全文获取类型
收费全文 | 7041篇 |
免费 | 254篇 |
国内免费 | 28篇 |
专业分类
化学 | 5028篇 |
晶体学 | 61篇 |
力学 | 180篇 |
数学 | 998篇 |
物理学 | 1056篇 |
出版年
2023年 | 61篇 |
2022年 | 193篇 |
2021年 | 210篇 |
2020年 | 172篇 |
2019年 | 163篇 |
2018年 | 120篇 |
2017年 | 122篇 |
2016年 | 273篇 |
2015年 | 218篇 |
2014年 | 248篇 |
2013年 | 372篇 |
2012年 | 434篇 |
2011年 | 554篇 |
2010年 | 304篇 |
2009年 | 289篇 |
2008年 | 421篇 |
2007年 | 394篇 |
2006年 | 399篇 |
2005年 | 371篇 |
2004年 | 339篇 |
2003年 | 262篇 |
2002年 | 240篇 |
2001年 | 97篇 |
2000年 | 92篇 |
1999年 | 91篇 |
1998年 | 64篇 |
1997年 | 63篇 |
1996年 | 81篇 |
1995年 | 50篇 |
1994年 | 51篇 |
1993年 | 44篇 |
1992年 | 51篇 |
1991年 | 45篇 |
1990年 | 25篇 |
1989年 | 24篇 |
1988年 | 20篇 |
1987年 | 22篇 |
1986年 | 23篇 |
1985年 | 31篇 |
1984年 | 33篇 |
1983年 | 25篇 |
1982年 | 25篇 |
1981年 | 23篇 |
1980年 | 20篇 |
1979年 | 33篇 |
1978年 | 13篇 |
1977年 | 22篇 |
1976年 | 24篇 |
1975年 | 14篇 |
1973年 | 12篇 |
排序方式: 共有7323条查询结果,搜索用时 15 毫秒
21.
Andrea Vietri 《Graphs and Combinatorics》2007,23(1):111-121
We analyse 3-subset difference families of Z2d+1⊕Z2d+1 arising as reductions (mod 2d+1) of particular families of 3-subsets of Z⊕Z. The latter structures, namely perfect d-families, can be viewed as 2-dimensional analogues of difference triangle sets having the least scope. Indeed, every perfect d-family is a set of base blocks which, under the natural action of the translation group Z⊕Z, cover all edges {(x,y),(x′,y′)} such that |x−x′|, |y−y′|≤d. In particular, such a family realises a translation invariant (G,K3)-design, where V(G)=Z⊕Z and the edges satisfy the above constraint. For that reason, we regard perfect families as part of the hereby defined translation designs, which comprise and slightly generalise many structures already existing in the literature. The geometric context allows
some suggestive additional definitions. The main result of the paper is the construction of two infinite classes of d-families. Furthermore, we provide two sporadic examples and show that a d-family may exist only if d≡0,3,8,11 (mod 12). 相似文献
22.
23.
Alberto Caprara Andrea Lodi Silvano Martello Michele Monaci 《Discrete Optimization》2006,3(4):317-326
We address the problem of packing a given set of rectangles into the minimum size square. We consider three versions of the problem, arising when the rectangles (i) are squares; (ii) have a fixed orientation; (iii) can be rotated by 90. For each case we study lower bounds, and analyze their worst-case performance ratio. In addition, we evaluate through computational experiments their average performance on instances from the literature. 相似文献
24.
Andrea Loi 《Differential Geometry and its Applications》2006,24(3):311-319
In this paper we study the Calabi diastasis function of Hermitian symmetric spaces. This allows us to prove that if a complete Hermitian locally symmetric space (M,g) admits a Kähler immersion into a globally symmetric space (S,G) then it is globally symmetric and the immersion is injective. Moreover, if (S,G) is symmetric of a specified type (Euclidean, noncompact, compact), then (M,g) is of the same type. We also give a characterization of Hermitian globally symmetric spaces in terms of their diastasis function. Finally, we apply our analysis to study the balanced metrics, introduced by Donaldson, in the case of locally Hermitian symmetric spaces. 相似文献
25.
26.
Michelina Soccio Lara Finelli Nadia Lotti Valentina Siracusa Tiberio A. Ezquerra Andrea Munari 《Journal of Polymer Science.Polymer Physics》2007,45(13):1694-1703
Poly(butylene naphthalate) (PBN), poly(diethylene naphthalate) (PDEN), and poly(thiodiethylene naphthalate) (PTDEN) were synthesized and characterized in terms of chemical structure and molecular weight. The polyesters were examined by TGA, DSC, and DMTA. All the polymers showed a good thermal stability, even though depending on chemical structure. At room temperature they appeared as semicrystalline materials; the effect of the introduction along the PBN polymer chain of ether oxygen atoms or sulfur ones was a lowering in the Tg value, a decrement of Tm, and a decrease of the crystallization rate. Changing in chemical structure also affects the main α absorption associated with the glass transition which moves to lower temperature and whose energetic requirements decrease. The results were explained as due to the presence of highly flexible C? S? C or C? O? C bonds in the polymeric chain. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1694–1703, 2007 相似文献
27.
28.
Three new tetracyclic diterpenes were isolated from the chloroform-soluble extract of Euphorbia villosa, together with one new and one known lathyrane diterpene. The structures were elucidated by means of various spectroscopic methods, including HREI-MS, HRFAB-MS, UV, and 1D and 2D NMR techniques. Spectral analyses revealed that two of the tetracyclic compounds contain the rare 5-6-6-4 fused ring system, while the third has a 5-6-7-3 fused diterpene core. Such diterpene skeletons have previously been found only in euphoractines A-E isolated from Euphorbia micractina. As a new structural feature, the diterpene framework described here has a C-2 epimer configuration. The new lathyrane diterpene is a diester of a hitherto unknown polyfunctional parent alcohol. 相似文献
29.
We are given a unique rectangular piece of stock material S, with height H and width W, and a list of m rectangular shapes to be cut from S. Each shape's type i (i = 1, ..., m) is characterized by a height , a width , a profit , and an upper bound ub
i
indicating the maximum number of items of type i which can be cut. We refer to the Two-Dimensional Knapsack (TDK) as the problem of determining a cutting pattern of S maximizing the sum of the profits of the cut items. In particular, we consider the classical variant of TDK in which the
maximum number of cuts allowed to obtain each item is fixed to 2, and we refer to this problem as 2-staged TDK (2TDK). For
the 2TDK problem we present two new Integer Linear Programming models, we discuss their properties, and we compare them with
other formulations in terms of the LP bound they provide. Finally, both models are computationally tested within a standard
branch-and-bound framework on a large set of instances from the literature by reinforcing them with the addition of linear
inequalities to eliminate symmetries.
Received: October 17, 2000 / Accepted: December 19, 2001 Published online: September 27, 2002
Key words. packing – cutting – integer linear programming 相似文献
30.
A ruthenium-catalyzed ring opening-ring closing metathesis reaction serves as the key step in the stereoselective synthesis of a new enantiopure 2-substituted-4,5-dehydropiperidine skeleton, a valuable intermediate for the synthesis of piperidine alkaloids (such as (−)-halosaline) and of hydroxylated quinolizidines (such as (2R,9aR)-(+)-2-hydroxy-quinolizidine). 相似文献