全文获取类型
收费全文 | 72133篇 |
免费 | 2381篇 |
国内免费 | 133篇 |
专业分类
化学 | 43665篇 |
晶体学 | 386篇 |
力学 | 1430篇 |
综合类 | 2篇 |
数学 | 12550篇 |
物理学 | 16614篇 |
出版年
2023年 | 407篇 |
2022年 | 458篇 |
2021年 | 814篇 |
2020年 | 1038篇 |
2019年 | 888篇 |
2018年 | 1440篇 |
2017年 | 1179篇 |
2016年 | 2132篇 |
2015年 | 1737篇 |
2014年 | 1907篇 |
2013年 | 4019篇 |
2012年 | 3658篇 |
2011年 | 4070篇 |
2010年 | 2309篇 |
2009年 | 2077篇 |
2008年 | 3705篇 |
2007年 | 3725篇 |
2006年 | 3334篇 |
2005年 | 3040篇 |
2004年 | 2530篇 |
2003年 | 2103篇 |
2002年 | 1929篇 |
2001年 | 1493篇 |
2000年 | 1374篇 |
1999年 | 1005篇 |
1998年 | 809篇 |
1997年 | 827篇 |
1996年 | 989篇 |
1995年 | 811篇 |
1994年 | 875篇 |
1993年 | 876篇 |
1992年 | 773篇 |
1991年 | 634篇 |
1990年 | 662篇 |
1989年 | 568篇 |
1988年 | 543篇 |
1987年 | 535篇 |
1986年 | 500篇 |
1985年 | 739篇 |
1984年 | 743篇 |
1983年 | 618篇 |
1982年 | 650篇 |
1981年 | 658篇 |
1980年 | 603篇 |
1979年 | 515篇 |
1978年 | 562篇 |
1977年 | 540篇 |
1976年 | 493篇 |
1975年 | 452篇 |
1973年 | 421篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
241.
On a recursive method for the estimation of unknown parameters of partially observed chaotic systems
We investigate a recently proposed method for on-line parameter estimation and synchronization in chaotic systems. This novel technique has been shown effective to estimate a single unknown parameter of a primary chaotic system with known functional form that is only partially observed through a scalar time series. It works by periodically updating the parameter of interest in a secondary system, with the same functional form as the primary one but no explicit coupling between their dynamic variables, in order to minimize a suitably defined cost function. In this paper, we review the basics of the method, and investigate its robustness and new extensions. In particular, we study the performance of the novel technique in the presence of noise (either observational, i.e., an additive contamination of the observed time series, or dynamical, i.e., a random perturbation of the system dynamics) and when there is a mismatch between the primary and secondary systems. Numerical results, including comparisons with other techniques, are presented. Finally, we investigate the extension of the original method to perform the estimation of two unknown parameters and illustrate its effectiveness by means of computer simulations. 相似文献
242.
Push-pull dithiafulvenes with reduced bond length alternation (BLA) and high optical nonlinearities have been prepared. The interplay between the proaromaticity of the donor and the structural and optical properties of these merocyanines is discussed. The donor ability of dithiafulvenes can reach that of ferrocene or dialkylaminophenyl groups. 相似文献
243.
244.
J. Fülöp 《Mathematical Methods of Operations Research》1991,35(1):1-13
This paper shows that any linear disjunctive program with a finite number of constraints can be transformed into an equivalent facial program. Based upon linear programming technique, a new, finite cutting plane method is presented for the facial programs.
Zusammenfassung Die Arbeit zeigt, daß jedes lineare disjunktive Optimierungsproblem mit endlich vielen Restriktionen in ein äquivalentes Fazetten-Problem transformiert werden kann. Auf der Grundlage von linearer Optimierungstechnik wird für das Fazetten-Problem ein neues, endliches Schnittebenenverfahren vorgestellt.相似文献
245.
Summary The influence of internal degrees of freedom on the behaviour of one-dimensional systems is discussed. For systems with half-filled
bands the coupling to internalviz. lattice coordinates decides whether Peierls distortion is caused by intramonomer coordinates or by a lattice coordinate.
Thereby the various intramonomer degrees of freedom act cooperatively. We show that there is a small regime of parameters
where both kinds of distortion exist simultaneously. For increasing temperature we find that distortions can also move from
the lattice coordinate to the intramonomer coordinate. 相似文献
246.
György Gát 《Analysis Mathematica》2006,32(1):25-48
Summary It is known that the Fejér means - with respect to the character system of the Walsh, and bounded Vilenkin groups - of an
integrable function converge to the function a.e. In this work we discuss analogous problems on the complete product of finite,
not necessarily Abelian groups with respect to the character system for functions that are constant on the conjugacy classes.
We find that the nonabelian case differs from the commutative case. We prove the a.e. convergence of the (C,1) means of the Fourier series of square integrable functions. We also prove the existence of a function f∊Lq for some q >1 such that sup |σn f | = + ∞ a.e. This is a sharp contrast between the Abelian and the nonabelian cases. 相似文献
247.
The objective of this paper is to investigate the role of the set of irrational numbers as the codomain of order-preserving
functions defined on topological totally preordered sets. We will show that although the set of irrational numbers does not
satisfy the Debreu property it is still nonetheless true that any lower (respectively, upper) semicontinuous total preorder
representable by a real-valued strictly isotone function (semicontinuous or not) also admits a representation by means of
a lower (respectively, upper) semicontinuous strictly isotone function that takes values in the set of irrational numbers.
These results are obtained by means of a direct construction. Moreover, they can be related to Cantor’s characterization of the real line to obtain much more general results on the semicontinuous
Debreu properties of a wide family of subsets of the real line.
相似文献
248.
Zbigniew Jaskólski 《Communications in Mathematical Physics》1991,139(2):353-375
Following the general scheme of the convariant path integral quantization of gauge systems, two alternative formulations of
the first quantized closed bosonic string in a position representation are presented. In both approaches the covariant path
integral representations of the propagator and of the higher order off-shell amplitudes are constructed. For a wide class
of gauges the explicit formulae for off-shell amplitudes are obtained. This paper is the continuation of our previous work
where the corresponding problems in the open string case were considered [20]. 相似文献
249.
We consider a retrial queue with a finite buffer of size N, with arrivals of ordinary units and of negative units (which cancel one ordinary unit), both assumed to be Markovian arrival
processes. The service requirements are of phase type. In addition, a PHL,N bulk service discipline is assumed. This means that the units are served in groups of size at least L, where 1≤ L≤ N. If at the completion of a service fewer than L units are present at the buffer, the server switches off and waits until the buffer length reaches the threshold L. Then it switches on and initiates service for such a group of units. On the contrary, if at the completion of a service
L or more units are present at the buffer, all units enter service as a group. Units arriving when the buffer is full are not
lost, but they join a group of unsatisfied units called “orbit”. Our interest is in the continuous-time Markov chain describing
the state of the queue at arbitrary times, which constitutes a level dependent quasi-birth-and-death process. We start by
analyzing a simplified version of our queueing model, which is amenable to numerical calculation and is based on spatially
homogeneous quasi-birth-and-death processes. This leads to modified matrix-geometric formulas that reveal the basic qualitative
properties of our algorithmic approach for computing performance measures.
AMS Subject Classification: Primary 60K25 Secondary 68M20 90B22. 相似文献
250.