全文获取类型
收费全文 | 650篇 |
免费 | 47篇 |
国内免费 | 60篇 |
专业分类
化学 | 28篇 |
晶体学 | 3篇 |
力学 | 62篇 |
综合类 | 22篇 |
数学 | 538篇 |
物理学 | 104篇 |
出版年
2024年 | 1篇 |
2023年 | 11篇 |
2022年 | 15篇 |
2021年 | 14篇 |
2020年 | 17篇 |
2019年 | 27篇 |
2018年 | 23篇 |
2017年 | 21篇 |
2016年 | 17篇 |
2015年 | 21篇 |
2014年 | 31篇 |
2013年 | 78篇 |
2012年 | 22篇 |
2011年 | 30篇 |
2010年 | 29篇 |
2009年 | 45篇 |
2008年 | 65篇 |
2007年 | 42篇 |
2006年 | 53篇 |
2005年 | 26篇 |
2004年 | 13篇 |
2003年 | 26篇 |
2002年 | 27篇 |
2001年 | 18篇 |
2000年 | 12篇 |
1999年 | 13篇 |
1998年 | 11篇 |
1997年 | 7篇 |
1996年 | 8篇 |
1995年 | 9篇 |
1994年 | 6篇 |
1992年 | 3篇 |
1991年 | 4篇 |
1989年 | 5篇 |
1986年 | 1篇 |
1982年 | 1篇 |
1981年 | 1篇 |
1980年 | 1篇 |
1979年 | 1篇 |
1978年 | 1篇 |
1959年 | 1篇 |
排序方式: 共有757条查询结果,搜索用时 31 毫秒
91.
1966年,Leo Moser提出了一个基本的几何问题,即Worm Problem.该问题是指:在平面上寻找一个面积最小的(凸)区域,使得任何一条长为1的平面曲线都能够通过旋转和平移完全放入该(凸)区域之中.对于要寻找的区域是凸的情形,本文将把目前所知道的最小的上界由0.2738086降低至0.270911861.在最后一部分,我们推广了Worm Problem,并初步给出了一些相应的结果. 相似文献
92.
We present an exact approach for solving the -interdiction median problem with fortification. Our approach consists of solving a greedy heuristic and a set cover problem iteratively that guarantees to find an optimal solution upon termination. The greedy heuristic obtains a feasible solution to the problem, and the set cover problem is solved to verify optimality of the solution and to provide a direction for improvement if not optimal. We demonstrate the performance of the algorithm in a computational study. 相似文献
93.
A clique in a graph is strong if it intersects all maximal independent sets. A graph is localizable if it has a partition of the vertex set into strong cliques. Localizable graphs were introduced by Yamashita and Kameda in 1999 and form a rich class of well-covered graphs that coincides with the class of well-covered graphs within the class of perfect graphs. In this paper, we give several equivalent formulations of the property of localizability and develop polynomially testable characterizations of localizable graphs within three non-perfect graph classes: triangle-free graphs, -free graphs, and line graphs. Furthermore, we use localizable graphs to construct an infinite family of counterexamples to a conjecture due to Zaare-Nahandi about -partite well-covered graphs having all maximal cliques of size . 相似文献
94.
Engin Büyükaşık Edgar Enochs J. R. García Rozas Gizem Kafkas-Demirci Sergio López-Permouth Luis Oyonarte 《代数通讯》2018,46(2):764-779
Relative notions of flatness are introduced as a mean to gauge the extent of the flatness of any given module. Every module is thus endowed with a flatness domain and, for every ring, the collection of flatness domains of all of its modules is a lattice with respect to class inclusion. This lattice, the flatness profile of the ring, allows us, in particular, to focus on modules which have a smallest flatness domain (namely, one consisting of all regular modules.) We establish that such modules exist over arbitrary rings and we call them Rugged Modules. Rings all of whose (cyclic) modules are rugged are shown to be precisely the von Neumann regular rings. We consider rings without a flatness middle class (i.e., rings for which modules must be either flat or rugged.) We obtain that, over a right Noetherian ring every left module is rugged or flat if and only if every right module is poor or injective if and only if R = S×T, where S is semisimple Artinian and T is either Morita equivalent to a right PCI-domain, or T is right Artinian whose Jacobson radical properly contains no nonzero ideals. Character modules serve to bridge results about flatness and injectivity profiles; in particular, connections between rugged and poor modules are explored. If R is a ring whose regular left modules are semisimple, then a right module M is rugged if and only if its character left module M+ is poor. Rugged Abelian groups are fully characterized and shown to coincide precisely with injectively poor and projectively poor Abelian groups. Also, in order to get a feel for the class of rugged modules over an arbitrary ring, we consider the homological ubiquity of rugged modules in the category of all modules in terms of the feasibility of rugged precovers and covers for arbitrary modules. 相似文献
95.
