全文获取类型
收费全文 | 5414篇 |
免费 | 426篇 |
国内免费 | 298篇 |
专业分类
化学 | 1332篇 |
晶体学 | 16篇 |
力学 | 1438篇 |
综合类 | 17篇 |
数学 | 1324篇 |
物理学 | 2011篇 |
出版年
2024年 | 4篇 |
2023年 | 104篇 |
2022年 | 71篇 |
2021年 | 78篇 |
2020年 | 153篇 |
2019年 | 112篇 |
2018年 | 103篇 |
2017年 | 169篇 |
2016年 | 181篇 |
2015年 | 154篇 |
2014年 | 295篇 |
2013年 | 390篇 |
2012年 | 271篇 |
2011年 | 356篇 |
2010年 | 257篇 |
2009年 | 404篇 |
2008年 | 399篇 |
2007年 | 363篇 |
2006年 | 303篇 |
2005年 | 244篇 |
2004年 | 230篇 |
2003年 | 196篇 |
2002年 | 174篇 |
2001年 | 134篇 |
2000年 | 148篇 |
1999年 | 95篇 |
1998年 | 101篇 |
1997年 | 84篇 |
1996年 | 66篇 |
1995年 | 51篇 |
1994年 | 59篇 |
1993年 | 57篇 |
1992年 | 43篇 |
1991年 | 35篇 |
1990年 | 29篇 |
1989年 | 34篇 |
1988年 | 29篇 |
1987年 | 18篇 |
1986年 | 20篇 |
1985年 | 23篇 |
1984年 | 18篇 |
1983年 | 9篇 |
1982年 | 16篇 |
1981年 | 17篇 |
1980年 | 10篇 |
1979年 | 12篇 |
1978年 | 6篇 |
1977年 | 5篇 |
1976年 | 3篇 |
1971年 | 2篇 |
排序方式: 共有6138条查询结果,搜索用时 15 毫秒
41.
First order dynamic inclusions on time scales 总被引:1,自引:0,他引:1
F.Merdivenci Atici D.C. Biles 《Journal of Mathematical Analysis and Applications》2004,292(1):222-237
In this paper, we study existence of solutions of first order dynamic inclusions on time scales with general boundary conditions. Both the ∇-derivative and Δ-derivative cases are considered. Examples are presented to illustrate that the Δ-derivative case needs more restrictive assumptions. 相似文献
42.
The development of perturbations of the parameters of a dense gaseous envelope traveling with an acceleration driven by a difference in the pressures on either side is investigated numerically. Plane and axisymmetric time-dependent flows of a compressible medium are considered. The effect of both the density of the envelope and the form of the initial perturbations of its shape and motion on the mass cumulation in the compactions formed is studied. The evolutions of the perturbations of a layer and the surface of a contact discontinuity accelerated by an impinging plane shock wave are compared. 相似文献
43.
Most adhesives and binders, including bitumen for asphalt mixture production, are presently produced from petrochemicals after
the refining of crude oil. The fact that crude oil reserves are a finite resource means that in the future, it may become
necessary to produce these materials from alternative and probably renewable sources. Suitable resources of this kind may
include polysaccharides, plant oils and proteins. This paper deals with the synthesis of polymer binders from monomers that
could, in future, be derived from renewable resources. These binders consist of polyethyl acrylate (PEA) of different molecular
weight, polymethyl acrylate (PMA) and polybutyl acrylate (PBA), which were synthesised from ethyl acrylate, methyl acrylate
and butyl acrylate, respectively, by atom transfer radical polymerisation. The rheological properties of these binders were
determined by means of oscillatory testing using a dynamic shear rheometer and combinations of stress/strain, temperature
and frequency sweeps. The results indicate that PEA can be produced to have rheological properties similar to that of ‘soft’
100/150 penetration grade bitumen, PMA with similar rheological properties to that of ‘hard’ 10/20 penetration grade bitumen,
while PBA, due to its highly viscous nature and low dynamic moduli, cannot be used on its own as a binder. The synthetic polymers
were found to be thermo-rheologically simple, and the shift factors, used to produce the dynamic moduli master curves, were
found to fit an Arrhenius function. 相似文献
44.
