全文获取类型
收费全文 | 9580篇 |
免费 | 856篇 |
国内免费 | 855篇 |
专业分类
化学 | 7657篇 |
晶体学 | 165篇 |
力学 | 240篇 |
综合类 | 72篇 |
数学 | 1217篇 |
物理学 | 1940篇 |
出版年
2024年 | 37篇 |
2023年 | 114篇 |
2022年 | 333篇 |
2021年 | 376篇 |
2020年 | 469篇 |
2019年 | 337篇 |
2018年 | 318篇 |
2017年 | 268篇 |
2016年 | 351篇 |
2015年 | 321篇 |
2014年 | 340篇 |
2013年 | 565篇 |
2012年 | 332篇 |
2011年 | 336篇 |
2010年 | 256篇 |
2009年 | 326篇 |
2008年 | 342篇 |
2007年 | 375篇 |
2006年 | 320篇 |
2005年 | 430篇 |
2004年 | 607篇 |
2003年 | 523篇 |
2002年 | 420篇 |
2001年 | 338篇 |
2000年 | 300篇 |
1999年 | 216篇 |
1998年 | 189篇 |
1997年 | 183篇 |
1996年 | 235篇 |
1995年 | 207篇 |
1994年 | 236篇 |
1993年 | 173篇 |
1992年 | 176篇 |
1991年 | 97篇 |
1990年 | 87篇 |
1989年 | 73篇 |
1988年 | 65篇 |
1987年 | 75篇 |
1986年 | 61篇 |
1985年 | 42篇 |
1984年 | 46篇 |
1982年 | 41篇 |
1981年 | 41篇 |
1980年 | 43篇 |
1979年 | 58篇 |
1978年 | 48篇 |
1977年 | 51篇 |
1976年 | 37篇 |
1974年 | 20篇 |
1973年 | 20篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
71.
K. L. Cherkasova V. S. Bogdanov V. A. Dorokhov O. V. Shishkin 《Russian Chemical Bulletin》1996,45(6):1437-1439
4-Methylthiopyrido[1,2-α]pyrimidin-2-one and 2-hydroxypyrido[1,2-α]pyrimidine-4-thione derivatives were synthesized by the addition ofN-(4-R-pyrid-2-yl)acetoacetamides (R = H, Me) to CS2 under phase-transfer conditions followed by the alkylation of the reaction products with Mel. The molecular structure of 3-acetyl-4-methylthiopyrido[1,2-α]pyrimidin-2-one is established by X-ray analysis. 相似文献
72.
Changes of the rheological properties of hyaluronic acid (sodium-magnesium salt) solutions after exposure to UV radiation
indicate a vigorous decrease in their viscosity, but its still strong shear rate dependence. Whereas the presence of the singlet
oxygen sensitizer (anthracene-1-sulphonic acid) brings about a loss of shear dependence; the studied solutions show newtonian
behavior. 相似文献
73.
N. S. Papageorgiou 《Journal of Optimization Theory and Applications》1991,68(2):305-320
In this paper, we consider a sequence of abstract optimal control problems by allowing the cost integrand, the partial differential operator, and the control constraint set all to vary simultaneously. Using the notions of -convergence of functions,G-convergence of operators, and Kuratowski-Mosco convergence of sets, we show that the values of the approximating problems converge to that of the limit problem. Also we show that a convergent sequence of optimal pairs for the approximating problems has a limit which is optimal for the limit problem. A concrete example of parabolic optimal control problems is worked out in detail.This research was supported by NSF Grant No. DMS-88-02688. 相似文献
74.
El-B. Yallaoui 《Periodica Mathematica Hungarica》1991,23(2):109-120
Let X be an abstract set and L a lattice of subsets of X. To each lattice regular measure µ, we associate two induced measures
and
on suitable lattices of the Wallman space IR(L) and another measure µ on the spaceIR(L). We will investigate the reflection of smoothness properties of p onto
,
and µ; and try to set some new criterion for repleteness and measure repleteness. 相似文献
75.
