全文获取类型
收费全文 | 420877篇 |
免费 | 2693篇 |
国内免费 | 922篇 |
专业分类
化学 | 200487篇 |
晶体学 | 6687篇 |
力学 | 22482篇 |
综合类 | 13篇 |
数学 | 61161篇 |
物理学 | 133662篇 |
出版年
2020年 | 3138篇 |
2019年 | 3741篇 |
2018年 | 11858篇 |
2017年 | 12469篇 |
2016年 | 10523篇 |
2015年 | 4515篇 |
2014年 | 6883篇 |
2013年 | 15124篇 |
2012年 | 14940篇 |
2011年 | 23425篇 |
2010年 | 16010篇 |
2009年 | 16283篇 |
2008年 | 21298篇 |
2007年 | 23946篇 |
2006年 | 11902篇 |
2005年 | 15292篇 |
2004年 | 12298篇 |
2003年 | 11658篇 |
2002年 | 9940篇 |
2001年 | 10030篇 |
2000年 | 7676篇 |
1999年 | 5686篇 |
1998年 | 4805篇 |
1997年 | 4746篇 |
1996年 | 4597篇 |
1995年 | 4132篇 |
1994年 | 4186篇 |
1993年 | 4012篇 |
1992年 | 4542篇 |
1991年 | 4582篇 |
1990年 | 4439篇 |
1989年 | 4446篇 |
1988年 | 4366篇 |
1987年 | 4385篇 |
1986年 | 4138篇 |
1985年 | 5355篇 |
1984年 | 5463篇 |
1983年 | 4678篇 |
1982年 | 4747篇 |
1981年 | 4561篇 |
1980年 | 4343篇 |
1979年 | 4819篇 |
1978年 | 4836篇 |
1977年 | 4881篇 |
1976年 | 4957篇 |
1975年 | 4693篇 |
1974年 | 4603篇 |
1973年 | 4880篇 |
1972年 | 3498篇 |
1971年 | 3072篇 |
排序方式: 共有10000条查询结果,搜索用时 9 毫秒
991.
The n-dimensional cube Qn is the graph whose vertices are the subsets of {1,…n}, with two vertices adjacent if and only if their symmetric difference is a singleton. Clearly Qn has diameter and radious n. Write M = n2n-1 = e(Qn) for the size of Qn. Let Q = (Qt)oM be a random Qn-process. Thus Qt is a spanning subgraph of Qn of size t, and Qt is obtained from Qt–1 by the random addition of an edge of Qn not in Qt–1, Let t(k) = τ(Q;δ?k) be the hitting time of the property of having minimal degree at least k. We show that the diameter dt = diam (Qt) of Qt in almost every Q? behaves as follows: dt starts infinite and is first finite at time t(1), it equals n + 1 for t(1) ? t(2) and dt, = n for t ? t(2). We also show that the radius of Qt, is first finite for t = t(1), when it assumes the value n. These results are deduced from detailed theorems concerning the diameter and radius of the almost surely unique largest component of Qt, for t = Ω(M). © 1994 John Wiley & Sons, Inc. 相似文献
992.
A nonlinear quasi-steady model for the analysis of the dynamics of a loosely supported cylinder, which takes into account position-dependent nonlinear fluid forces as well as nonuniform flow, is formulated. The model includes an approximation for the equivalent viscous damping associated with energy dissipation on impact at the support. The nonlinear model shows reasonably good agreement with experiments, in predicting the observed bifurcations in the cylinder response. Comparison criteria include the standard orbital plots, time traces and response spectra. A borderline chaotic response is found to be predominant over the test velocity range. In this chaotic regime, the theoretical results were verified via attractor fractal-dimension calculations and saddle orbit distributions; theoretical values of these invariant measures compare reasonably well with their experimental counterparts. Two mechanisms leading to chaos have been identified for this system. The first is a switching mechanism , at the onset of impacting. The second, and more prevalent, is the type I intermittency route to chaos. 相似文献
993.
