全文获取类型
收费全文 | 8002篇 |
免费 | 464篇 |
国内免费 | 33篇 |
专业分类
化学 | 3963篇 |
晶体学 | 71篇 |
力学 | 161篇 |
数学 | 437篇 |
物理学 | 1517篇 |
无线电 | 2350篇 |
出版年
2024年 | 11篇 |
2023年 | 93篇 |
2022年 | 126篇 |
2021年 | 227篇 |
2020年 | 154篇 |
2019年 | 170篇 |
2018年 | 142篇 |
2017年 | 163篇 |
2016年 | 222篇 |
2015年 | 224篇 |
2014年 | 300篇 |
2013年 | 484篇 |
2012年 | 517篇 |
2011年 | 536篇 |
2010年 | 369篇 |
2009年 | 396篇 |
2008年 | 549篇 |
2007年 | 495篇 |
2006年 | 496篇 |
2005年 | 414篇 |
2004年 | 396篇 |
2003年 | 345篇 |
2002年 | 348篇 |
2001年 | 253篇 |
2000年 | 174篇 |
1999年 | 156篇 |
1998年 | 109篇 |
1997年 | 71篇 |
1996年 | 71篇 |
1995年 | 78篇 |
1994年 | 55篇 |
1993年 | 49篇 |
1992年 | 36篇 |
1991年 | 25篇 |
1990年 | 41篇 |
1989年 | 43篇 |
1988年 | 21篇 |
1987年 | 18篇 |
1985年 | 25篇 |
1984年 | 11篇 |
1983年 | 12篇 |
1982年 | 15篇 |
1981年 | 17篇 |
1980年 | 10篇 |
1979年 | 4篇 |
1978年 | 4篇 |
1977年 | 5篇 |
1975年 | 3篇 |
1974年 | 3篇 |
1969年 | 3篇 |
排序方式: 共有8499条查询结果,搜索用时 31 毫秒
21.
Bandwidth Adaptation Algorithms for Adaptive Multimedia Services in Mobile Cellular Networks 总被引:3,自引:0,他引:3
The fluctuation of available link bandwidth in mobilecellular networks motivates the study of adaptive multimediaservices, where the bandwidth of an ongoing multimedia call can bedynamically adjusted. We analyze the diverse objectives of theadaptive multimedia framework and propose two bandwidth adaptationalgorithms (BAAs) that can satisfy these objectives. The firstalgorithm, BAA-RA, takes into consideration revenue and``anti-adaptation' where anti-adaptation means that a user feelsuncomfortable whenever the bandwidth of the user's call ischanged. This algorithm achieves near-optimal total revenue withmuch less complexity compared to an optimal BAA. The secondalgorithm, BAA-RF, considers revenue and fairness, and aims at themaximum revenue generation while satisfying the fairnessconstraint defined herein. Comprehensive simulation experimentsshow that the difference of the total revenue of BAA-RA and thatof an optimal BAA is negligible. Also, numerical results revealthat there is a conflicting relationship between anti-adaptationand fairness. 相似文献
22.
In this paper, we shall find the uniquerational price associated with the exchange option. Also, we find the decomposition of Snell envelope and value function of the American exchange option. 相似文献
23.
Quantization of Underdamped, Critically Damped, and Overdamped Electric Circuits with a Power Source
Jeong-Ryeol Choi 《International Journal of Theoretical Physics》2002,41(10):1931-1939
We have investigated the quantum mechanical effect of the underdamped, critically damped, and overdamped electric circuits with a power source. The charge of the underdamped circuit oscillates while those of the critically damped and overdamped ones don't. The wave function of the system of overdamped circuit represented parabolic cylinder function while underdamped circuit was represented by well-known Hermite polynomial. The eigenvalues of underdamped circuit is discrete while those of the critically damped and overdamped ones are given as continuously. 相似文献
24.
