全文获取类型
收费全文 | 35004篇 |
免费 | 1103篇 |
国内免费 | 265篇 |
专业分类
化学 | 24665篇 |
晶体学 | 280篇 |
力学 | 684篇 |
综合类 | 1篇 |
数学 | 5960篇 |
物理学 | 4782篇 |
出版年
2023年 | 167篇 |
2022年 | 324篇 |
2021年 | 431篇 |
2020年 | 611篇 |
2019年 | 599篇 |
2018年 | 417篇 |
2017年 | 389篇 |
2016年 | 943篇 |
2015年 | 831篇 |
2014年 | 1005篇 |
2013年 | 1706篇 |
2012年 | 2180篇 |
2011年 | 2462篇 |
2010年 | 1329篇 |
2009年 | 1181篇 |
2008年 | 2274篇 |
2007年 | 2093篇 |
2006年 | 2117篇 |
2005年 | 1968篇 |
2004年 | 1687篇 |
2003年 | 1380篇 |
2002年 | 1335篇 |
2001年 | 486篇 |
2000年 | 469篇 |
1999年 | 408篇 |
1998年 | 397篇 |
1997年 | 418篇 |
1996年 | 449篇 |
1995年 | 314篇 |
1994年 | 368篇 |
1993年 | 335篇 |
1992年 | 303篇 |
1991年 | 278篇 |
1990年 | 244篇 |
1989年 | 210篇 |
1988年 | 210篇 |
1987年 | 221篇 |
1986年 | 180篇 |
1985年 | 376篇 |
1984年 | 331篇 |
1983年 | 280篇 |
1982年 | 342篇 |
1981年 | 277篇 |
1980年 | 288篇 |
1979年 | 240篇 |
1978年 | 227篇 |
1977年 | 217篇 |
1976年 | 208篇 |
1975年 | 186篇 |
1974年 | 160篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
Andrey Semichaevsky Markus E. Testorf Robert V. McGahan Michael A. Fiddy 《Waves in Random and Complex Media》2004,14(2):S415-S434
A linear spectral estimation technique, the PDFT algorithm, is used as part of a nonlinear iterative reconstruction scheme to obtain improved radar images. The iterative PDFT algorithm is used to address the limited resolution problem inherent to imaging objects buried in soil and hidden under foliage. This is achieved by subsequent application of two properties of the PDFT algorithm: the energy parameter of the PDFT algorithm is used to determine the target shape, while the shape information in turn is used to obtain super-resolved images. We describe algorithms able to exploit both properties automatically and without manual intervention. Two methods are investigated in particular, one iteratively optimizing the constraints by monitoring the energy parameter, the other method computing energy values for all points, from which a weighted prior function is determined. In addition, we discuss variants of both algorithm which provide an optimized trade-off between computation time and performance. Additional attention is given to situations, where a known target is embedded in an unknown background. Imaging results are presented for both synthetic and measured data. 相似文献
72.
Let G be a d‐regular graph G on n vertices. Suppose that the adjacency matrix of G is such that the eigenvalue λ which is second largest in absolute value satisfies λ = o(d). Let Gp with p = α/d be obtained from G by including each edge of G independently with probability p. We show that if α < 1, then whp the maximum component size of Gp is O(log n) and if α > 1, then Gp contains a unique giant component of size Ω(n), with all other components of size O(log n). © 2003 Wiley Periodicals, Inc. Random Struct. Alg., 2004 相似文献
73.
74.
Microemulsions are becoming increasingly complex systems by containing more sophisticated surfactants, polymers, biomolecules, inorganic nanoparticles, etc. The detailed understanding of such more complex systems requires increasingly more refined and comprehensive characterisation. This is typically done by the combination of complementary techniques and is aided by the fact that several experimental methods have been improved (such as electron microscopy) in recent times, new ones have become available (such as fluorescence correlation spectroscopy), and the theoretical understanding of structural data is advancing. 相似文献
75.
