全文获取类型
收费全文 | 495540篇 |
免费 | 4951篇 |
国内免费 | 1442篇 |
专业分类
化学 | 259924篇 |
晶体学 | 8009篇 |
力学 | 22075篇 |
综合类 | 8篇 |
数学 | 56159篇 |
物理学 | 155758篇 |
出版年
2021年 | 3941篇 |
2020年 | 4268篇 |
2019年 | 4714篇 |
2018年 | 6261篇 |
2017年 | 6453篇 |
2016年 | 9339篇 |
2015年 | 5662篇 |
2014年 | 9038篇 |
2013年 | 22468篇 |
2012年 | 16637篇 |
2011年 | 20207篇 |
2010年 | 14484篇 |
2009年 | 14321篇 |
2008年 | 18142篇 |
2007年 | 18258篇 |
2006年 | 16910篇 |
2005年 | 14863篇 |
2004年 | 13760篇 |
2003年 | 12192篇 |
2002年 | 12113篇 |
2001年 | 14456篇 |
2000年 | 10998篇 |
1999年 | 8597篇 |
1998年 | 7136篇 |
1997年 | 7053篇 |
1996年 | 6767篇 |
1995年 | 5980篇 |
1994年 | 5861篇 |
1993年 | 5705篇 |
1992年 | 6459篇 |
1991年 | 6483篇 |
1990年 | 6274篇 |
1989年 | 6230篇 |
1988年 | 5931篇 |
1987年 | 6085篇 |
1986年 | 5725篇 |
1985年 | 7443篇 |
1984年 | 7479篇 |
1983年 | 6246篇 |
1982年 | 6526篇 |
1981年 | 6229篇 |
1980年 | 6105篇 |
1979年 | 6443篇 |
1978年 | 6748篇 |
1977年 | 6557篇 |
1976年 | 6460篇 |
1975年 | 6144篇 |
1974年 | 6061篇 |
1973年 | 6090篇 |
1972年 | 4206篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
31.
A numerical procedure, with an exact strain-displacement relationship of twisted and curved cylindrical panels having variable thickness derived by considering the Green strain tensor on general shell theory, is presented using the principle of virtual work and the Rayleigh-Ritz method with algebraic polynomials as in-plane and transverse displacement functions. The accuracy and applicability of the procedure are verified by comparing the present results with previous experimental and theoretical results for several panels. The effects of variation ratio of thickness in chordwise and lengthwise directions, twist, and curvature both in two directions aforementioned on vibrations of cylindrical panels are studied in detail, and typical vibration mode shapes are plotted to demonstrate the effects. 相似文献
32.
The Wheeler–DeWitt equation of arbitrary Hartle–Hawking factor ordering for several minisuperspace universe models, such as the pure gravity Friedmann–Robertson–Walker and Taub ones, is mapped onto the dynamics of corresponding classical oscillators. The latter ones are studied by the classical Ermakov invariant method, which is a natural approach in this context. For the more realistic case of a minimally coupled massive scalar field, one can study, within the same type of approach, the corresponding squeezing features as a possible means of describing cosmological evolution. Finally, we comment on the analogy with the accelerator physics. 相似文献
33.
A.-S. F. Obada M. Darwish H. H. Salah 《International Journal of Theoretical Physics》2002,41(9):1755-1768
In this paper a new state called odd-excited binomial state (OEBS) is introduced. It interpolates between the odd number state and the odd-excited coherent state. We discuss some statistical properties, such as the Glauber second-order correlation function and squeezing phenomenon (normal and amplitude-squared squeezing) for this state. The quasiprobability distribution functions (Husimi Q-function and Wigner W-function) are also examined. 相似文献
34.
Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 54, No. 5, pp. 815–819, May, 1991. 相似文献
35.
36.
A. S. Merkur'ev 《Journal of Mathematical Sciences》1991,57(6):3489-3497
This article considers the splitting properties of finite-dimensional division rings over universal splitting fields of quadratic forms. An example of a field with u-invariant equal to 6 is constructed, which contradicts Kaplansky's conjecture concerning u-invariants.Translated from Zapiski Nauchnykh Seminarov Leningradskogo Otdeleniya Matematicheskogo Instituta im, V. A. Steklova Akademii Nauk SSSR, Vol. 175, pp. 75–89, 1989. 相似文献
37.
38.
The dynamics of multiphoton transitions in a two-level spin system excited by transverse microwave and longitudinal RF fields with the frequencies ωmw and ωrf, respectively, is analyzed. The effective time-independent Hamiltonian describing the “dressed” spin states of the “spin + bichromatic field” system is obtained by using the Krylov-Bogoliubov-Mitropolsky averaging method. The direct detection of the time behavior of the spin system by the method of nonstationary nutations makes it possible to identify the multiphoton transitions for resonances ω0 = ωmw + rωrf (ω0 is the central frequency of the EPR line, r = 1, 2), to measure the amplitudes of the effective fields of these transitions, and to determine the features generated by the inhomogeneous broadening of the EPR line. It is shown that the Bloch-Siegert shifts for multiphoton resonances at the inhomogeneous broadening of spectral lines reduce only the nutation amplitude but do not change their frequencies. 相似文献
39.
The structural properties of polycrystalline silicon films, prepared by plasma enhanced chemical vapor deposition system, with different flow rates of SiH4/SiF4 mixtures at 300 °C were investigated. This study indicates that the low hydrogen coverage on the growing surface, under optimum fluorine radicals, will be leaded to an improvement of crystallized area as compared with case of high hydrogen coverage surface. Moreover, the studies of the role of SiH4 and SiF4 radicals show that the SiH4 radicals are important in the nucleation and growth of grains. However, SiF4 radicals are effective in the structural change of grain boundaries regions and by this way, in the present system, establish the growth of grains under the dominant 〈1 1 0〉 direction. The stress investigation indicates that addition of high flow rate of SiF4 in amorphous film, results in the nearly stress free films. Finally, we found that the changes in g-value reflect the changes in the intrinsic compressive and tensile stress in the both polycrystalline and amorphous silicon films. 相似文献
40.
M. A. H. Dempster 《Journal of Mathematical Sciences》2006,133(4):1422-1444
This paper gives a comprehensive treatment of EVPI-based sequential importance sampling algorithms for dynamic (multistage)
stochastic programming problems. Both theory and computational algorithms are discussed. Under general assumptions it is shown
that both an expected value of perfect information (EVPI) process and the corresponding marginal EVPI process (the supremum
norm of the conditional expectation of its generalized derivative) are nonanticipative nonnegative supermartingales. These
processes are used as importance criteria in the class of sampling algorithms treated in the paper. When their values are
negligible at a node of the current sample problem scenario tree, scenarios descending from the node are replaced by a single
scenario at the next iteration. On the other hand, high values lead to increasing the number of scenarios descending from
the node. Both the small sample and asymptotic properties of the sample problem estimates arising from the algorithms are
established, and the former are evaluated numerically in the context of a financial planning problem. Finally, current and
future research is described. Bibliography: 49 titles.
__________
Published in Zapiski Nauchnykh Seminarov POMI, Vol. 312, 2004, pp. 94–129. 相似文献