全文获取类型
收费全文 | 24375篇 |
免费 | 651篇 |
国内免费 | 141篇 |
专业分类
化学 | 16701篇 |
晶体学 | 123篇 |
力学 | 575篇 |
数学 | 4334篇 |
物理学 | 3434篇 |
出版年
2022年 | 177篇 |
2021年 | 259篇 |
2020年 | 301篇 |
2019年 | 317篇 |
2018年 | 233篇 |
2017年 | 194篇 |
2016年 | 481篇 |
2015年 | 453篇 |
2014年 | 490篇 |
2013年 | 1300篇 |
2012年 | 1137篇 |
2011年 | 1429篇 |
2010年 | 725篇 |
2009年 | 707篇 |
2008年 | 1256篇 |
2007年 | 1279篇 |
2006年 | 1251篇 |
2005年 | 1210篇 |
2004年 | 1052篇 |
2003年 | 910篇 |
2002年 | 816篇 |
2001年 | 347篇 |
2000年 | 278篇 |
1999年 | 248篇 |
1998年 | 276篇 |
1997年 | 307篇 |
1996年 | 358篇 |
1995年 | 239篇 |
1994年 | 279篇 |
1993年 | 262篇 |
1992年 | 237篇 |
1991年 | 240篇 |
1990年 | 191篇 |
1989年 | 231篇 |
1988年 | 237篇 |
1987年 | 201篇 |
1986年 | 195篇 |
1985年 | 291篇 |
1984年 | 336篇 |
1983年 | 218篇 |
1982年 | 366篇 |
1981年 | 336篇 |
1980年 | 316篇 |
1979年 | 317篇 |
1978年 | 322篇 |
1977年 | 292篇 |
1976年 | 280篇 |
1975年 | 250篇 |
1974年 | 248篇 |
1973年 | 234篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
82.
Near-infrared calibrations for water, reducing sugar, glycerin, propylene glycol, nicotine, and menthol in tobacco blends were first obtained with a monochromator instrument. Transfer of calibrations to a tilting-filter instrument for factory trials is shown to be facilitated greatly by the computer-assisted scheme MTRAN. By means of this software, calibration equations were established for a Neotec 51A instrument that is sufficiently rugged for factory use. The predictive capabilities of the transferred calibrations are similar to those of the research monochromator. 相似文献
83.
The qualitative behavior of buckled states of two different models of elastic beams is studied. It is assumed that random imperfections affect the governing nonlinear equations. It is shown that near the first critical value of the buckling load the stochastic bifurcation is described asymptotically by an algebraic equation whose coeffficients are Gaussian random variables. The corresponding asymptotic expansion for the displacement is to lowest order a Gaussian stochastic process.Work supported by NSF Grant No. DCR81-14726.Work supported by NSF Grant No. DMS87-01895. 相似文献
84.
Numerical methods for ordinary initial value problems that do not depend on special properties of the system are usually found in the class of linear multistage multivalue methods, first formulated by J.C. Butcher. Among these the explicit methods are easiest to implement. For these reasons there has been considerable research activity devoted to generating methods of this class which utilize independent function evaluations that can be performed in parallel. Each such group of concurrent function evaluations can be regarded as a stage of the method. However, it turns out that parallelism affords only limited opportunity for reducing the computing time with such methods. This is most evident for the simple linear homogeneous constant-coefficient test problem, whose solution is essentially a matter of approximating the exponential by an algebraic function. For a given number of stages and a given number of saved values, parallelism offers a somewhat enlarged set of algebraic functions from which to choose. However, there is absolutely no benefit in having the degree of parallelism (number of processors) exceed the number of saved values of the method. Thus, in particular, parallel one-step methods offer no speedup over serial one-step methods for the standard linear test problem. Although the implication of this result for general nonlinear problems is unclear, there are indications that dramatic speedups are not possible in general. Also given are some results relevant to the construction of methods.Work supported in part by National Science Foundation grants DMS 89 11410 and DMS 90 15533 and by US Department of Energy grant DOE DEFG02-87ER25026. Work of the second author was completed while at the University of Illinois. 相似文献
85.
