全文获取类型
收费全文 | 1795篇 |
免费 | 84篇 |
国内免费 | 8篇 |
专业分类
化学 | 1119篇 |
晶体学 | 11篇 |
力学 | 107篇 |
数学 | 326篇 |
物理学 | 324篇 |
出版年
2023年 | 12篇 |
2022年 | 21篇 |
2021年 | 48篇 |
2020年 | 48篇 |
2019年 | 34篇 |
2018年 | 33篇 |
2017年 | 37篇 |
2016年 | 68篇 |
2015年 | 75篇 |
2014年 | 62篇 |
2013年 | 100篇 |
2012年 | 120篇 |
2011年 | 147篇 |
2010年 | 68篇 |
2009年 | 81篇 |
2008年 | 131篇 |
2007年 | 105篇 |
2006年 | 89篇 |
2005年 | 91篇 |
2004年 | 76篇 |
2003年 | 57篇 |
2002年 | 51篇 |
2001年 | 22篇 |
2000年 | 24篇 |
1999年 | 11篇 |
1998年 | 12篇 |
1997年 | 20篇 |
1996年 | 14篇 |
1995年 | 10篇 |
1994年 | 15篇 |
1993年 | 15篇 |
1992年 | 11篇 |
1991年 | 12篇 |
1990年 | 13篇 |
1989年 | 16篇 |
1988年 | 12篇 |
1987年 | 5篇 |
1986年 | 7篇 |
1985年 | 9篇 |
1984年 | 11篇 |
1983年 | 8篇 |
1982年 | 9篇 |
1981年 | 13篇 |
1980年 | 9篇 |
1979年 | 11篇 |
1978年 | 3篇 |
1977年 | 4篇 |
1974年 | 5篇 |
1970年 | 3篇 |
1901年 | 2篇 |
排序方式: 共有1887条查询结果,搜索用时 15 毫秒
991.
Nonsmooth optimization via quasi-Newton methods 总被引:1,自引:0,他引:1
We investigate the behavior of quasi-Newton algorithms applied to minimize a nonsmooth function f, not necessarily convex. We introduce an inexact line search that generates a sequence of nested intervals containing a set of points of nonzero measure that satisfy the Armijo and Wolfe conditions if f is absolutely continuous along the line. Furthermore, the line search is guaranteed to terminate if f is semi-algebraic. It seems quite difficult to establish a convergence theorem for quasi-Newton methods applied to such general classes of functions, so we give a careful analysis of a special but illuminating case, the Euclidean norm, in one variable using the inexact line search and in two variables assuming that the line search is exact. In practice, we find that when f is locally Lipschitz and semi-algebraic with bounded sublevel sets, the BFGS (Broyden–Fletcher–Goldfarb–Shanno) method with the inexact line search almost always generates sequences whose cluster points are Clarke stationary and with function values converging R-linearly to a Clarke stationary value. We give references documenting the successful use of BFGS in a variety of nonsmooth applications, particularly the design of low-order controllers for linear dynamical systems. We conclude with a challenging open question. 相似文献
992.
This paper studies periodic traveling gravity waves at the free surface of water in a flow of constant vorticity over a flat
bed. Using conformal mappings the free-boundary problem is transformed into a quasilinear pseudodifferential equation for
a periodic function of one variable. The new formulation leads to a regularity result and, by use of bifurcation theory, to
the existence of waves of small amplitude even in the presence of stagnation points in the flow. 相似文献
993.
Adrian Williams 《代数通讯》2013,41(5):1599-1613
994.
995.
We consider those graphs G that admit decompositions into copies of a fixed graph F, each copy being an induced subgraph of G. We are interested in finding the extremal graphs with this property, that is, those graphs G on n vertices with the maximum possible number of edges. We discuss the cases where F is a complete equipartite graph, a cycle, a star, or a graph on at most four vertices. 相似文献
996.
Adrian E. Raftery Xiaoyue Niu Peter D. Hoff Ka Yee Yeung 《Journal of computational and graphical statistics》2013,22(4):901-919
Multilevel modeling is a popular statistical technique for analyzing data in hierarchical format, and thus naturally fits within a distributed database framework. We consider the computational aspects of multilevel modeling across distributed databases. In addition, we consider these aspects under a generalization of the multilevel model where the distributed groups (or databases) are allowed to specify different models at both level-1 (individual) and level-2 (group). For a variety of scenarios, we develop the distributed computation algorithm for two-step least squares (LS) estimators and also for iterative MLE estimators of the parameters of interest; in particular, we determine the required data structure at each computing site, the necessary information (original data, cross-product matrices, coefficient vectors), and the order in which such information needs to be passed between sites. Finally, we discuss recursive updating, fault tolerance, and security issues. 相似文献
997.
Evans CL Morris GA Davis AL 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2002,154(2):325-328
Sample convection can severely attenuate the signals observed in pulsed field gradient spin--echo experiments such as those used for gradient shimming. A new class of pulse sequences is proposed, in which a double spin--echo refocuses the phase errors caused by sample convection, enabling gradient shimming to be performed reliably over a wide range of temperatures. 相似文献
998.
A function, F, on the space of n×n real symmetric matrices is called spectral if it depends only on the eigenvalues of its argument, that is F(A)=F(UAUT) for every orthogonal U and symmetric A in its domain. Spectral functions are in one-to-one correspondence with the symmetric functions on
: those that are invariant under arbitrary swapping of their arguments. In this paper we show that a spectral function has a quadratic expansion around a point A if and only if its corresponding symmetric function has quadratic expansion around λ(A) (the vector of eigenvalues). We also give a concise and easy to use formula for the ‘Hessian' of the spectral function. In the case of convex functions we show that a positive definite ‘Hessian' of f implies positive definiteness of the ‘Hessian' of F. 相似文献
999.
Ionel Stavarache Ana-Maria Lepadatu Nicoleta G. Gheorghe Ruxandra M. Costescu George E. Stan Dan Marcov Adrian Slav Gheorghe Iordache Tionica F. Stoica Vladimir Iancu Valentin S. Teodorescu Cristian M. Teodorescu Magdalena Lidia Ciurea 《Journal of nanoparticle research》2011,13(1):221-232
Transmission electron microscopy and X-ray photoelectron spectroscopy analyses are performed to investigate Ge nanoparticles embedded in an amorphous SiO2 matrix. GeSiO thin films are prepared by two methods, sol?Cgel and radio frequency magnetron sputtering. After the deposition, the sol?Cgel films are annealed in either N2 (at 1 atm and 800 °C) or H2 (at 2 atm and 500 °C), and the sputtered films in H2 (at 2 atm and 500 °C), to allow Ge segregation. Amorphous Ge-rich nanoparticles (3?C7 nm size) are observed in sol?Cgel films. Crystalline Ge nanoparticles in the high pressure tetragonal phase (10?C50 nm size) are identified in the sputtered films. The size of the nanoparticles increases with Ge concentration in the volume of the film. At the film surface, the Ge concentration is much larger that in the volume for both sol?Cgel and sputtered films. At the same time, at the film surface, only oxidized Ge is observed. 相似文献
1000.
Dougherty Steven T. Gildea Joe Korban Adrian Kaya Abidin 《Designs, Codes and Cryptography》2021,89(7):1615-1638
Designs, Codes and Cryptography - In this work, we define composite matrices which are derived from group rings. We extend the idea of G-codes to composite G-codes. We show that these codes are... 相似文献