We have investigated the effect of extended dislocations (0.5-3 μm) on charge distribution in GaN epilayer grown by metalorganic chemical vapor deposition on (0001) sapphire using atomic force microscopy (AFM) and scanning surface potential microscopy (SSPM). It has been observed for the surface at the extended dislocations present in undoped GaN film to be negatively charged showing 0.04-0.2 V higher potential relative to regions that contain no dislocations. In addition to the higher potential at the dislocation core, the surrounding surfaces, including the edge of the dislocations, are also negatively charged in a symmetric way around the dislocations revealing crater-shaped higher potential regions (∼0.04 V) relative to surrounding dislocation-free area. The experimental results show that the protrusion-type of dislocation is also negatively charged and its potential is dependent on the size of dislocation. 相似文献
25.
Myungjin Jeon Dongsoong Han Kyeongsu Park Gundon Choi 《Journal of Applied Mathematics and Computing》2005,18(1-2):235-246
We present an interpolating, univariate subdivision scheme which preserves the discrete curvature and tangent direction at each step of subdivision. Since the polygon have a geometric information of some original (in some sense) curve as a discrete curvature, we can expect that the limit curve has the same curvature at each vertex as the control polygon. We estimate the curvature bound of odd vertices and give an error estimate for restoring a curve from sampled vertices on curves. 相似文献
26.
Jong-Hwan Kim Yeon-Chan Hong Sung-Jun Lee Keh-Kun Choi 《Industrial Electronics, IEEE Transactions on》1989,36(3):361-364
A direct adaptive control scheme is proposed for nonminimum-phase systems in which controller parameters are estimated from the recursive least-squares algorithm and additional auxiliary parameters are obtained from the proposed polynomial identity. A local convergence is guaranteed without any extra condition. Integral action is incorporated into the adaptive controller to eliminate the steady-state error and to satisfy a condition of the unique solution for the polynomial identity. The control law used in this scheme is based on the set-point-on-I-only proportional-integral-derivative (PID) structure 相似文献
27.
Jeong Ryeol Choi 《International Journal of Theoretical Physics》2004,43(4):947-958
We derived the WKB wave function for the general time-dependent quadratic Hamiltonian system using a unitary transformation method. We applied our research to sinusodially drived Caldirola–Kanai oscillator and confirmed that the time evolution of our approximated WKB wave function is similar to that of the exact one. This wave function can be used to analyze the interference between the probability amplitudes contributed by the area of overlap in phase space of quantum states. 相似文献
28.
A W-band divide-by-3 frequency divider with wide bandwidth and low power dissipation is presented using harmonic injection-locking technique. A cascode FET is employed for a self-oscillating second-harmonic mixer which is injection-locked by third-harmonic input to obtain the division order of three. The fabricated frequency divider using 0.1 /spl mu/m GaAs metamorphic HEMT technology shows superior performance such as large bandwidth of 6.1 GHz around 83.1 GHz (7.3%) under small DC power consumption of 12 mW. 相似文献
29.
Seyeong Choi Alouini M.-S. Qaraqe K.A. Hong-Chuan Yang 《Wireless Communications, IEEE Transactions on》2008,7(2):495-499
We propose and analyze new finger assignment techniques that are applicable for RAKE receivers in the soft handover (SHO) region. Specifically, extending the results for the case of two-base station (BS), we consider the multi-BS situation, attack the statistics of several correlated generalized selection combining (GSC) stages, and provide closed-form expressions for the statistics of the output signal-to-noise ratio (SNR). By investigating the tradeoff among the error performance, the average number of required path estimations/comparisons, and the SHO overhead, we show through numerical examples that the new schemes offer commensurate performance in comparison with more complicated GSC-based diversity systems while requiring a smaller estimation load and SHO overhead. 相似文献
30.
Ultrasonic longitudinal velocity and adsorption have been measured in ceramic superconductors YBa2Cu3Oy with various porosity and also in BiSrCaCu2Oy. A velocity drop of about 400 ppm was found at Tc only in the measurements on cooling. The magnitude of the velocity drop is anomalously large compared with the value expected from the thermodynamics. A hysteresis of velocity with respect to temperature was observed in all the samples studied. It is suggested that some structural change at pore size level is responsible for this phenomenon. 相似文献