全文获取类型
收费全文 | 5882篇 |
免费 | 127篇 |
国内免费 | 34篇 |
专业分类
化学 | 4050篇 |
晶体学 | 88篇 |
力学 | 134篇 |
数学 | 1022篇 |
物理学 | 749篇 |
出版年
2022年 | 36篇 |
2021年 | 42篇 |
2020年 | 41篇 |
2019年 | 62篇 |
2018年 | 58篇 |
2017年 | 47篇 |
2016年 | 113篇 |
2015年 | 113篇 |
2014年 | 113篇 |
2013年 | 314篇 |
2012年 | 262篇 |
2011年 | 320篇 |
2010年 | 148篇 |
2009年 | 145篇 |
2008年 | 287篇 |
2007年 | 313篇 |
2006年 | 265篇 |
2005年 | 301篇 |
2004年 | 250篇 |
2003年 | 281篇 |
2002年 | 234篇 |
2001年 | 102篇 |
2000年 | 88篇 |
1999年 | 84篇 |
1998年 | 63篇 |
1997年 | 86篇 |
1996年 | 95篇 |
1995年 | 93篇 |
1994年 | 85篇 |
1993年 | 64篇 |
1992年 | 56篇 |
1991年 | 61篇 |
1990年 | 51篇 |
1989年 | 68篇 |
1988年 | 73篇 |
1987年 | 58篇 |
1986年 | 53篇 |
1985年 | 84篇 |
1984年 | 102篇 |
1983年 | 69篇 |
1982年 | 92篇 |
1981年 | 96篇 |
1980年 | 92篇 |
1979年 | 75篇 |
1978年 | 80篇 |
1977年 | 73篇 |
1976年 | 68篇 |
1975年 | 59篇 |
1974年 | 50篇 |
1973年 | 36篇 |
排序方式: 共有6043条查询结果,搜索用时 0 毫秒
971.
The 1,4-addition of the lithium enolate of methyldithioacetate (LMDTA) to (+/-)-4-O-TBS-2-cyclohexenone (3) can be varied from being highly 3,4-trans selective to being highly 3,4-cis selective simply by varying the reaction temperature. This stereodivergency allows expedient syntheses of the corresponding trans and cis methyl esters 6t and 6c and derived bicyclic ketolactones 7t and 7c. 相似文献
972.
Alan Dow 《Topology and its Applications》2010,157(8):1379-1857
We consider generalizations of a well-known class of spaces, called by S. Mrówka, N∪R, where R is an infinite maximal almost disjoint family (MADF) of countable subsets of the natural numbers N. We denote these generalizations by ψ=ψ(κ,R) for κ?ω. Mrówka proved the interesting theorem that there exists an R such that |βψ(ω,R)?ψ(ω,R)|=1. In other words there is a unique free z-ultrafilter p0 on the space ψ. We extend this result of Mrówka to uncountable cardinals. We show that for κ?c, Mrówka's MADF R can be used to produce a MADF M⊂ω[κ] such that |βψ(κ,M)?ψ(κ,M)|=1. For κ>c, and every M⊂ω[κ], it is always the case that |βψ(κ,M)?ψ(κ,M)|≠1, yet there exists a special free z-ultrafilter p on ψ(κ,M) retaining some of the properties of p0. In particular both p and p0 have a clopen local base in βψ (although βψ(κ,M) need not be zero-dimensional). A result for κ>c, that does not apply to p0, is that for certain κ>c, p is a P-point in βψ. 相似文献
973.
Damien Weidmann Chad B. Roller Clive Oppenheimer Alan Fried Frank K. Tittel 《Isotopes in environmental and health studies》2013,49(4):293-302
Recent developments of two mid-infrared tunable laser spectrometers dedicated to carbon isotope ratio determination are presented. First, a field deployable quantum cascade laser-based sensor is described, along with line selection strategy for 13/12CO2 ratio measurements. Secondly, an instrument architecture based on difference frequency generation is presented. The analyses of fundamental limitations, specifically temperature and pressure stability, and water vapor collision broadening, are detailed. 相似文献
974.
975.
Alan V. Lair 《Applicable analysis》2013,92(5):431-437
We show that the equation Δu = p(x)f(u) has a positive solution on R N , N ≥ 3, satisfying <artwork name="GAPA31011ei1"> <artwork name="GAPA31011ei2"> if and only if <artwork name="GAPA31011ei3"> when ψ(r) = min{p(x): |x| = r}. The nondecreasing continuous function f satisfies f(0) = 0, f (s) > 0 for s > 0, and sup s ≥ 1 f(s)/s<∞, and the nonnegative continuous function p is required to be asymptotically radial. This extends previous results which required the function p to be constant or radial. 相似文献
976.
Murat Alan 《代数通讯》2013,41(11):4089-4099
Let R be a commutative ring with identity. R is a finite factorization ring (FFR) if every nonzero nonunit of R has only a finite number of factorizations up to order and associates. In this article, we give a characterization of R for R[X] and R[[X]] to be an FFR. 相似文献
977.
In this paper we show that e/n is the sharp threshold for the existence of tight Hamilton cycles in random k ‐uniform hypergraphs, for all k ≥ 4. When k = 3 we show that 1/n is an asymptotic threshold. We also determine thresholds for the existence of other types of Hamilton cycles. © 2012 Wiley Periodicals, Inc. Random Struct. Alg., 2013 相似文献
978.
Eric T. Bradlow Alan M. Zaslavsky 《Journal of computational and graphical statistics》2013,22(3):314-331
Abstract We demonstrate how case influence analysis, commonly used in regression, can be applied to Bayesian hierarchical models. Draws from the joint posterior distribution of parameters are importance weighted to reflect the effect of deleting each observation in turn; the ensuing changes in the posterior distribution of each parameter are displayed graphically. The procedure is particularly useful when drawing a sample from the posterior distribution requires extensive calculations (as with a Markov Chain Monte Carlo sampler). The structure of hierarchical models, and other models with local dependence, makes the importance weights inexpensive to calculate with little additional programming. Some new alternative weighting schemes are described that extend the range of problems in which reweighting can be used to assess influence. Applications to a growth curve model and a complex hierarchical model for opinion data are described. Our focus on case influence on parameters is complementary to other work that measures influence by distances between posterior or predictive distributions. 相似文献
979.
Chisheng Huang John Alan McDonald Werner Stuetzle 《Journal of computational and graphical statistics》2013,22(4):383-396
Abstract Scatterplots are the method of choice for displaying the distribution of points in two dimensions. They are used to discover patterns such as holes, outliers, modes, and association between the two variables. A common problem is overstriking—the overlap on the plotting surface of glyphs representing individual observations. Overstriking can create a misleading impression of the data distribution. The variable resolution bivariate plots (Varebi plots) proposed in this article deal with the problem of overstriking by mixing display of a density estimate and display of individual observations. The idea is to determine the display format by analyzing the actual amount of overstriking on the screen. Thus, the display format will depend on the sample size, the distribution of the observations, the size and shape of individual icons, and the size of the window. It may change automatically when the window is resized. Varebi plots reveal detail wherever possible, and show the overall trend when displaying detail is not feasible. 相似文献
980.
A necessary condition is given for a very wide separable p-group to be derermined by its n-socle (the set of elements of order at most p n). A group G is very wide if it has a direct summand of final rank ‖G‖ which itself is a direct sum of cyclic groups. In the case, n=1 (as Shelat has shown)this condition is sufficient but for n>1 examples are given to show that this condition is not sufficient. 相似文献