调制不稳定性对级联放大光纤传输系统信噪比的影响 总被引:1,自引:0,他引:1
在考虑光纤损耗和级联放大器的情况下,推导了调制不稳定性的产生条件和增益的普适解析表达式,分析了调制不稳定性对信噪比的影响,给出了一个新的计算信噪比的表达式。 相似文献
45.
46.
Faison P. Gibson 《Computational & Mathematical Organization Theory》2007,13(1):39-61
An important aspect of learning is the ability to transfer knowledge to new contexts. However, in dynamic decision tasks, such as bargaining, firefighting, and process control, where decision makers must make repeated decisions under time pressure and outcome feedback may relate to any of a number of decisions, such transfer has proven elusive. This paper proposes a two-stage connectionist model which hypothesizes that decision makers learn to identify categories of evidence requiring similar decisions as they perform in dynamic environments. The model suggests conditions under which decision makers will be able to use this ability to help them in novel situations. These predictions are compared against those of a one-stage decision model that does not learn evidence categories, as is common in many current theories of repeated decision making. Both models' predictions are then tested against the performance of decision makers in an Internet bargaining task. Both models correctly predict aspects of decision makers' learning under different interventions. The two-stage model provides closer fits to decision maker performance in a new, related bargaining task and accounts for important features of higher-performing decision makers' learning. Although frequently omitted in recent accounts of repeated decision making, the processes of evidence category formation described by the two-stage model appear critical in understanding the extent to which decision makers learn from feedback in dynamic tasks.
Faison (Bud) Gibson is an Assistant Professor at College of Business, Eastern Michigan University. He has extensive experience developing and empirically testing models of decision behavior in dynamic decision environments. 相似文献
47.
48.
C. Ruyer-Quil P. Manneville 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,6(2):277-292
A new model of film flow down an inclined plane is derived by a method combining results of the classical long wavelength
expansion to a weighted-residuals technique. It can be expressed as a set of three coupled evolution equations for three slowly
varying fields, the thickness h, the flow-rate q, and a new variable that measures the departure of the wall shear from the shear predicted by a parabolic velocity profile. Results of a preliminary
study are in good agreement with theoretical asymptotic properties close to the instability threshold, laboratory experiments
beyond threshold and numerical simulations of the full Navier-Stokes equations.
Received: 16 April 1998 / Revised: 29 June 1998 / Accepted: 2 July 1998 相似文献
49.
Smooth Solutions to Optimal Investment Models with Stochastic Volatilities and Portfolio Constraints
Pham 《Applied Mathematics and Optimization》2002,46(1):55-78
Abstract. This paper deals with an extension of Merton's optimal investment problem to a multidimensional model with stochastic volatility
and portfolio constraints. The classical dynamic programming approach leads to a characterization of the value function as
a viscosity solution of the highly nonlinear associated Bellman equation. A logarithmic transformation expresses the value
function in terms of the solution to a semilinear parabolic equation with quadratic growth on the derivative term. Using a
stochastic control representation and some approximations, we prove the existence of a smooth solution to this semilinear
equation. An optimal portfolio is shown to exist, and is expressed in terms of the classical solution to this semilinear equation.
This reduction is useful for studying numerical schemes for both the value function and the optimal portfolio. We illustrate
our results with several examples of stochastic volatility models popular in the financial literature. 相似文献
50.
In this paper, we propose the treatment of complex reservoir operation problems via our newly developed tool of fuzzy criterion
decision processes. This novel approach has been shown to be a more flexible and useful analysis tool especially when it is
desirable to incorporate an expert’s knowledge into the decision models. Additionally, it has been demonstrated that this
form of decision models will usually result in an optimal solution, which guarantees the highest satisfactory degree. We provide
a practical exemplification procedure for the models presented as well as an application example. 相似文献