全文获取类型
收费全文 | 102176篇 |
免费 | 5967篇 |
国内免费 | 13133篇 |
专业分类
化学 | 71913篇 |
晶体学 | 1232篇 |
力学 | 3040篇 |
综合类 | 1132篇 |
数学 | 22285篇 |
物理学 | 21674篇 |
出版年
2024年 | 130篇 |
2023年 | 805篇 |
2022年 | 2041篇 |
2021年 | 2002篇 |
2020年 | 2439篇 |
2019年 | 2451篇 |
2018年 | 2120篇 |
2017年 | 3005篇 |
2016年 | 3146篇 |
2015年 | 2626篇 |
2014年 | 3806篇 |
2013年 | 7377篇 |
2012年 | 6656篇 |
2011年 | 5631篇 |
2010年 | 4829篇 |
2009年 | 6551篇 |
2008年 | 6833篇 |
2007年 | 7065篇 |
2006年 | 6379篇 |
2005年 | 5454篇 |
2004年 | 4963篇 |
2003年 | 4242篇 |
2002年 | 5405篇 |
2001年 | 3196篇 |
2000年 | 3059篇 |
1999年 | 2846篇 |
1998年 | 2491篇 |
1997年 | 1977篇 |
1996年 | 1665篇 |
1995年 | 1494篇 |
1994年 | 1351篇 |
1993年 | 1099篇 |
1992年 | 1032篇 |
1991年 | 707篇 |
1990年 | 586篇 |
1989年 | 546篇 |
1988年 | 411篇 |
1987年 | 319篇 |
1986年 | 291篇 |
1985年 | 263篇 |
1984年 | 278篇 |
1983年 | 151篇 |
1982年 | 240篇 |
1981年 | 193篇 |
1980年 | 216篇 |
1979年 | 205篇 |
1978年 | 183篇 |
1977年 | 135篇 |
1976年 | 116篇 |
1973年 | 71篇 |
排序方式: 共有10000条查询结果,搜索用时 34 毫秒
111.
介绍了光码分多址系统中常用地址码(一维扩时码、二维码和三维码)的特点,并对它们各自的互相关均值和方差进行了理论分析。基于非相干光码分多址系统中光学相关接收机的基本原理,结合不同的用户地址码,对系统误码率性能进行了分析,得到了接收机最佳判决阈值与地址码基本特性参数和系统同时用户数间的关系。最后,给出了数值仿真结果。结果表明,对于采用特定地址码的光码分多址系统,只有选择合适的接收机判决阈值,系统的误码率性能才能达到最佳。研究结果对光码分多址系统中接收机判决阈值的选取具有一定的参考作用。 相似文献
112.
非傍轴平顶高斯光束M2因子两种定义的比较研究 总被引:1,自引:1,他引:0
基于功率密度的二阶矩方法,推导出了非傍轴平顶高斯(FG)光束束宽和远场发散角的解析表达式.研究表明,当w0/λ→0时,远场发散角趋于渐近值θmax=63.435°,与阶数无关.使用非傍轴高斯光束代替傍轴高斯光束作为理想光束,研究了非傍轴FG光束的M2因子,并与传统定义的M2因子作了比较.在非傍轴范畴,非傍轴FG光束的M2因子不仅与阶数N有关,而且与w0/λ有关.按照定义,当w0/λ→0时,非傍轴FG光束的M2因子不等于0,对阶数N=1, 2, 3时,M2因子分别趋于0.913,0.882和0.886.当N→∞时,M2因子取最小值M2min=0.816. 相似文献
113.
114.
115.
具有某种断面的半群的研究进展 总被引:1,自引:0,他引:1
本文综述了几类具有特殊断面的半群的近期研究结果。在介绍逆半群和正则半群的一般结构之后,概述了具有逆断面的正则半群的结构和同余格的研究成果。总结了作为逆断面的推广的可裂断面,纯正断面,正则^*-断面和恰当断面。提出了可以进一步研究的重要的问题。 相似文献
116.
We consider some Sobolev-type spaces and obtain a necessary and sufficient condition for their embedding in a Lebesgue space. 相似文献
117.
This paper gives some global and uniform convergence estimates for a class of subspace correction (based on space decomposition) iterative methods applied to some unconstrained convex optimization problems. Some multigrid and domain decomposition methods are also discussed as special examples for solving some nonlinear elliptic boundary value problems.
118.
Software failures have become the major factor that brings the system down or causes a degradation in the quality of service. For many applications, estimating the software failure rate from a user's perspective helps the development team evaluate the reliability of the software and determine the release time properly. Traditionally, software reliability growth models are applied to system test data with the hope of estimating the software failure rate in the field. Given the aggressive nature by which the software is exercised during system test, as well as unavoidable differences between the test environment and the field environment, the resulting estimate of the failure rate will not typically reflect the user‐perceived failure rate in the field. The goal of this work is to quantify the mismatch between the system test environment and the field environment. A calibration factor is proposed to map the failure rate estimated from the system test data to the failure rate that will be observed in the field. Non‐homogeneous Poisson process models are utilized to estimate the software failure rate in both the system test phase and the field. For projects that have only system test data, use of the calibration factor provides an estimate of the field failure rate that would otherwise be unavailable. For projects that have both system test data and previous field data, the calibration factor can be explicitly evaluated and used to estimate the field failure rate of future releases as their system test data becomes available. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
119.
This paper investigates the large scale travelling ionospheric disturbances (LSTIDs) using the observation data of an HF Doppler array located in Central China. The data observed in a high solar activity year (year 1989) are analyzed to obtain the main propagation parameters of LSTIDs such as period, horizontal phase velocity and propagating direction. Results are outlined as follows: Most of the LSTIDs propagate southward; others tend to propagate northward, mostly in summer; dispersion of most LSTIDs is matched with that of Lamb pseudomode, while others have the dispersion of long period gravity wave mode. The horizontal phase velocities of these two modes are about 220 and 450 m/s respectively. The analysis shows that LSTIDs are strongly pertinent to solar activity and space magnetic storms; thus the results presented here are significant for the research of ionospheric weather in mid-low latitude region. 相似文献
120.
Monte Carlo simulation within the grand canonical ensemble, the histogram reweighting technique, and finite size scaling analysis are used to explore the phase behaviour of heteronuclear dimers, composed of A and B type atoms, on a square lattice. We have found that for the models with attractive BB and AB nearest-neighbour energy, uBB=uAB=−1, and for non-repulsive energy between AA nearest-neighbour sites, uAA<0, the system belongs to the universality class of the two-dimensional Ising model. However, when uAA>0, the system exhibits a non-universal critical behaviour. We have evaluated the dependences of the critical point characteristics on the value of uAA. 相似文献