全文获取类型
收费全文 | 58860篇 |
免费 | 851篇 |
国内免费 | 293篇 |
专业分类
化学 | 27359篇 |
晶体学 | 1027篇 |
力学 | 3199篇 |
综合类 | 8篇 |
数学 | 4662篇 |
物理学 | 23749篇 |
出版年
2022年 | 495篇 |
2021年 | 472篇 |
2020年 | 446篇 |
2019年 | 424篇 |
2018年 | 570篇 |
2017年 | 496篇 |
2016年 | 896篇 |
2015年 | 634篇 |
2014年 | 990篇 |
2013年 | 2495篇 |
2012年 | 2334篇 |
2011年 | 3010篇 |
2010年 | 2124篇 |
2009年 | 2173篇 |
2008年 | 2755篇 |
2007年 | 2588篇 |
2006年 | 2453篇 |
2005年 | 2176篇 |
2004年 | 1988篇 |
2003年 | 1764篇 |
2002年 | 1650篇 |
2001年 | 3008篇 |
2000年 | 2149篇 |
1999年 | 1554篇 |
1998年 | 1083篇 |
1997年 | 1058篇 |
1996年 | 884篇 |
1995年 | 784篇 |
1994年 | 704篇 |
1993年 | 626篇 |
1992年 | 954篇 |
1991年 | 947篇 |
1990年 | 849篇 |
1989年 | 749篇 |
1988年 | 734篇 |
1987年 | 795篇 |
1986年 | 657篇 |
1985年 | 883篇 |
1984年 | 833篇 |
1983年 | 569篇 |
1982年 | 562篇 |
1981年 | 539篇 |
1980年 | 496篇 |
1979年 | 621篇 |
1978年 | 652篇 |
1977年 | 660篇 |
1976年 | 577篇 |
1975年 | 484篇 |
1974年 | 522篇 |
1973年 | 452篇 |
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
221.
For any two points p and q in the Euclidean plane, define LUNpq = { v | v ∈ R2, dpv < dpq and dqv < dpq}, where duv is the Euclidean distance between two points u and v . Given a set of points V in the plane, let LUNpq(V) = V ∩ LUNpq. Toussaint defined the relative neighborhood graph of V, denoted by RNG(V) or simply RNG, to be the undirected graph with vertices V such that for each pair p,q ∈ V, (p,q) is an edge of RNG(V) if and only if LUNpq (V) = ?. The relative neighborhood graph has several applications in pattern recognition that have been studied by Toussaint. We shall generalize the idea of RNG to define the k-relative neighborhood graph of V, denoted by kRNG(V) or simply kRNG, to be the undirected graph with vertices V such that for each pair p,q ∈ V, (p,q) is an edge of kRNG(V) if and only if | LUNpq(V) | < k, for some fixed positive number k. It can be shown that the number of edges of a kRNG is less than O(kn). Also, a kRNG can be constructed in O(kn2) time. Let Ec = {epq| p ∈ V and q ∈ V}. Then Gc = (V,Ec) is a complete graph. For any subset F of Ec, define the maximum distance of F as maxepq∈Fdpq. A Euclidean bottleneck Hamiltonian cycle is a Hamiltonian cycle in graph Gc whose maximum distance is the minimum among all Hamiltonian cycles in graph Gc. We shall prove that there exists a Euclidean bottleneck Hamiltonian cycle which is a subgraph of 20RNG(V). Hence, 20RNGs are Hamiltonian. 相似文献
222.
223.
224.
Idealization of a decomposition theorem 总被引:1,自引:1,他引:0
In 1986, Tong [13] proved that a function f : (X,τ)→(Y,φ) is continuous if and only if it is α-continuous and A-continuous. We extend this decomposition of continuity in terms of ideals. First, we introduce the notions of regular-I-closed sets, A
I-sets and A
I -continuous functions in ideal topological spaces and investigate their properties. Then, we show that a function f : (X,τ,I)→(Y, φ) is continuous if and only if it is α-I-continuous and A
I-continuous.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
225.
A. Y. M. Chin 《Acta Mathematica Hungarica》2004,102(4):337-342
Let R be an associative ring with unit and let N(R) denote the set of nilpotent elements of R. R is said to be stronglyπ-regular if for each x∈R, there exist a positive integer n and an element y∈R such that x
n=x
n
+1
y and xy=yx. R is said to be periodic if for each x∈R there are integers m,n≥ 1 such that m≠n and x
m=x
n. Assume that the idempotents in R are central. It is shown in this paper that R is a strongly π-regular ring if and only if N(R) coincides with the Jacobson radical of R and R/N(R) is regular. Some similar conditions for periodic rings are also obtained.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
226.
Montgomery and Vaughan improved a theorem of Erd?s and Fuchs for an arbitrary sequence. Sárközy extended this theorem of Erd?s and Fuchs for two arbitrary sequences which are "near" in a certain sense. Using the idea of Jurkat (differentiation of the generating function), we will extend similarly the result of Montgomery and Vaughan for "sufficiently near" sequences. 相似文献
227.
The electronic states of the Cr overlayers on TiO2(0 0 1) surfaces have been investigated using angle-resolved and resonant photoemission spectroscopy with synchrotron radiation. At lower coverages, Cr deposition on TiO2(0 0 1) creates two well separated in-gap emissions due to the formation of surface Ti3+ (3d1) ions and Cr3+ (3d3) ions. At higher coverages, the in-gap emission is developed into the 2-peak-structure emission of Cr 3d character. The corresponding state is considered to be of metallic nature from the viewpoint of the high ability of oxygen adsorption, but has no Fermi edge, indicating a possibility of forming small Cr clusters on TiO2(0 0 1) at this stage. 相似文献
228.
229.
A general framework is developed to treat inverse problems with parameters that are random fields. It involves a sampling
method that exploits the sensitivity derivatives of the control variable with respect to the random parameters. As the sensitivity
derivatives are computed only at the mean values of the relevant parameters, the related extra cost of the present method
is a fraction of the total cost of the Monte Carlo method. The effectiveness of the method is demonstrated on an example problem
governed by the Burgers equation with random viscosity. It is specifically shown that this method is two orders of magnitude
more efficient compared to the conventional Monte Carlo method. In other words, for a given number of samples, the present
method yields two orders of magnitude higher accuracy than its conventional counterpart. 相似文献
230.
An exponentially fitted special second-order finite difference method is presented for solving singularly perturbed two-point boundary value problems with the boundary layer at one end (left or right) point. A fitting factor is introduced in a tri-diagonal finite difference scheme and is obtained from the theory of singular perturbations. Thomas Algorithm is used to solve the system and its stability is investigated. To demonstrate the applicability of the method, we have solved several linear and non-linear problems. From the results, it is observed that the present method approximates the exact solution very well. 相似文献