全文获取类型
收费全文 | 10765篇 |
免费 | 1683篇 |
国内免费 | 482篇 |
专业分类
化学 | 1924篇 |
晶体学 | 44篇 |
力学 | 1721篇 |
综合类 | 208篇 |
数学 | 5658篇 |
物理学 | 3375篇 |
出版年
2024年 | 16篇 |
2023年 | 145篇 |
2022年 | 259篇 |
2021年 | 307篇 |
2020年 | 259篇 |
2019年 | 244篇 |
2018年 | 244篇 |
2017年 | 462篇 |
2016年 | 530篇 |
2015年 | 469篇 |
2014年 | 591篇 |
2013年 | 806篇 |
2012年 | 573篇 |
2011年 | 582篇 |
2010年 | 526篇 |
2009年 | 646篇 |
2008年 | 680篇 |
2007年 | 685篇 |
2006年 | 532篇 |
2005年 | 424篇 |
2004年 | 427篇 |
2003年 | 437篇 |
2002年 | 379篇 |
2001年 | 276篇 |
2000年 | 290篇 |
1999年 | 278篇 |
1998年 | 245篇 |
1997年 | 190篇 |
1996年 | 182篇 |
1995年 | 161篇 |
1994年 | 123篇 |
1993年 | 99篇 |
1992年 | 76篇 |
1991年 | 81篇 |
1990年 | 68篇 |
1989年 | 55篇 |
1988年 | 59篇 |
1987年 | 49篇 |
1986年 | 40篇 |
1985年 | 68篇 |
1984年 | 36篇 |
1983年 | 28篇 |
1982年 | 36篇 |
1981年 | 30篇 |
1980年 | 47篇 |
1979年 | 46篇 |
1978年 | 27篇 |
1977年 | 37篇 |
1976年 | 38篇 |
1974年 | 13篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
901.
902.
Fritz Colonius Wolfgang Kliemann 《Transactions of the American Mathematical Society》2008,360(11):5667-5682
This paper proposes a topological framework for the analysis of the time shift on behaviors. It is shown that controllability is not a property of the time shift, while chain controllability is. This also leads to a global decomposition of behaviors.
903.
In high power laser systems, the stability of the seed laser is vitally important. This paper presents the work on the design and manufacture of a highly stable LD pumped solid state seed laser which has the advantages of being all solid state, compact, easy to manufacture and low cost. The laser was composed of two parts. One was an LD pumped 0.5 mm thin Nd: YLF laser system in which the instability was around±6% (3σ). The other was a close-looped laser power control system using external cavity power feedback. Using an electro-optic modulator, a 1×2 optical fiber splitter and a signal processing circuit, we successfully realized the highly stable LD pumped Nd: YLF laser in which the instability of laser output power was lower than±0.2 %/hour (3σ). A digital PID (Proportion, Integration and Differentiation) control method was used with a control frequency of 40 kHz. The 3 mW of highly stable laser power through the optical fiber was obtained. 相似文献
904.
We propose a novel approach to modeling advertising dynamics for a firm operating over a distributed market domain based on
controlled partial differential equations of the diffusion type. Using our model, we consider a general type of finite-horizon
profit maximization problem in a monopoly setting. By reformulating this profit maximization problem as an optimal control
problem in infinite dimensions, we derive sufficient conditions for the existence of its optimal solutions under general profit
functions, as well as state and control constraints, and provide a general characterization of the optimal solutions. Sharper,
feedback-form characterizations of the optimal solutions are obtained for two variants of the general problem.
The first author gratefully acknowledges financial support by the NSF, the DAAD, the SFB 611 (Bonn), and the Max-Planck-Institut
für Mathematik (Leipzig) through an IPDE fellowship. 相似文献
905.
Wolfgang J. Philipp Pierre van Iwaarden Heinz Schimmel Nele Meeus Nadine Kollmorgen 《Accreditation and quality assurance》2007,12(3-4):134-138
The reliability of reference materials (RMs) depends on properties such as fitness, robustness, commutability, stability and
homogeneity. The development of RMs for microbiological analysis is especially challenged through questions around the stabilisation
and recovery of viable cells, the dispersion of precise numbers of cells, matrix effects and, when using molecular techniques,
the presence of nucleic acids (e.g. DNA) of dead and live target organisms. However, RMs are indispensable tools for quality
control in microbiological analysis. The Institute for Reference Materials and Measurements (IRMM), as part of the European
Commission, concentrates its efforts on the development of RMs to support the development, implementation and monitoring of
EU legislation. A special focus is given to highly precise RMs for presence/absence and enumeration tests in microbiological
food and water analysis. Another group of new RMs certified by the IRMM comprise DNA-based materials to control the identity
of micro-organisms in qualitative assays. All of these activities serve to improve quality control in microbiological analysis.
Presented at ‘BERM-10’, April 2006, Charleston, SC, USA. 相似文献
906.
This paper addresses an application of ORTHOMIN and GMRES to petroleum reservoir simulation using the black oil model on unstructured grids. Comparisons between these two algorithms are presented in terms of storage and total flops per restart step. Numerical results indicate that GMRES is faster than ORTHOMIN for all tested petroleum reservoir problems, particularly for large scale problems. The control volume function approximation method is utilized in the discretization of the governing equations of the black oil model. This method can accurately approximate both the pressure and velocity in the simulation of multiphase flow in porous media, effectively reduce grid orientation effects, and be easily applied to arbitrarily shaped control volumes. It is particularly suitable for hybrid grid reservoir simulation. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
907.
908.
This paper concerns a due-date matching problem in a single-stage manufacturing system. Given a finite sequence of jobs and their service order, and given the delivery due date of each job, the problem is to choose the jobs release (arrival) times so as to match as closely as possible their completion times to their respective due dates. The system is modelled as a deterministic single-server FIFO queue with an output buffer for storing jobs whose service is completed prior to their due dates. The output buffer has a finite capacity; when it is full, the server is being blocked. Associated with each job there is a convex cost function penalizing its earliness as well as tardiness. The due-date matching problem is cast as an optimal control problem, whose objective is to minimize the sum of the above cost functions by the choice of the jobs arrival (release) times. Time-box upper-bound and lower-bound constraints are imposed on the jobs output (delivery) times. The optimal-control setting brings to bear on the development of fast and efficient algorithms having intuitive geometric appeal and potential for online implementation.Communicated by W. B. GongResearch supported in part by the National Science Foundation under Grant ECS-9979693 and by the Georgia Tech Manufacturing Research Center under Grant B01-D06. 相似文献
909.
910.
Amir Azaron Reza Tavakkoli-Moghaddam 《European Journal of Operational Research》2007,180(3):1186-1200
We develop a multi-objective model for the time–cost trade-off problem in a dynamic PERT network using an interactive approach. The activity durations are exponentially distributed random variables and the new projects are generated according to a renewal process and share the same facilities. Thus, these projects cannot be analyzed independently. This dynamic PERT network is represented as a network of queues, where the service times represent the durations of the corresponding activities and the arrival stream to each node follows a renewal process. At the first stage, we transform the dynamic PERT network into a proper stochastic network and then compute the project completion time distribution by constructing a continuous-time Markov chain. At the second stage, the time–cost trade-off problem is formulated as a multi-objective optimal control problem that involves four conflicting objective functions. Then, the STEM method is used to solve a discrete-time approximation of the original problem. Finally, the proposed methodology is extended to the generalized Erlang activity durations. 相似文献