全文获取类型
收费全文 | 4434篇 |
免费 | 258篇 |
国内免费 | 106篇 |
专业分类
化学 | 3315篇 |
晶体学 | 21篇 |
力学 | 237篇 |
综合类 | 2篇 |
数学 | 525篇 |
物理学 | 698篇 |
出版年
2024年 | 4篇 |
2023年 | 24篇 |
2022年 | 96篇 |
2021年 | 130篇 |
2020年 | 157篇 |
2019年 | 207篇 |
2018年 | 253篇 |
2017年 | 202篇 |
2016年 | 357篇 |
2015年 | 229篇 |
2014年 | 305篇 |
2013年 | 558篇 |
2012年 | 378篇 |
2011年 | 346篇 |
2010年 | 273篇 |
2009年 | 217篇 |
2008年 | 247篇 |
2007年 | 174篇 |
2006年 | 123篇 |
2005年 | 93篇 |
2004年 | 105篇 |
2003年 | 65篇 |
2002年 | 54篇 |
2001年 | 22篇 |
2000年 | 23篇 |
1999年 | 20篇 |
1998年 | 13篇 |
1997年 | 3篇 |
1996年 | 11篇 |
1995年 | 12篇 |
1994年 | 7篇 |
1993年 | 7篇 |
1992年 | 9篇 |
1991年 | 4篇 |
1990年 | 5篇 |
1989年 | 7篇 |
1988年 | 5篇 |
1987年 | 2篇 |
1986年 | 3篇 |
1985年 | 7篇 |
1984年 | 6篇 |
1983年 | 6篇 |
1982年 | 3篇 |
1981年 | 3篇 |
1980年 | 4篇 |
1979年 | 2篇 |
1977年 | 4篇 |
1975年 | 3篇 |
1937年 | 2篇 |
1927年 | 3篇 |
排序方式: 共有4798条查询结果,搜索用时 31 毫秒
951.
952.
The degree pattern of a finite group has been introduced in [18].A group M is called k-fold OD- characterizable if there exist exactly k non-isomorphic finite groups having the same order and degree pattern as M .In particular,a 1-fold OD-characterizable group is simply called OD-characterizable.It is shown that the alternating groups A m and A m+1 ,for m = 27,35,51,57,65,77,87,93 and 95,are OD-characterizable,while their automorphism groups are 3-fold OD-characterizable.It is also shown that the symmetric groups S m+2 ,for m = 7,13,19,23,31,37,43,47,53,61,67,73,79,83,89 and 97,are 3-fold OD-characterizable.From this,the following theorem is derived.Let m be a natural number such that m 100.Then one of the following holds: (a) if m = 10,then the alternating groups A m are OD-characterizable,while the symmetric groups S m are OD- characterizable or 3-fold OD-characterizable;(b) the alternating group A 10 is 2-fold OD-characterizable;(c) the symmetric group S 10 is 8-fold OD-characterizable.This theorem completes the study of OD-characterizability of the alternating and symmetric groups A m and S m of degree m 100. 相似文献
953.
One of the most significant discussions in the field of machine learning today is on the clustering ensemble. The clustering ensemble combines multiple partitions generated by different clustering algorithms into a single clustering solution. Genetic algorithms are known for their high ability to solve optimization problems, especially the problem of the clustering ensemble. To date, despite the major contributions to find consensus cluster partitions with application of genetic algorithms, there has been little discussion on population initialization through generative mechanisms in genetic-based clustering ensemble algorithms as well as the production of cluster partitions with favorable fitness values in first phase clustering ensembles. In this paper, a threshold fuzzy C-means algorithm, named TFCM, is proposed to solve the problem of diversity of clustering, one of the most common problems in clustering ensembles. Moreover, TFCM is able to increase the fitness of cluster partitions, such that it improves performance of genetic-based clustering ensemble algorithms. The fitness average of cluster partitions generated by TFCM are evaluated by three different objective functions and compared against other clustering algorithms. In this paper, a simple genetic-based clustering ensemble algorithm, named SGCE, is proposed, in which cluster partitions generated by the TFCM and other clustering algorithms are used as the initial population used by the SGCE. The performance of the SGCE is evaluated and compared based on the different initial populations used. The experimental results based on eleven real world datasets demonstrate that TFCM improves the fitness of cluster partitions and that the performance of the SGCE is enhanced using initial populations generated by the TFCM. 相似文献
954.
In this paper we have considered a red-shift based linear correction in derivative of action in the context of vacuum f (R) gravity. Here we have found out that the linear correction may describe the late time acceleration which is appeared by
SuperNovae Ia with no need of dark energy. Also we have calculated the asymptotic action for the desired correction. The value
of all solutions may reduce to de’ Sitter universe in the absence of correction term. 相似文献
955.
In this paper, a necessary condition is first presented for the existence of limit cycles in nonlinear systems, then four
theorems are presented for the stability, instability, and semistabilities of limit cycles in second order nonlinear systems.
