全文获取类型
收费全文 | 6431篇 |
免费 | 226篇 |
国内免费 | 35篇 |
专业分类
化学 | 4652篇 |
晶体学 | 51篇 |
力学 | 141篇 |
数学 | 894篇 |
物理学 | 954篇 |
出版年
2023年 | 40篇 |
2022年 | 72篇 |
2021年 | 141篇 |
2020年 | 115篇 |
2019年 | 100篇 |
2018年 | 85篇 |
2017年 | 99篇 |
2016年 | 227篇 |
2015年 | 143篇 |
2014年 | 172篇 |
2013年 | 391篇 |
2012年 | 466篇 |
2011年 | 529篇 |
2010年 | 290篇 |
2009年 | 234篇 |
2008年 | 465篇 |
2007年 | 427篇 |
2006年 | 375篇 |
2005年 | 403篇 |
2004年 | 310篇 |
2003年 | 249篇 |
2002年 | 234篇 |
2001年 | 99篇 |
2000年 | 101篇 |
1999年 | 60篇 |
1998年 | 79篇 |
1997年 | 51篇 |
1996年 | 96篇 |
1995年 | 52篇 |
1994年 | 44篇 |
1993年 | 58篇 |
1992年 | 36篇 |
1991年 | 41篇 |
1990年 | 53篇 |
1989年 | 38篇 |
1988年 | 46篇 |
1987年 | 37篇 |
1986年 | 21篇 |
1985年 | 36篇 |
1984年 | 29篇 |
1983年 | 18篇 |
1982年 | 24篇 |
1981年 | 13篇 |
1980年 | 17篇 |
1979年 | 18篇 |
1978年 | 11篇 |
1977年 | 9篇 |
1976年 | 5篇 |
1975年 | 5篇 |
1974年 | 7篇 |
排序方式: 共有6692条查询结果,搜索用时 31 毫秒
21.
Sharp upper and lower bounds are obtained for the reliability functions and the expectations of lifetimes of coherent systems based on dependent exchangeable absolutely continuous components with a given marginal distribution function, by use of the concept of Samaniego's signature. We first show that the distribution of any coherent system based on exchangeable components with absolutely continuous joint distribution is a convex combination of distributions of order statistics (equivalent to the k-out-of-n systems) with the weights identical with the values of the Samaniego signature of the system. This extends the Samaniego representation valid for the case of independent and identically distributed components. Combining the representation with optimal bounds on linear combinations of distribution functions of order statistics from dependent identically distributed samples, we derive the corresponding reliability and expectation bounds, dependent on the signature of the system and marginal distribution of dependent components. We also present the sequences of exchangeable absolutely continuous joint distributions of components which attain the bounds in limit. As an application, we obtain the reliability bounds for all the coherent systems with three and four exchangeable components, expressed in terms of the parent marginal reliability function and specify the respective expectation bounds for exchangeable exponential components, comparing them with the lifetime expectations of systems with independent and identically distributed exponential components. 相似文献
22.
José A. Anquela Teresa Cortés Miguel Gómez-Lozano Mercedes Siles-Molina 《Acta Mathematica Hungarica》2004,103(3):177-196
We investigate the basic properties of the different socles that can be considered in not necessarily semiprime associative
systems. Among other things, we show that the socle defined as the sum of minimal (or minimal and trivial) inner ideals is
always an ideal. When trivial inner ideals are included, this inner socle contains the socles defined in terms of minimal
left or right ideals.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
23.
24.
25.
A noncooperative game governed by a distributed-parameter predator-prey system is considered, assuming that two players control
initial conditions for predator and prey, respectively. Existence of a Nash equilibrium is shown under the condition that
the desired population profiles and the environmental carrying capacity for the prey are sufficiently small. A conceptual
approximation algorithm is proposed and analyzed. Finally, numerical simulations are performed, too. 相似文献
26.
We consider a single machine scheduling problem with two min-sum objective functions: the sum of completion times and the sum of weighted completion times. We propose a simple polynomial time (1+(1/γ),1+γ)-approximation algorithm, and show that for γ>1, there is no (x,y)-approximation with 1<x<1+(1/γ) and 1<y<1+(γ-1)/(2+γ). 相似文献
27.
Parking Capacity and Pricing in Park'n Ride Trips: A Continuous Equilibrium Network Design Problem 总被引:2,自引:0,他引:2
In this paper we consider the problem of designing parking facilities for park'n ride trips. We present a new continuous equilibrium network design problem to decide the capacity and fare of these parking lots at a tactical level. We assume that the parking facilities have already been located and other topological decisions have already been taken.The modeling approach proposed is mathematical programming with equilibrium constraints. In the outer optimization problem, a central Authority evaluates the performance of the transport network for each network design decision. In the inner problem a multimodal traffic assignment with combined modes, formulated as a variational inequality problem, generates the share demand for modes of transportation, and for parking facilities as a function of the design variables of the parking lots. The objective is to make optimal parking investment and pricing decisions in order to minimize the total travel cost in a subnetwork of the multimodal transportation system.We present a new development in model formulation based on the use of generalized parking link cost as a design variable.The bilevel model is solved by a simulated annealing algorithm applied to the continuous and non-negative design decision variables. Numerical tests are reported in order to illustrate the use of the model, and the ability of the approach to solve applications of moderate size. 相似文献
28.
29.
Push-pull dithiafulvenes with reduced bond length alternation (BLA) and high optical nonlinearities have been prepared. The interplay between the proaromaticity of the donor and the structural and optical properties of these merocyanines is discussed. The donor ability of dithiafulvenes can reach that of ferrocene or dialkylaminophenyl groups. 相似文献
30.
We analyze new forms of aberrations affecting a time dependent magnetic field lens recently proposed. The lens consist of an axially symmetric ellipsoidal solenoid producing a spatially uniform but time pulsating homogeneous field. The dynamics of electrons emitted by some object and injected into this coil shows that this system is capable of focusing a parallel beam or produce images free of geometrical aberrations. The cardinal elements of this lens change periodically in positions and therefore to obtain a stationary image it is necessary to employ a chopped electron beam. Two forms of aberrations originate from the finite duration of the electron pulse and the energy dispersion of the incoming beam (chromatic aberration). Their image degradation effects are quite similar and result from recording different axially displaced images at some fixed stationary plane. We show how, by adjusting the parameters characterizing the magnetic pulse and introducing small apertures that increase the depth of focus, their effects can be reduced or in some cases eliminated. Other forms of instrumental aberrations characteristic of this time dependent magnetic field lens are briefly discussed. 相似文献