全文获取类型
收费全文 | 249498篇 |
免费 | 3913篇 |
国内免费 | 1558篇 |
专业分类
化学 | 142314篇 |
晶体学 | 3410篇 |
力学 | 9590篇 |
综合类 | 61篇 |
数学 | 29529篇 |
物理学 | 70065篇 |
出版年
2020年 | 2204篇 |
2019年 | 2336篇 |
2018年 | 2834篇 |
2017年 | 2873篇 |
2016年 | 4388篇 |
2015年 | 3073篇 |
2014年 | 4263篇 |
2013年 | 10425篇 |
2012年 | 9288篇 |
2011年 | 11441篇 |
2010年 | 7663篇 |
2009年 | 7363篇 |
2008年 | 10529篇 |
2007年 | 10673篇 |
2006年 | 10127篇 |
2005年 | 9339篇 |
2004年 | 8229篇 |
2003年 | 7266篇 |
2002年 | 7097篇 |
2001年 | 7055篇 |
2000年 | 5418篇 |
1999年 | 3978篇 |
1998年 | 3330篇 |
1997年 | 3332篇 |
1996年 | 3437篇 |
1995年 | 2912篇 |
1994年 | 2986篇 |
1993年 | 2815篇 |
1992年 | 3065篇 |
1991年 | 3107篇 |
1990年 | 2891篇 |
1989年 | 2787篇 |
1988年 | 2767篇 |
1987年 | 2672篇 |
1986年 | 2684篇 |
1985年 | 3605篇 |
1984年 | 3669篇 |
1983年 | 3024篇 |
1982年 | 3323篇 |
1981年 | 3112篇 |
1980年 | 2884篇 |
1979年 | 3049篇 |
1978年 | 3222篇 |
1977年 | 3237篇 |
1976年 | 3242篇 |
1975年 | 2981篇 |
1974年 | 3101篇 |
1973年 | 3148篇 |
1972年 | 2392篇 |
1971年 | 1913篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
161.
Afraimovich É. L. Vodyannikov V. V. Voyeikov S. V. Perevalova N. P. Yakovets A. F. 《Radiophysics and Quantum Electronics》2002,45(10):741-757
Using the COPHASE method and the GPS interferometry method for travelling ionospheric disturbances, we analyze in detail the spatio-temporal properties of travelling wave packets (TWP) of total electron content (TEC) disturbances. The analysis is performed on the example of a clearest TWP manifestation observed in California, USA, in October 18, 2001, using the GLOBDET technique, developed at the Institute of Solar-Terrestrial Physics of the Siberian Branch of RAS for global detection and monitoring of natural and technogenic ionospheric disturbances on the basis of TEC variations retrieved from the global network of GPS receivers. In the time domain, TWPs are quasi-periodic TEC oscillations of duration about 1 h, period of 10–20 min, and amplitude exceeding that of the background TEC fluctuations by at least one order of magnitude. The velocity and direction of TWP motion are similar to those of mid-latitude mesoscale travelling ionospheric disturbances, as obtained earlier from the analysis of phase parameters of HF radio signals and the signals of geostationary satellites and discrete space radio sources. 相似文献
162.
163.
David B. Wilson 《Random Structures and Algorithms》2002,21(2):182-195
There has been much recent interest in the satisfiability of random Boolean formulas. A random k‐SAT formula is the conjunction of m random clauses, each of which is the disjunction of k literals (a variable or its negation). It is known that when the number of variables n is large, there is a sharp transition from satisfiability to unsatisfiability; in the case of 2‐SAT this happens when m/n → 1, for 3‐SAT the critical ratio is thought to be m/n ≈ 4.2. The sharpness of this transition is characterized by a critical exponent, sometimes called ν = νk (the smaller the value of ν the sharper the transition). Experiments have suggested that ν3 = 1.5 ± 0.1. ν4 = 1.25 ± 0.05, ν5 = 1.1 ± 0.05, ν6 = 1.05 ± 0.05, and heuristics have suggested that νk → 1 as k → ∞. We give here a simple proof that each of these exponents is at least 2 (provided the exponent is well defined). This result holds for each of the three standard ensembles of random k‐SAT formulas: m clauses selected uniformly at random without replacement, m clauses selected uniformly at random with replacement, and each clause selected with probability p independent of the other clauses. We also obtain similar results for q‐colorability and the appearance of a q‐core in a random graph. © 2002 Wiley Periodicals, Inc. Random Struct. Alg., 21: 182–195, 2002 相似文献
164.
165.
166.
167.
L. T. Ashchepkov 《Computational Mathematics and Mathematical Physics》2006,46(7):1168-1175
A general nonlinear programming problem with interval functions is considered. Two reductions of this problem to the deterministic nonlinear programming problem are proposed, and illustrative examples are discussed. 相似文献
168.
A Modified Quasi-Newton Method for Structured Optimization with Partial Information on the Hessian 总被引:2,自引:0,他引:2
This paper develops a modified quasi-Newton method for structured unconstrained optimization with partial information on the
Hessian, based on a better approximation to the Hessian in current search direction. The new approximation is decided by both
function values and gradients at the last two iterations unlike the original one which only uses the gradients at the last
two iterations. The modified method owns local and superlinear convergence. Numerical experiments show that the proposed method
is encouraging comparing with the methods proposed in [4] for structured unconstrained optimization
Presented at the 6th International Conference on Optimization: Techniques and Applications, Ballarat, Australia, December
9–11, 2004 相似文献
169.
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
170.
C Falugi A Diaspro C Angelini M L Pedrotti M Raimondo M Robello 《Micron (Oxford, England : 1993)》2002,33(3):233-239
Confocal laser scanning microscopy (CLSM) was used to examine molecules related to the cholinergic neurotransmission system and detected at all the larval stages of Paracentrotus lividus, by histochemical and immunohistochemical methods. CLSM, providing spatial resolution of the cells located both at the larval surface and at depth, allows a complete mapping in a three-dimensional volumetric frame. At early larval stages acetylcholinesterase- as well as choline acetyltransferase-like molecules were found mainly in the gut wall cells, and along the ciliary bands of the arms, together with muscarinic acetylcholine receptors. At perimetamorphic stages, cholinergic molecules were present in the ciliate strands along the arms, in the larval body and in the rudiment. At metamorphosis, positivity to cholinergic molecules translocated to the juvenile, where a high frequency of mAChR- and ChAT-like positive cells was found. 相似文献