全文获取类型
收费全文 | 19802篇 |
免费 | 2029篇 |
国内免费 | 837篇 |
专业分类
化学 | 14527篇 |
晶体学 | 144篇 |
力学 | 648篇 |
综合类 | 47篇 |
数学 | 2497篇 |
物理学 | 4805篇 |
出版年
2023年 | 257篇 |
2022年 | 243篇 |
2021年 | 361篇 |
2020年 | 497篇 |
2019年 | 507篇 |
2018年 | 389篇 |
2017年 | 369篇 |
2016年 | 766篇 |
2015年 | 801篇 |
2014年 | 907篇 |
2013年 | 1475篇 |
2012年 | 1710篇 |
2011年 | 1629篇 |
2010年 | 1048篇 |
2009年 | 886篇 |
2008年 | 1365篇 |
2007年 | 1235篇 |
2006年 | 1241篇 |
2005年 | 1082篇 |
2004年 | 905篇 |
2003年 | 738篇 |
2002年 | 659篇 |
2001年 | 370篇 |
2000年 | 309篇 |
1999年 | 245篇 |
1998年 | 208篇 |
1997年 | 223篇 |
1996年 | 249篇 |
1995年 | 175篇 |
1994年 | 168篇 |
1993年 | 166篇 |
1992年 | 117篇 |
1991年 | 99篇 |
1990年 | 102篇 |
1989年 | 76篇 |
1988年 | 65篇 |
1987年 | 55篇 |
1986年 | 85篇 |
1985年 | 102篇 |
1984年 | 76篇 |
1983年 | 48篇 |
1982年 | 61篇 |
1981年 | 61篇 |
1980年 | 64篇 |
1979年 | 49篇 |
1978年 | 53篇 |
1977年 | 39篇 |
1976年 | 51篇 |
1975年 | 29篇 |
1973年 | 28篇 |
排序方式: 共有10000条查询结果,搜索用时 187 毫秒
71.
Simple expressions are given for the mean delay, mean waiting time, and mean busy period length in a multiplexer. Data streams with active periods having a general distribution are permitted, and the data rate during the active periods can be random. Data can also arrive in batches. The key restrictions of the model are that the sources are independent, idle periods are exponentially distributed, and a source generates at least enough data during an active period to keep the server busy throughout the period. The exact formulas allow evaluation of the error in approximations such as a heavy traffic diffusion approximation.Both continuous and discrete time models are considered. The discrete-time model includes that studied by Viterbi and subsequently generalized by Neuts. The Pollaczek-Khinchine formula for the mean amount of work in anM/GI/1 queue is retrieved as a limiting case.Preliminary version presented at IEEE INFOCOM, San Francisco, April 1993. 相似文献
72.
Summary We consider a one-dimensional linear wave equation with a small mean zero dissipative field and with the boundary condition imposed by the so-called Goursat problem. In order to observe the effect of the randomness on the solution we perform a space-time rescaling and we rewrite the problem in a diffusion approximation form for two parameter processes. We prove that the solution converges in distribution toward the solution of a two-parameter stochastic differential equation which we identify. The diffusion approximation results for oneparameter processes are well known and well understood. In fact, the solution of the one-parameter analog of the problem we consider here is immediate. Unfortunately, the situation is much more complicated for two-parameter processes and we believe that our result is the first one of its kind.Partially supported by ONR N00014-91-J-1010 相似文献
73.
We analyze the well-posedness of the initial value problem for the dissipative quasi-geostrophic equations in the subcritical
case. Mild solutions are obtained in several spaces with the right homogeneity to allow the existence of self-similar solutions.
While the only small self-similar solution in the strong
Lp{\cal L}^{p}
space is the null solution, infinitely many self-similar solutions do exist in weak-
Lp{\cal L}^{p}
spaces and in a recently introduced [7] space of tempered distributions. The asymptotic stability of solutions is obtained
in both spaces, and as a consequence, a criterion of self-similarity persistence at large times is obtained. 相似文献
74.
All-optical wavelength conversion of 10-Gb/s signal based on four-wave mixing is experimentally demonstrated in a 30-m-long dispersion-flattened microstructure fiber with small positive dispersion. For an average pump power of 26 dBm, the conversion efficiency was around -19.5 dB with the fluctuation less than ±1.4 dB, covering a conversion bandwidth of 20 nm. The eye diagram of the converted signal shows good eye opening. 相似文献
75.
The difference between the classical treatment offlexible body impact and the treatment of impact in flexiblemultibody dynamics is due to several fundamental reasons. Inthe classical impact theory, simple structures such as beamsand plates are used. Infinite dimensional models can bedeveloped for these simple structural elements to study theimpact dynamics and the wave propagation problem. Flexiblemultibody impact problems, on the other hand, involve bodieswith complex geometry that cannot be modeled using infinitenumber of degrees of freedom. Furthermore, the classicalimpact theory has been mainly concerned with the impactbetween a rigid mass that moves without constraints beforeit impacts a simple flexible structure. This is not amultibody simulation scenario in which the impact occursbetween kinematically constrained bodies that are subjectedto impulsive constraint forces in addition to the impactforces. These constraint forces can influence the motion ofthe two bodies immediately after impact, and as aconsequence, the simple classical theory scenario of impactdoes not apply. It is the objective of this paper to discussthe use of the restitution condition in flexible multibodyimpact problems and demonstrate that the use of thisapproach does not exclude the classical formulation.Nonetheless, the impulse momentum balance approach can serveas an effective and efficient procedure for solving theimpact problem in finite dimensional models that do not obeythe classical wave theory. Energy results of simplestructural elements are presented in order to demonstratethe consistency of using the impulse momentum balanceapproach in solving impact problems in finite dimensionalflexible body applications. 相似文献
76.
77.
Inmaculada García Cuesta José Sánchez-Marín Alfredo M J Sánchez de Merás 《Chemphyschem》2006,7(2):508-513
Investigations into the charge-separated states and electron-transfer transitions in tetracyanoethylene (TCNE) complexes have recently generated much interest. In this work we present theoretical calculations showing that the most stable structure of the dianion TCNE2- has D2d symmetry in vacuum as well as in the solvents dichloromethane and acetonitrile. By means of the coupled cluster linear response, we compute the vertical electronic spectrum in both the gas phase and solution. The theoretical results are compared to the experimental data and good agreement is achieved. 相似文献
78.
The thermal stability and measurement temperature dependence of Schottky contact characteristics on n-GaN using a W2B5/Ti/Au metallization scheme was studied using current-voltage (I-V), scanning electron microscopy (SEM) and Auger electron spectroscopy (AES) measurements. The elemental profile obtained from samples annealed at 350 °C showed some titanium diffusion into the gold layer but little other difference from the as-deposited wafer. Annealing at 700 °C produced significant diffusion of titanium. The Schottky barrier height increased with anneal temperature up to 200 °C, reaching a maximum value of 0.65 eV, but decreased at higher annealing temperatures. The reverse breakdown voltage from diodes fabricated using the W2B5-based contacts showed a similar dependence. The reverse current magnitude was larger than predicted by thermionic emission alone. The barrier height showed only minor changes with measurement temperature up to 150 °C. 相似文献
79.
In this paper a cubic lattice L(S) is endowed with a symmetric implication structure and it is proved that L(S) \ {0} is a power of the three-element simple symmetric implication algebra. The Metropolis–Rota’s symmetries are obtained as partial terms in the language of symmetric implication algebras. 相似文献
80.