It is proved that a semiperfect module is lifting if and only if it has a projective cover preserving direct summands. Three corollaries are obtained: (1) every cyclic module over a ring R is lifting if and only if every cyclic R-module has a projective cover preserving direct summands; (2) a ring R is artinian serial with Jacobson radical square-zero if and only if every (2-generated) R-module has a projective cover preserving direct summands; (3) a ring R is a right (semi-)perfect ring if and only if (cyclic) lifting R-module has a projective cover preserving direct summands, if and only if every (cyclic) R-module having a projective cover preserving direct summands is lifting. It is also proved that every cyclic module over a ring R is ⊕-supplemented if and only if every cyclic R-module is a direct sum of local modules. Consequently, a ring R is artinian serial if and only if every left and right R-module is a direct sum of local modules. 相似文献
96.
1000多年前,英国著名学者Alcuin曾提出一个古老的渡河问题,即狼、羊和卷心菜的渡河问题。2006年,Prisner把该问题推广到任意的冲突图上,考虑了一类情况更一般的渡河运输问题。所谓冲突图是指一个图G=(V,E),这里V代表某些物品的集合,V中的两个点有边连结当且仅当这两个点是冲突的,即在无人监管的情况下不允许留在一起的点。图G=(V,E)的一个可行运输方案是指在保证不发生任何冲突的前提下,把V的点所代表的物品全部摆渡到河对岸的一个运输方案。图G的Alcuin数定义为它存在可行运输方案时所需船的最小容量。本文讨论了覆盖数不超过3的连通图的Alcuin数,给出了该类图Alcuin数的完全刻画。 相似文献
97.
Paul Ramsden 《Journal of Functional Analysis》2010,258(12):3988-4009
Let S be a semigroup. In this paper we investigate the injectivity of ?1(S) as a Banach right module over ?1(S). For weakly cancellative S this is the same as studying the flatness of the predual left module c0(S). For such semigroups S, we also investigate the projectivity of c0(S). We prove that for many semigroups S for which the Banach algebra ?1(S) is non-amenable, the ?1(S)-module ?1(S) is not injective. The main result about the projectivity of c0(S) states that for a weakly cancellative inverse semigroup S, c0(S) is projective if and only if S is finite. 相似文献
98.
Bojan Mohar 《Discrete Mathematics》2010,310(20):2595-2599
A “folklore conjecture, probably due to Tutte” (as described in [P.D. Seymour, Sums of circuits, in: Graph Theory and Related Topics (Proc. Conf., Univ. Waterloo, 1977), Academic Press, 1979, pp. 341-355]) asserts that every bridgeless cubic graph can be embedded on a surface of its own genus in such a way that the face boundaries are cycles of the graph. Sporadic counterexamples to this conjecture have been known since the late 1970s. In this paper we consider closed 2-cell embeddings of graphs and show that certain (cubic) graphs (of any fixed genus) have closed 2-cell embedding only in surfaces whose genus is very large (proportional to the order of these graphs), thus providing a plethora of strong counterexamples to the above conjecture. The main result yielding such counterexamples may be of independent interest. 相似文献
99.
100.
In a pushout-pullback diagram,which consists of four morphisms f : A → B,g : A → C,α : C → D and β : B → D,we give some relations among the covers of these four modules.If kerf ∈ I(L ),then g : A → C is L -covering if and only if β : B → D is L -covering.If every module has an L -precover and kerf ∈ I(L ),then A and C have isomorphic L -precovers if and only if B and D have isomorphic L -precovers. 相似文献