全文获取类型
收费全文 | 13055篇 |
免费 | 249篇 |
国内免费 | 10篇 |
专业分类
化学 | 47篇 |
力学 | 119篇 |
综合类 | 1篇 |
数学 | 12656篇 |
物理学 | 457篇 |
无线电 | 34篇 |
出版年
2022年 | 22篇 |
2021年 | 14篇 |
2020年 | 27篇 |
2019年 | 257篇 |
2018年 | 305篇 |
2017年 | 137篇 |
2016年 | 116篇 |
2015年 | 122篇 |
2014年 | 300篇 |
2013年 | 799篇 |
2012年 | 347篇 |
2011年 | 807篇 |
2010年 | 638篇 |
2009年 | 820篇 |
2008年 | 975篇 |
2007年 | 986篇 |
2006年 | 681篇 |
2005年 | 496篇 |
2004年 | 394篇 |
2003年 | 307篇 |
2002年 | 232篇 |
2001年 | 211篇 |
2000年 | 218篇 |
1999年 | 327篇 |
1998年 | 292篇 |
1997年 | 236篇 |
1996年 | 298篇 |
1995年 | 301篇 |
1994年 | 289篇 |
1993年 | 254篇 |
1992年 | 235篇 |
1991年 | 169篇 |
1990年 | 145篇 |
1989年 | 159篇 |
1988年 | 98篇 |
1987年 | 115篇 |
1986年 | 103篇 |
1985年 | 165篇 |
1984年 | 157篇 |
1983年 | 86篇 |
1982年 | 144篇 |
1981年 | 138篇 |
1980年 | 84篇 |
1979年 | 60篇 |
1978年 | 87篇 |
1977年 | 63篇 |
1976年 | 47篇 |
1975年 | 15篇 |
1974年 | 11篇 |
1973年 | 10篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
A self-avoiding polygon (SAP) on a graph is an elementary cycle. Counting SAPs on the hypercubic lattice ℤ
d
withd≥2, is a well-known unsolved problem, which is studied both for its combinatorial and probabilistic interest and its connections
with statistical mechanics. Of course, polygons on ℤ
d
are defined up to a translation, and the relevant statistic is their perimeter.
A SAP on ℤ
d
is said to beconvex if its perimeter is “minimal”, that is, is exactly twice the sum of the side lengths of the smallest hyper-rectangle containing
it. In 1984, Delest and Viennot enumerated convex SAPs on the square lattice [6], but no result was available in a higher
dimension.
We present an elementar approach to enumerate convex SAPs in any dimension. We first obtain a new proof of Delest and Viennot's
result, which explains combinatorially the form of the generating function. We then compute the generating function for convex
SAPs on the cubic lattice. In a dimension larger than 3, the details of the calculations become very cumbersome. However,
our method suggests that the generating function for convex SAPs on ℤ
d
is always a quotient ofdifferentiably finite power series. 相似文献
52.
Sans résumé
相似文献
相似文献
53.
Properties of nonparametric estimators of autocovariance for stationary random fields 总被引:1,自引:0,他引:1
Summary We introduce nonparametric estimators of the autocovariance of a stationary random field. One of our estimators has the property that it is itself an autocovatiance. This feature enables the estimator to be used as the basis of simulation studies such as those which are necessary when constructing bootstrap confidence intervals for unknown parameters. Unlike estimators proposed recently by other authors, our own do not require assumptions such as isotropy or monotonicity. Indeed, like nonparametric function estimators considered more widely in the context of curve estimation, our approach demands only smoothness and tail conditions on the underlying curve or surface (here, the autocovariance), and moment and mixing conditions on the random field. We show that by imposing the condition that the estimator be a covariance function we actually reduce the numerical value of integrated squared error. 相似文献
54.
Lévy processes in matrix Lie groups are studied. Subordination (random time change) is used to show that quasi-invariance of the Brownian motion in a Lie group induces absolute continuity of the laws of the corresponding pure jump processes. These results are applied to several examples which are discussed in detail. 相似文献
55.
Roberto E. Madariaga-Garcia 《Topology and its Applications》2007,154(7):1470-1480
Tkachenko showed in 1990 the existence of a countably compact group topology on the free Abelian group of size c using CH. Koszmider, Tomita and Watson showed in 2000 the existence of a countably compact group topology on the free Abelian group of size c2 using a forcing model in which CH holds.Wallace's question from 1955, asks whether every both-sided cancellative countably compact semigroup is a topological group. A counterexample to Wallace's question has been called a Wallace semigroup. In 1996, Robbie and Svetlichny constructed a Wallace semigroup under CH. In the same year, Tomita constructed a Wallace semigroup from MAcountable.In this note, we show that the examples of Tkachenko, Robbie and Svetlichny, and Koszmider, Tomita and Watson can be obtained using a family of selective ultrafilters. As a corollary, the constructions presented here are compatible with the total failure of Martin's Axiom. 相似文献
56.
We classify all the global phase portraits of the quadratic polynomial vector fields having a rational first integral of degree 2. In other words we characterize all the global phase portraits of the quadratic polynomial vector fields having all their orbits contained in conics. For such a vector field there are exactly 25 different global phase portraits in the Poincaré disc, up to a reversal of sense. 相似文献
57.
M. Frigon 《Journal of Fixed Point Theory and Applications》2007,1(2):189-194
We present results on the global existence of analytic solutions to the Cauchy problem in starshaped or convex complex domains.
No growth conditions are imposed. Our results rely on a notion of solution-tube that we introduce.
à la mémoire de Jean Leray 相似文献
58.
This paper establishes a link between a generalized matrix Matsumoto-Yor (MY) property and the Wishart distribution. This link highlights certain conditional independence properties within blocks of the Wishart and leads to a new characterization of the Wishart distribution similar to the one recently obtained by Geiger and Heckerman but involving independences for only three pairs of block partitionings of the random matrix.In the process, we obtain two other main results. The first one is an extension of the MY independence property to random matrices of different dimensions. The second result is its converse. It extends previous characterizations of the matrix generalized inverse Gaussian and Wishart seen as a couple of distributions.We present two proofs for the generalized MY property. The first proof relies on a new version of Herz's identity for Bessel functions of matrix arguments. The second proof uses a representation of the MY property through the structure of the Wishart. 相似文献
59.
We introduce symmetric Boundary Value Methods for the solution of second order initial and boundary value problems (in particular
Hamiltonian problems). We study the conditioning of the methods and link it to the boundary loci of the roots of the associated
characteristic polynomial. Some numerical tests are provided to assess their reliability.
Dedicated to the memory of Professor Aldo Cossu 相似文献
60.
The authors give a consistent affirmative response to a question of Juhász, Soukup and Szentmiklóssy: If GCH fails, there are (many) extraresolvable, not maximally resolvable Tychonoff spaces. They show also in ZFC that for ω<λ?κ, no maximal λ-independent family of λ-partitions of κ is ω-resolvable. In topological language, that theorem translates to this: A dense, ω-resolvable subset of a space of the form (DI(λ)) is λ-resolvable. 相似文献