In this paper a statistical multiplexer that processes a mixture of real-time and non-real-time traffic is studied under bursts of traffic. Different scheduling algorithms are compared under conditions when one of the classes of traffic has a sudden increase in its arrival rate during a short period of time. The results show a difference in the way the scheduling disciplines studied behave under short overloads of traffic even though the scheduling algorithms had been set up to give similar performance under steady-state arrivals. The lifetime of real-time packets is shown to have a great effect on the way in which the performance of the scheduling algorithms compare.Robert Lackman is an IBM employee in the IBM Resident Study Program. 相似文献
86.
Robert J. Daverman 《Topology and its Applications》2007,155(2):105-111
This paper sets forth three mismatch properties, strictly ordered in strength, about sewings of crumpled n-cubes. The strongest is a sufficient but not a necessary condition for a sewing to yield Sn, and the weakest, a necessary but not sufficient one. We show that when both crumpled cubes satisfies the Disjoint Disks Property, then the weakest property implies the sewing yields Sn, and we also show that the intermediate property leads to the same conclusion when just one of the crumpled cubes possesses the Disjoint Disks Property. In addition, we develop examples that confirm sharpness of the relevant Disjoint Disks conditions. 相似文献
87.
Physical aspects of an operation of the GaAs-based InGaAs/GaAs quantum-well (QW) VCSELs with the intentionally detuned optical
cavities have been considered in the present paper using the comprehensive three-dimensional self-consistent optical–electrical–thermal-gain
simulation. In GaAs-based structures, very good DBR resonator mirrors and a very efficient methods to confine radially both
the current spreading and the electromagnetic field with the aid of oxide apertures may be applied. It has been found using
the above simulation that even currently available immature technology enables manufacturing the above devices emitting radiation
of wavelengths over 1.20 μm. In particular, while the room-temperature 1.30-μm lasing emission is still beyond possibilities
of the InGaAs/GaAs QW VCSELs, these structures may offer analogous 1.25-μm emission, especially for the high-power and/or
high-temperature operation. 相似文献
88.
We evaluate the practical relevance of two measures of conic convex problem complexity as applied to second-order cone problems solved using the homogeneous self-dual (HSD) embedding model in the software SeDuMi. The first measure we evaluate is Renegar's data-based condition measure C(d), and the second measure is a combined measure of the optimal solution size and the initial infeasibility/optimality residuals denoted by S (where the solution size is measured in a norm that is naturally associated with the HSD model). We constructed a set of 144 second-order cone test problems with widely distributed values of C(d) and S and solved these problems using SeDuMi. For each problem instance in the test set, we also computed estimates of C(d) (using Peña’s method) and computed S directly. Our computational experience indicates that SeDuMi iteration counts and log (C(d)) are fairly highly correlated (sample correlation R = 0.675), whereas SeDuMi iteration counts are not quite as highly correlated with S (R = 0.600). Furthermore, the experimental evidence indicates that the average rate of convergence of SeDuMi iterations is affected by the condition number C(d) of the problem instance, a phenomenon that makes some intuitive sense yet is not directly implied by existing theory. 相似文献
89.
Robert McKelvey 《Natural Resource Modeling》1986,1(1):171-189
Fishing vessels, harvesting geographically dispersed and heterogeneous multispecies aggregates, are presumed to target on the most profitable pools. But, this behavior may lead to depletion of biologically valuable species, and consequent early closure by the management agency of the entire fishery. In this article, a simple model is analyzed in order to understand the dynamics of such targeting behavior. Further analysis explores the theoretical extent to which targeting might be altered through economic rationalization measures, landings taxes or quota markets, to achieve a more nearly optimal pattern of exploitation. 相似文献
90.