全文获取类型
收费全文 | 20060篇 |
免费 | 404篇 |
国内免费 | 509篇 |
专业分类
化学 | 2577篇 |
晶体学 | 109篇 |
力学 | 186篇 |
综合类 | 30篇 |
数学 | 15866篇 |
物理学 | 2205篇 |
出版年
2023年 | 58篇 |
2022年 | 105篇 |
2021年 | 132篇 |
2020年 | 113篇 |
2019年 | 412篇 |
2018年 | 400篇 |
2017年 | 269篇 |
2016年 | 190篇 |
2015年 | 252篇 |
2014年 | 432篇 |
2013年 | 1441篇 |
2012年 | 641篇 |
2011年 | 1181篇 |
2010年 | 977篇 |
2009年 | 1243篇 |
2008年 | 1517篇 |
2007年 | 1572篇 |
2006年 | 1277篇 |
2005年 | 873篇 |
2004年 | 739篇 |
2003年 | 580篇 |
2002年 | 468篇 |
2001年 | 463篇 |
2000年 | 444篇 |
1999年 | 477篇 |
1998年 | 400篇 |
1997年 | 355篇 |
1996年 | 363篇 |
1995年 | 414篇 |
1994年 | 400篇 |
1993年 | 309篇 |
1992年 | 267篇 |
1991年 | 173篇 |
1990年 | 164篇 |
1989年 | 159篇 |
1988年 | 116篇 |
1987年 | 141篇 |
1986年 | 133篇 |
1985年 | 184篇 |
1984年 | 133篇 |
1983年 | 95篇 |
1982年 | 156篇 |
1981年 | 130篇 |
1980年 | 138篇 |
1979年 | 86篇 |
1978年 | 120篇 |
1977年 | 100篇 |
1976年 | 98篇 |
1975年 | 23篇 |
1974年 | 29篇 |
排序方式: 共有10000条查询结果,搜索用时 11 毫秒
41.
LetA be anM-matrix in standard lower block triangular form, with diagonal blocksA
ii irreducible. LetS be the set of indices such that the diagonal blockA
is singular. We define the singular graph ofA to be the setS with partial order defined by > if there exists a chain of non-zero blocksA
i, Aij, , Al.Let 1 be the set of maximal elements ofS, and define thep-th level
p
,p = 2, 3, , inductively as the set of maximal elements ofS
\(
1
p-1). Denote by
p
the number of elements in
p
. The Weyr characteristic (associated with 0) ofA is defined to be (A) = (
1, 2,,
h
), where
1 + +
p
= dim KerA
p
,p = 1, 2, , and
h > 0, h+1 = 0.Using a special type of basis, called anS-basis, for the generalized eigenspaceE(A) of 0 ofA, we associate a matrixD withA. We show that(A) = (
1, , h) if and only if certain submatricesD
p,p+1
,p = 1, , h – 1, ofD have full column rank. This condition is also necessary and sufficient forE(A) to have a basis consisting of non-negative vectors, which is a Jordan basis for –A.
We also consider a given finite partially ordered setS, and we find a necessary and sufficient condition that allM-matricesA with singular graphS have(A) = (
1, , h). This condition is satisfied ifS is a rooted forest.The work of the second-named author was partly supported by the National Science Foundation, under grant MPS-08618 A02. 相似文献
42.
B. Gyires 《Periodica Mathematica Hungarica》1978,9(1-2):109-113
This paper contains applications of theorems of [1] for quadratic statistics which have constant regression on linear statistics. Two theorems are proved. The first is a sufficient condition which assumes that the characteristic function of a sample is an entire function. The second gives a new characterization of the normal distribution. 相似文献
43.
Many polysaccharide chains can adopt ordered helical and ribbon-like secondary structures. It seems however that these chains are often so stiff and extended that the cooperative interactions necessary for stability in the solvent environment can only be achieved when inter-chain as well as intra-chain interactions are favorable. Hence we commonly find two-or more-stranded associations of helices, of ribbons, or of helices with ribbons. These can be regarded as tertiary and higher levels of structure. The ordered secondary structure characteristically requires a regular repeating sequence of sugar residues, and the termination of this sequence by insertion of a residue of different type may also terminate the secondary structure and hence the association in which it is involved. This is the mechanism by which native polysaccharides may link up to form three dimensional networks, or gels, in which state they perform their natural roles in maintaining the hydration and integrity of biological tissues. For several polysaccharides there is evidence that the mechanism of biological control over the fine topology and properties of the gel network is mediated by enzymes which modify sugar residues at the polymer level to change the pattern of “interrupting” sugar residues. 相似文献
44.
Shie-Shien Yang 《Annals of the Institute of Statistical Mathematics》1981,33(1):463-470
Summary Let (X
1,Y
1), (X
2,Y
2),…, (X
n,Y
n) be i.i.d. as (X, Y). TheY-variate paired with therth orderedX-variateX
rn is denoted byY
rn and terms the concomitant of therth order statistic. Statistics of the form
are considered. The asymptotic normality ofT
n is established. The asymptotic results are used to test univariate and bivariate normality, to test independence and linearity
ofX andY, and to estimate regression coefficient based on complete and censored samples. 相似文献
45.
H. Groemer 《Aequationes Mathematicae》1981,22(1):215-222
In the euclidean planeE
2 letS
1,S
2, ... be a sequence of strips of widthsw
1,w
2, .... It is shown thatE
2 can be covered by translates of the stripsS
i if w
1
3/2
= . Further results concern conditions in order that a compact convex domain inE
2 can be covered by translates ofS
1,S
2, ....This research was supported by National Science Foundation Research Grant MCS 76-06111. 相似文献
46.
47.
B. Rendi 《Periodica Mathematica Hungarica》1977,8(3-4):225-228
Sans résumé
The article is the text of a talk given at the Symposium on Differential Geometry in Debrecen, Hungary, on August 28–September 3, 1975. 相似文献
48.
Frank J.S. Wang 《Stochastic Processes and their Applications》1977,5(2):173-193
Consider a population consisting of one type of individual living in a fixed region with area A. In [8], we constructed a stochastic population model in which the death rate is affected by the age of the individual and the birth rate is affected by the population density PA(t), i.e., the population size divided by the area A of the given region. In [8], we proposed a continuous deterministic model which in general is a nonlinear Volterra type integral equation and proved that under appropriate conditions the sequence PA(t) would converge to the solution P(t) of our integral equation in the sense that .In this paper, we obtain a “central limit theorem” for the random element √A(PA(t)?P(t)). We prove that under appropriate conditions √A(PA(t)?P(t)) will converge to a Gaussian process. (See Theorem 3.4 for the explicit formula of this Gaussian process.) 相似文献
49.
50.
The molecular structure of 1-formyl-3-phenyl-Δ2-pyrazoline was determined by X-ray crystallography (triclinic, P-1). The geometry thus obtained was compared with that obtained by DFT calculations. The GIAO method was used to calculate absolute shieldings, which agree conveniently with those measured by 13C and 15N NMR. The title compound appears to be an essentially planar molecule. 相似文献