≤5, aritmeticamente Cohen-Macaulay e non di tipo generale. Nel contempo descriviamo alcune classi di sottovarietà non estendibili. Diamo poi due nuove dimostrazioni della classificazione degli scrolls inP 4; infine consideriamo superfici lisce inP 4, non di tipo generale, associate a fibrati di rango tre.相似文献
994.
LetB1:
n×
N1→
m1,B2:
n×
N2→
m2andQ:
m2→
m1be bilinear forms which are related as follows: ifμandνsatisfyB1(ξ, μ)=0 andB2(ξ, ν)=0 for someξ≠0, thenμτQν=0. Supposep−1+q−1=1. Coifman, Lions, Meyer and Semmes proved that, ifuLp(
n) andvLq(
n), and the first order systemsB1(D, u)=0,B2(D, v)=0 hold, thenuτQvbelongs to the Hardy spaceH1(
n), provided that both (i)p=q=2, and (ii) the ranks of the linear mapsBj(ξ, ·) :
Nj→
m1are constant. We apply the theory of paracommutators to show that this result remains valid when only one of the hypotheses (i), (ii) is postulated. The removal of the constant-rank condition whenp=q=2 involves the use of a deep result of Lojasiewicz from singularity theory. 相似文献
995.
996.
The constrained optimization problem with a quadratic cost functional and two quadratic equality constraints has been studied by Bar-on and Grasse, with positive-definite matrix in the objective. In this note, we shall relax the matrix in the objective to be positive semidefinite. A necessary and sufficient condition to characterize a local optimal solution to be global is established. Also, a perturbation scheme is proposed to solve this generalized problem. 相似文献
997.
Summary Th(IV) was quantitatively extracted from 1 . 10-3M HNO3 using 1 . 10-3M Cyanex302 in xylene and was stripped from the organic phase with 5M HCl. The effect of different parameters affecting the
extraction was systematically studied to achieve optimum conditions for the extraction of thorium. Based on the data some
separations of thorium from binary and complex mixtures and its recovery from monazite sand were achieved. The method is reproducible
with a relative standard deviation of 0.4%. 相似文献
998.
We study the spectrum and eigenmodes of the QCD Dirac operator in a gauge background given by an instanton liquid model (ILM) at temperatures around the chiral phase transition. Generically we find the Dirac eigenvectors become more localized as the temperature is increased. At the chiral phase transition, both the low lying eigenmodes and the spectrum of the QCD Dirac operator undergo a transition to localization similar to the one observed in a disordered conductor. This suggests that Anderson localization is the fundamental mechanism driving the chiral phase transition. We also find an additional temperature dependent mobility edge (separating delocalized from localized eigenstates) in the bulk of the spectrum which moves toward lower eigenvalues as the temperature is increased. In both regions, the origin and the bulk, the transition to localization exhibits features of a 3D Anderson transition including multifractal eigenstates and spectral properties that are well described by critical statistics. Similar results are obtained in both the quenched and the unquenched case though the critical temperature in the unquenched case is lower. Finally we argue that our findings are not in principle restricted to the ILM approximation and may also be found in lattice simulations. 相似文献
999.
Alexis C. Kaporis Lefteris M. Kirousis Yannis C. Stamatiou Malvina Vamvakari Michele Zito 《Discrete Applied Mathematics》2007,155(12):1525-1538
The problem of determining the unsatisfiability threshold for random 3-SAT formulas consists in determining the clause to variable ratio that marks the experimentally observed abrupt change from almost surely satisfiable formulas to almost surely unsatisfiable. Up to now, there have been rigorously established increasingly better lower and upper bounds to the actual threshold value. In this paper, we consider the problem of bounding the threshold value from above using methods that, we believe, are of interest on their own right. More specifically, we show how the method of local maximum satisfying truth assignments can be combined with results for the occupancy problem in schemes of random allocation of balls into bins in order to achieve an upper bound for the unsatisfiability threshold less than 4.571. In order to obtain this value, we establish a bound on the q-binomial coefficients (a generalization of the binomial coefficients). No such bound was previously known, despite the extensive literature on q-binomial coefficients. Finally, to prove our result we had to establish certain relations among the conditional probabilities of an event in various probabilistic models for random formulas. It turned out that these relations were considerably harder to prove than the corresponding ones for unconditional probabilities, which were previously known. 相似文献
1000.
N. I. Kavallaris A. A. Lacey C. V. Nikolopoulos D. E. Tzanetis 《Mathematical Methods in the Applied Sciences》2007,30(13):1507-1526
We estimate the blow‐up time for the reaction diffusion equation ut=Δu+ λf(u), for the radial symmetric case, where f is a positive, increasing and convex function growing fast enough at infinity. Here λ>λ*, where λ* is the ‘extremal’ (critical) value for λ, such that there exists an ‘extremal’ weak but not a classical steady‐state solution at λ=λ* with ∥w(?, λ)∥∞→∞ as 0<λ→λ*?. Estimates of the blow‐up time are obtained by using comparison methods. Also an asymptotic analysis is applied when f(s)=es, for λ?λ*?1, regarding the form of the solution during blow‐up and an asymptotic estimate of blow‐up time is obtained. Finally, some numerical results are also presented. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献