全文获取类型
收费全文 | 105028篇 |
免费 | 19533篇 |
国内免费 | 1690篇 |
专业分类
化学 | 104781篇 |
晶体学 | 577篇 |
力学 | 3199篇 |
数学 | 9783篇 |
物理学 | 7911篇 |
出版年
2024年 | 381篇 |
2023年 | 4175篇 |
2022年 | 1678篇 |
2021年 | 2949篇 |
2020年 | 6144篇 |
2019年 | 5199篇 |
2018年 | 2901篇 |
2017年 | 1532篇 |
2016年 | 7864篇 |
2015年 | 7868篇 |
2014年 | 7423篇 |
2013年 | 9087篇 |
2012年 | 6757篇 |
2011年 | 5005篇 |
2010年 | 6183篇 |
2009年 | 5960篇 |
2008年 | 4938篇 |
2007年 | 4147篇 |
2006年 | 3387篇 |
2005年 | 3423篇 |
2004年 | 2951篇 |
2003年 | 2631篇 |
2002年 | 3217篇 |
2001年 | 1754篇 |
2000年 | 1640篇 |
1999年 | 716篇 |
1998年 | 388篇 |
1997年 | 455篇 |
1996年 | 563篇 |
1995年 | 485篇 |
1994年 | 383篇 |
1993年 | 480篇 |
1992年 | 361篇 |
1985年 | 453篇 |
1984年 | 449篇 |
1982年 | 441篇 |
1981年 | 480篇 |
1980年 | 496篇 |
1979年 | 465篇 |
1978年 | 446篇 |
1977年 | 537篇 |
1976年 | 566篇 |
1975年 | 664篇 |
1974年 | 679篇 |
1973年 | 493篇 |
1972年 | 450篇 |
1971年 | 429篇 |
1970年 | 582篇 |
1969年 | 460篇 |
1968年 | 490篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
Bounds are given on the size of the parameter-space decomposition induced by multiple sequence alignment problems where phylogenetic information may be given or inferred. It is shown that many of the usual formulations of these problems fall within the same integer parametric framework, implying that the number of distinct optima obtained as the parameters are varied across their ranges is polynomially bounded in the length and number of sequences. 相似文献
102.
David S. Dummit Brett A. Tangedal Paul B. van Wamelen. 《Mathematics of Computation》2004,73(247):1525-1546
Systematic computation of Stark units over nontotally real base fields is carried out for the first time. Since the information provided by Stark's conjecture is significantly less in this situation than the information provided over totally real base fields, new techniques are required. Precomputing Stark units in relative quadratic extensions (where the conjecture is already known to hold) and coupling this information with the Fincke-Pohst algorithm applied to certain quadratic forms leads to a significant reduction in search time for finding Stark units in larger extensions (where the conjecture is still unproven). Stark's conjecture is verified in each case for these Stark units in larger extensions and explicit generating polynomials for abelian extensions over complex cubic base fields, including Hilbert class fields, are obtained from the minimal polynomials of these new Stark units.
103.
Günter Graumann Prof. Dr. 《ZDM》2005,37(3):190-198
Problem fields with one or two generating problems and possibilities of varying existing problems give a good chance for self-activities of students and can be used for reaching different general aims. In this paper some topics concerning quadrilaterals will be presented. I hope they will animate teachers for more problem orientation in mathematics education. First we will reflect about different types of convex and non-convex quadrilaterals and possibilities of ordering them. Then we focus on middle-quadrilaterals and types of quadrilaterals with special middle-quadrilaterals as well as their logical ordering. Finally we investigate the analogies in space to the parallelogram and its sub-types and order them in the “house of parallelepipeds”. 相似文献
104.
An m‐covering of a graph G is a spanning subgraph of G with maximum degree at most m. In this paper, we shall show that every 3‐connected graph on a surface with Euler genus k ≥ 2 with sufficiently large representativity has a 2‐connected 7‐covering with at most 6k ? 12 vertices of degree 7. We also construct, for every surface F2 with Euler genus k ≥ 2, a 3‐connected graph G on F2 with arbitrarily large representativity each of whose 2‐connected 7‐coverings contains at least 6k ? 12 vertices of degree 7. © 2003 Wiley Periodicals, Inc. J Graph Theory 43: 26–36, 2003 相似文献
105.
By optimizing pump power ratio between 1st order backward pump and 2nd order forward pump on discrete Raman amplifier, we demonstrated over 2dB noise figure improvement without excessive non-linearity degradation. 相似文献
106.
David E. Anderson 《Proceedings of the American Mathematical Society》2004,132(9):2517-2527
With an explicit example, we confirm a conjecture by Neumann and Wahl that there exist cusps with no Galois cover by a complete intersection. Some computational techniques are reviewed, and a method for deciding whether a given cusp has a complete intersection Galois cover is developed.
107.
David M. Evans 《组合设计杂志》2004,12(6):459-465
For all ‘reasonable’ finite t, k, and s, we construct a t‐(?0, k, 1) design and a group of automorphisms which is transitive on blocks and has s orbits on points. In particular, there is a 2‐(?0, 4, 1) design with a block‐transitive group of automorphisms having two point orbits. This answers a question of P. J. Cameron and C. E. Praeger. The construction is presented in a purely combinatorial way, but is a by‐product of a new way of looking at a model‐theoretic construction of E. Hrushovski. © 2004 Wiley Periodicals, Inc. 相似文献
108.
We exhibit timelike geodesic paths for a metric, introduced by Bonnor [11] and considered also by Steadman [13], and show that coordinate time runs backward along a portion of these geodesics. 相似文献
109.
5‐Methylene‐2‐oxazolidinone (1) and 5‐methylene‐1,3‐thiazolidine‐2‐thione (4) react with various isocyanates to give the corresponding urethanes 3 and 5 in high yields in the presence of palladium(0) or palladium(II) catalyst under mild reaction conditions. A mechanism is proposed. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
110.
One of the open questions in the geometry of line arrangements is to what extent does the incidence lattice of an arrangement determine its fundamental group. Line arrangements of up to 6 lines were recently classified by K.M. Fan (Michigan Math. J. 44(2) (1997) 283), and it turns out that the incidence lattice of such arrangements determines the projective fundamental group. We use actions on the set of wiring diagrams, introduced in (Garber et al. (J. Knot Theory Ramf.), to classify real arrangements of up to 8 lines. In particular, we show that the incidence lattice of such arrangements determines both the affine and the projective fundamental groups. 相似文献