全文获取类型
收费全文 | 4934篇 |
免费 | 723篇 |
国内免费 | 484篇 |
专业分类
化学 | 1284篇 |
晶体学 | 70篇 |
力学 | 767篇 |
综合类 | 104篇 |
数学 | 1661篇 |
物理学 | 2255篇 |
出版年
2024年 | 8篇 |
2023年 | 72篇 |
2022年 | 88篇 |
2021年 | 80篇 |
2020年 | 114篇 |
2019年 | 95篇 |
2018年 | 102篇 |
2017年 | 174篇 |
2016年 | 203篇 |
2015年 | 168篇 |
2014年 | 230篇 |
2013年 | 448篇 |
2012年 | 309篇 |
2011年 | 329篇 |
2010年 | 243篇 |
2009年 | 330篇 |
2008年 | 273篇 |
2007年 | 329篇 |
2006年 | 296篇 |
2005年 | 213篇 |
2004年 | 232篇 |
2003年 | 223篇 |
2002年 | 200篇 |
2001年 | 165篇 |
2000年 | 166篇 |
1999年 | 148篇 |
1998年 | 126篇 |
1997年 | 89篇 |
1996年 | 99篇 |
1995年 | 78篇 |
1994年 | 76篇 |
1993年 | 68篇 |
1992年 | 51篇 |
1991年 | 40篇 |
1990年 | 37篇 |
1989年 | 27篇 |
1988年 | 26篇 |
1987年 | 26篇 |
1986年 | 18篇 |
1985年 | 28篇 |
1984年 | 18篇 |
1983年 | 8篇 |
1982年 | 14篇 |
1981年 | 13篇 |
1980年 | 11篇 |
1979年 | 10篇 |
1978年 | 9篇 |
1977年 | 6篇 |
1976年 | 10篇 |
1975年 | 4篇 |
排序方式: 共有6141条查询结果,搜索用时 0 毫秒
11.
I. Peshko G. Galich M. Lopiychuk A. Khizhnyak V. Nakvasuk 《Optics & Laser Technology》1997,29(4):211-215
A laser system containing four CW Nd:YAG double-rod lasers, transforming optics and a fibre with input-output optics has been designed and investigated. It is shown that reduction of the beam parameter product of the non-Gaussian beams in the image space of the lens has made it possible to collect the emission of four lasers to the same fibre. Independent laser operation has provided the output power control in the range of 10–900 W with a stable light spot size on the input end of the fibre. 相似文献
12.
A highly sensitive method for infrared radiation detection based on thermal resonance in an active bolometer is set forth. An active bolometer is a self-oscillating system consisting of an IR-sensitive cell in a feedback circuit of an adjustable proportional controller. The analysis of an active bolometer autonomous (dark) dynamics reveals that with a generalized gain factor A variation the system evolves from relaxation type towards oscillating and self-oscillating type. When A=Ac, where Ac is a critical value of the generalized gain factor A, the steady state loses stability through self-excited thermal oscillations. The resonance in a system weakly perturbed by IR radiation modulated at self-oscillation frequency q0[1+exp(iωct)] is considered. It is shown that in a small precritical vicinity =(A−Ac)/Ac of the gain factor the amplitude of forced thermal oscillations is proportional to q0/Ac. The D* calculation reveals that the detection power of a passive (A=0) bolometer increases with feedback introduction by a factor of 1/||. The detection powers of feasible versions of an active bolometer are compared. 相似文献
13.
Summary We consider the problem of maximizing the discounted net profit of a firm which purchases a quantity of some product at a
given time and afterwards advertises and sells the product progressively. We distinguish among the three possibilities of
assuming the final time to be either fixed, or bounded, or free. In all cases, after stating the problem in the optimal control
theory framework, we prove the existence of an optimal solution and characterize it using the Maximum Principle necessary
conditions. Furthermore, we prove that the convexity of the purchase cost function is a sufficient condition for the uniqueness
of the optimal solution.
Partially supported by MURST. 相似文献
14.
Identification of nonlinear elliptic equations 总被引:1,自引:0,他引:1
An optimization theoretic approach and algorithm for the estimation of state-dependent coefficients in nonlinear elliptic equation is presented. It is based on a splitting method combined with convex analysis techniques. Convergence of the algorithm is established and numerical examples are included.The research of K. Kunisch was supported in part by a grant from the Bundesministerium für Wissenschaft und Forschung, Austria. 相似文献
15.
In this work two non-local problems for the parabolic-hyperbolic type equation with non-characteristic line of changing type
are considered. Unique solvability of these problems is proven. The uniqueness of the solution is proven by the method of
energy integrals and the existence is proven by the method of integral equations. 相似文献
16.
A. B. Vasil’eva A. A. Plotnikov 《Computational Mathematics and Mathematical Physics》2006,46(5):762-767
A singularly perturbed parabolic equation with a nonlinear right-hand side of a special form is examined. A numerical analytical study of such equations is performed. 相似文献
17.
18.
Djamel Meraghni Abdelhakim Necir 《Methodology and Computing in Applied Probability》2007,9(4):557-572
The characteristic exponent α of a Lévy-stable law S
α
(σ, β, μ) was thoroughly studied as the extreme value index of a heavy tailed distribution. For 1 < α < 2, Peng (Statist. Probab. Lett. 52: 255–264, 2001) has proposed, via the extreme value approach, an asymptotically normal estimator for the location parameter μ. In this paper, we derive by the same approach, an estimator for the scale parameter σ and we discuss its limiting behavior.
相似文献
19.
20.
H. P. Gunnlaugsson 《Hyperfine Interactions》2006,167(1-3):851-854
The general problem of finding a distribution in hyperfine interaction parameters from experimental Mössbauer spectra is outlined. Existing methods may lack flexibility to be easily applicable to simple problems. A line shape for hyperfine parameter distributions is given, which is based on linear segments in the probability function. This method is applied in the analysis of samples containing iron in a silicate glass. 相似文献