Necessary and sufficient conditions are given in terms of the signs of first and second derivatives of a continuously differentiable
positive function at the vicinity of the limit cycle. Two examples considering nonlinear systems with familiar limit cycles
are presented to illustrate the theorems. 相似文献
956.
Upscaled Unstructured Computational Grids for Efficient Simulation of Flow in Fractured Porous Media
Muhammad Sahimi R. Darvishi Manouchehr Haghighi M. Reza Rasaei 《Transport in Porous Media》2010,83(1):195-218
Discrete fracture modeling (DFM) is currently the most promising approach for modeling of naturally fractured reservoirs and
simulation of multiphase fluid flow therein. In contrast with the classical double-porosity/double permeability models, in
the DFM approach all the interactions and fluid flow in and between the fractures and within the matrix are modeled in a unified
manner, using the same computational grid. There is no need for computing the shape factors, which are crucial to the accuracy
of the double-porosity models. We have exploited this concept in order to develop a new method for the generation of unstructured
computational grids. In the new approach the geological model (GM) of the reservoir is first generated, using square or cubic
grid blocks. The GM is then upscaled using a method based on the multiresolution wavelet transformations that we recently
developed. The upscaled grid contains a distribution of the square or cubic blocks of various sizes. A map of the blocks’
centers is then used with an optimized Delauney triangulation method and the advancing-front technique, in order to generate
the final unstructured triangulated grid suitable for use in any general reservoir simulator with any number of fluid phases. The new method also includes an algorithm for generating fractures
that, contrary to the previous methods, does not require modifying their paths due to the complexities that may arise in spatial
distribution of the grid blocks. It also includes an effective partitioning of the simulation domain that results in large
savings in the computation times. The speed-up in the computations with the new upscaled unstructured grid is about three
orders of magnitude over that for the initial GM. Simulation of waterflooding indicates that the agreement between the results
obtained with the GM and the upscaled unstructured grid is excellent. The method is equally applicable to the simulations
of multiphase flow in unfractured, but highly heterogeneous, reservoirs. 相似文献
957.
Generating one-, two-, three- and four-scroll attractors from a novel four-dimensional smooth autonomous chaotic system 下载免费PDF全文
A new four-dimensional quadratic smooth autonomous chaotic
system is presented in this paper, which can exhibit periodic orbit
and chaos under the conditions on the system parameters.
Importantly, the system can generate one-, two-, three- and
four-scroll chaotic attractors with appropriate choices of
parameters. Interestingly, all the attractors are generated only by
changing a single parameter. The dynamic analysis approach in the
paper involves time series, phase portraits, Poincar\'{e} maps,
a bifurcation diagram, and Lyapunov exponents, to investigate some
basic dynamical behaviours of the proposed four-dimensional system. 相似文献
958.
Numerical investigation of turbulent free jet flows issuing from rectangular nozzles: the influence of small aspect ratio 总被引:1,自引:0,他引:1
Ehsan Faghani Reza Maddahian Pedram Faghani Bijan Farhanieh 《Archive of Applied Mechanics (Ingenieur Archiv)》2010,80(7):727-745
In this research, the fluid and thermal characteristics of a rectangular turbulent jet flow is studied numerically. The results
of three-dimensional jet issued from a rectangular nozzle are presented. A numerical method employing control volume approach
with collocated grid arrangement was employed. Velocity and pressure fields are coupled with SIMPLEC algorithm. The turbulent
stresses are approximated using k–e{\varepsilon} model with two different inlet conditions. The velocity and temperature fields are presented and the rates of their decay
at the jet centerline are noted. The velocity vectors of the main flow and the secondary flow are illustrated. Also, effect
of aspect ratio on mixing in rectangular cross-section jets is considered. The aspect ratios that were considered for this
work were 1:1 to 1:4. The results showed that the jet entrains more with smaller AR. Special attention has been drawn to the
influence of the Reynolds number (based on hydraulic diameter) as well as the inflow conditions on the evolution of the rectangular
jet. An influence on the jet evolution is found for smaller Re, but the jet is close to a converged state for higher Reynolds numbers. The inflow conditions have considerable influence
on the jet characteristics. 相似文献
959.
Zahra Mansourpour Sedighe Karimi Reza Zarghami Navid Mostoufi Rahmat Sotudeh-Gharebagh 《Particuology》2010,8(5):407-414
A numerical simulation was conducted to study the effect of pressure on bubble dynamics in a gas–solid fluidized bed. The gas flow was modeled using the continuum theory and the solid phase, by the discrete element method (DEM). To validate the simulation results, calculated local pressure fluctuations were compared with corresponding experimental data of 1-mm polyethylene particles. It was shown that the model successfully predicts the hydrodynamic features of the fluidized bed as observed in the experiments. Influence of pressure on bubble rise characteristics such as bubble rise path, bubble stability, average bubbles diameter and bubble velocity through the bed was investigated. The simulation results are in conformity with current hydrodynamic theories and concepts for fluidized beds at high pressures. The results show further that elevated pressure reduces bubble growth, velocity and stability and enhances bubble gyration through the bed, leading to change in bed flow structure. 相似文献
960.