Michael C. W. Chan Dr. 《化学:亚洲杂志》2008,3(1):18-27
The notion of weak attractive ligand–polymer interactions is introduced, and its potential application, importance, and conceptual links with “cooperative” ligand–substrate interactions are discussed. Synthetic models of weak attractive ligand–polymer interactions are described, in which intramolecular weak C? H???F? C interactions (the existence of which remains contentious) have been detected by NMR spectroscopy and neutron and X‐ray diffraction experiments. These C? H???F? C interactions carry important implications for the design of catalysts for olefin polymerization, because they provide support for the practical feasibility of ortho‐F???Hβ ligand–polymer contacts proposed for living Group 4 fluorinated phenoxyimine catalysts. The notion of weak attractive noncovalent interactions between an “active” ligand and the growing polymer chain is a novel concept in polyolefin catalysis. 相似文献
76.
Let Hn be an n-dimensional Haar subspace of
and let Hn−1 be a Haar subspace of Hn of dimension n−1. In this note we show (Theorem 6) that if the norm of a minimal projection from Hn onto Hn−1 is greater than 1, then this projection is an interpolating projection. This is a surprising result in comparison with Cheney and Morris (J. Reine Angew. Math. 270 (1974) 61 (see also (Lecture Notes in Mathematics, Vol. 1449, Springer, Berlin, Heilderberg, New York, 1990, Corollary III.2.12, p. 104) which shows that there is no interpolating minimal projection from C[a,b] onto the space of polynomials of degree n, (n2). Moreover, this minimal projection is unique (Theorem 9). In particular, Theorem 6 holds for polynomial spaces, generalizing a result of Prophet [(J. Approx. Theory 85 (1996) 27), Theorem 2.1]. 相似文献
77.
This paper investigates regression quantiles (RQ) for unstable autoregressive models. The uniform Bahadur representation of the RQ process is obtained. The joint asymptotic distribution of the RQ process is derived in a unified manner for all types of characteristic roots on or outside the unit circle. It involves stochastic integrals in terms of a sequence of independent and identically distributed multivariate Brownian motions with correlated components. The related L-estimator is also discussed. The asymptotic distributions of the RQ and the L-estimator corresponding to the nonstationary componentwise arguments can be transformed into a function of a normal random variable and a sequence of i.i.d. univariate Brownian motions. This is different from the analysis based on the LSE in the literature. As an auxiliary theorem, a weak convergence of a randomly weighted residual empirical process to the stochastic integral of a Kiefer process is established. The results obtained in this paper provide an asymptotic theory for nonstationary time series processes, which can be used to construct robust unit root tests. 相似文献
78.
79.
Classical electrodynamics based on the Maxwell–Born–Infeld field equations coupled with a Hamilton–Jacobi law of point charge motion is partially quantized. The Hamilton–Jacobi phase function is supplemented by a dynamical amplitude field on configuration space. Both together combine into a single complex wave function satisfying a relativistic Klein–Gordon equation that is self-consistently coupled to the evolution equations for the point charges and the electromagnetic fields. Radiation-free stationary states exist. The hydrogen spectrum is discussed in some detail. Upper bounds for Born's “aether constant” are obtained. In the limit of small velocities of and negligible radiation from the point charges, the model reduces to Schrödinger's equation with Coulomb Hamiltonian, coupled with the de Broglie–Bohm guiding equation. 相似文献
80.
A graph with n vertices that contains no triangle and no 5-cycle and minimum degree exceeding n/4 contains an independent set with at least (3n)/7 vertices. This is best possible. The proof proceeds by producing a homomorphism to the 7-cycle and invoking the No Homomorphism Lemma. For k ≥ 4, a graph with n vertices, odd girth 2k+1, and minimum degree exceeding n/(k+1) contains an independent set with at least kn/(2k+1) vertices; however, we suspect this is not best possible. © 1993 John Wiley & Sons, Inc. 相似文献