994.
Plasma B-6 vitamer and plasma and urinary 4-pyridoxic acid concentrations of 21 young white women, 21-27 years, having radiomonitored pyridoxal 5'-phosphate and coenzyme stimulation of erythrocyte alanine aminotransferase activities indicative of adequate vitamin B-6 status were determined in an effort to establish normal ranges for plasma B-6 vitamers. B-6 vitamers and 4-pyridoxic acid were quantitated using reversed phase high performance liquid chromatography with fluorometric and ultraviolet detection. Pyridoxal phosphate values obtained by radioenzymatic and chromatographic, fluorometric and ultraviolet, assays were highly correlated as were pyridoxine phosphate values determined using both detectors. The B-6 vitamer and 4-pyridoxic acid values of these subjects should be of use in the establishment of normal ranges of these congeners in women. 相似文献
995.
Ingrid Persson Folke Tjerneld Bärbel Hähn-Hagerdal 《Applied biochemistry and biotechnology》1991,27(1):9-25
Cellulolytic enzyme production in aqueous two-phase systems withTrichoderma reesei Rutgers C30 has been investigated. The influ ence of different phase systems, as well as addition of media compo nents and substrate on enzyme production have been studied. Extractive enzyme production in fed-batch cultivations was per formed in a phase system composed of PEG 8000 5%-Dextran T500 7% with 1% Solka-Floc BW 200 as substrate. The cellulolytic enzyme system was intermittently withdrawn with the top phase. Addition of media components every 24 h and cellulose every 72 h gave an aver age enzyme activity in the withdrawn top phase of 2.2 FPU/mL dur ing 170 h cultivation. The corresponding productivity was 18 FPU/lh. The productivity was increased to 24 FPU/l.h when media compo nents and cellulose were added every 72 h. The average enzyme con centration was then 1.6 FPU/mL. The results are discussed in relation to methods for cellulolytic enzyme production involving immobiliza tion and cell recycling. 相似文献
996.
B. Griepink E. A. Maier P. Quevauviller H. Muntau 《Fresenius' Journal of Analytical Chemistry》1991,339(9):599-603
Summary The Community Bureau of Reference (BCR) has undertaken the production of a wide range of certified reference materials (CRMs) for the improvement of accuracy and measurement harmony in various areas of concern at European Community level. This paper provides an overview of the sectors of the BCR-programme in the field of environment i.e. for water systems (water, sediment, biota), soil analysis (soil, fertiliser, waste), plant analysis (aquatic and terrestrial plants), analysis related to animals (e.g. muscle or liver) or man (e.g. industry and food). 相似文献
997.
998.
999.
Christian Bär 《Communications in Mathematical Physics》1997,188(3):709-721
We prove that the nodal set (zero set) of a solution of a generalized Dirac equation on a Riemannian manifold has codimension
2 at least. If the underlying manifold is a surface, then the nodal set is discrete. We obtain a quick proof of the fact that
the nodal set of an eigenfunction for the Laplace-Beltrami operator on a Riemannian manifold consists of a smooth hypersurface
and a singular set of lower dimension. We also see that the nodal set of a Δ-harmonic differential form on a closed manifold
has codimension 2 at least; a fact which is not true if the manifold is not closed. Examples show that all bounds are optimal.
Received: 28 October 1996 / Accepted: 3 March 1997 相似文献
1000.
In this paper, we consider a real-life vehicle routeing problem that occurs in a major Swiss company producing pet food and flour. In contrast with usual hypothetical problems, a large variety of restrictions has to be considered. The main constraints are relative to the accessibility and the time windows at customers, the carrying capacities of vehicles, the total duration of routes and the drivers' breaks. To find good solutions to this problem, we propose two heuristic methods: a fast straightforward insertion procedure and a method based on tabu search techniques. Next, the produced solutions are compared with the routes actually covered by the company. Our outcomes indicate that the total distance travelled can be reduced significantly when such methods are used. 相似文献