Flos Spieksma 《Annals of Operations Research》1991,28(1):273-295
Recently Dekker and Hordijk [3,4] introduced conditions for the existence of deterministic Blackwell optimal policies in denumerable Markov decision chains with unbounded rewards. These conditions include- uniform geometric recurrence.The-uniform geometric recurrence property also implies the existence of average optimal policies, a solution to the average optimality equation with explicit formula's and convergence of the value iteration algorithm for average rewards. For this reason, the verification of-uniform geometric convergence is also useful in cases where average and-discounted rewards are considered.On the other hand,-uniform geometric recurrence is a heavy condition on the Markov decision chain structure for negative dynamic programming problems. The verification of-uniform geometric recurrence for the Markov chain induced by some deterministic policy together with results by Sennott [14] yields the existence of a deterministic policy that minimizes the expected average cost for non-negative immediate cost functions.In this paper-uniform geometric recurrence will be proved for two queueing models: theK competing queues and the two centre open Jackson network with control of the service rates.The research of the author is supported by the Netherlands Organization for Scientific Research N.W.O. 相似文献
76.
The rate-of-convergence problem in the central limit theorem is considered for -stable distributions on the noncommutative group of motions in d. The method used here is based on the theory of probability metrics. 相似文献
77.
We consider the accumulation of immobile particles landing on a one-dimensional lattice and annihilating via the A + B 0 bimolecular reaction. Here we focus on short-range interactions with cutoff. We investigate through computer simulations both the kinetics of the particles' accumulation and also their spatial distribution. The relation between the exponents describing the growth of the particles' concentration and the correlation length of their distribution shows that the kinetics of accumulation is fluctuation-dominated.This work is dedicated to Prof. George H. Weiss. 相似文献
78.
I. I. Kandror M. A. Galkina I. O. Bragina B. D. Lavrukhin Yu. N. Belokon' Yu. G. Gololobov 《Russian Chemical Bulletin》1992,41(10):1928-1930
Ethyl -cyanocarylate undergoes the Michael reaction with strong CH-acids (pK
a < 13) in the presence of strong basic amines such as butylamine, piperidine, and triethylamine. Only polymerization of ethyl -cyanoacrylate occurs under the same conditions with weaker CH-acids in the presence of less basic amines such as aniline, N-methylaniline, and pyridine.A. N. Nesmeyanov Institute of Organometallic Compounds, Russian Academy of Sciences, 117813 Moscow. Translated from Izvestiya Akademii Nauk, Seriya Khimicheskaya, No. 10, pp. 2449–2452, October, 1992. 相似文献
79.
Bromination of -aminovinyl ketones (AVK) with geminal arrangement of the fluoroalkyl and amino groups gives N-bromo derivatives. Regioisomeric AVK with the amino groups in the -position relative to the fluoroalkyl group undergo bromination under analogous conditions to give a mixture of the -bromo and ,N-dibromo derivatives. -Bromo derivatives of AVK disproportionate upon storage.Division of Fine Chemical Synthesis, Institute of Organic Chemistry, Urals Branch, Russian Academy of Sciences, 620219 Ekaterinburg. Translated from Izvestiya Akademii Nauk, Seriya Khimicheskaya, No. 3, pp. 746–749, March, 1992. 相似文献
80.
I. I. Kandror B. D. Lavrukhin M. A. Galkina Yu. G. Gololobov 《Russian Chemical Bulletin》1992,41(1):176-178
Syntheses are reported for representatives of a new class of derivatives of -cyanoacrylic acid, namely, its salts with brucine, tert-butyldimethylamine, and diisopropylmethylamine.A. N. Nesmeyanov Institute of Organometallic Compounds, Russian Academy of Sciences, 117813 Moscow. Translated from Izvestiya Akademii Nauk, Seriya Khimicheskaya, No. 1, pp. 212–214, January, 1992. 相似文献