全文获取类型
收费全文 | 5610篇 |
免费 | 752篇 |
国内免费 | 272篇 |
专业分类
化学 | 784篇 |
晶体学 | 18篇 |
力学 | 273篇 |
综合类 | 81篇 |
数学 | 1579篇 |
物理学 | 1449篇 |
无线电 | 2450篇 |
出版年
2024年 | 17篇 |
2023年 | 89篇 |
2022年 | 133篇 |
2021年 | 145篇 |
2020年 | 169篇 |
2019年 | 164篇 |
2018年 | 161篇 |
2017年 | 200篇 |
2016年 | 258篇 |
2015年 | 245篇 |
2014年 | 383篇 |
2013年 | 391篇 |
2012年 | 344篇 |
2011年 | 356篇 |
2010年 | 276篇 |
2009年 | 332篇 |
2008年 | 358篇 |
2007年 | 321篇 |
2006年 | 261篇 |
2005年 | 260篇 |
2004年 | 233篇 |
2003年 | 223篇 |
2002年 | 188篇 |
2001年 | 150篇 |
2000年 | 169篇 |
1999年 | 160篇 |
1998年 | 110篇 |
1997年 | 81篇 |
1996年 | 73篇 |
1995年 | 49篇 |
1994年 | 49篇 |
1993年 | 49篇 |
1992年 | 31篇 |
1991年 | 40篇 |
1990年 | 23篇 |
1989年 | 14篇 |
1988年 | 17篇 |
1987年 | 13篇 |
1986年 | 15篇 |
1985年 | 18篇 |
1984年 | 10篇 |
1983年 | 8篇 |
1982年 | 15篇 |
1981年 | 10篇 |
1980年 | 9篇 |
1979年 | 3篇 |
1977年 | 2篇 |
1976年 | 3篇 |
1970年 | 1篇 |
1936年 | 2篇 |
排序方式: 共有6634条查询结果,搜索用时 15 毫秒
11.
Necessary conditions for the Gâteaux differentiability of the distance function to a set are considered. A series of characterizing results is obtained. 相似文献
12.
空间遥感相机长条反射镜较圆反射镜的整体刚度相差很大,整个组件在随火箭发射过程中因力学环境引起的自身位置改变导致面形的微小畸变从而对成像质量造成严重影响。因此,反射镜组件的设计就成为整个相机系统研制的重点与难点之一。对其进行科学、严谨的力学环境仿真是设计中的有效辅助手段。对某相机的长条反射镜组件动力学响应进行了较为深入的研究,借助MSC-patran,用有限元法进行了求解。获得了组件的动力学特性。 相似文献
13.
Summary Considered here are model equations for weakly nonlinear and dispersive long waves, which feature general forms of dispersion and pure power nonlinearity. Two variants of such equations are introduced, one of Korteweg-de Vries type and one of regularized long-wave type. It is proven that solutions of the pure initial-value problem for these two types of model equations are the same, to within the order of accuracy attributable to either, on the long time scale during which nonlinear and dispersive effects may accumulate to make an order-one relative difference to the wave profiles.This research was supported in part by the National Science Foundation. A considerable portion of the project was completed while the first author was resident at the Institute for Mathematics and Its Applications, University of Minnesota. 相似文献
14.
R. J. Saykally K. M. Evenson D. A. Jennings L. R. Zink A. Scalabrin 《International Journal of Infrared and Millimeter Waves》1987,8(6):653-662
Twenty new cw FIR laser lines in CD3OH, optically pumped by a CO2 laser, are reported. The frequencies of 39 of the stronger laser lines were measured relative to stabilized CO2 lasers with a fractional uncertainty, as determined by the reproducibility of the FIR frequency itself, of 2 parts in 107.Contribution of the U.S. Government, not subject to copyright. 相似文献
15.
Tor Schoenmeyr David Yu Zhang 《Journal of Algorithms in Cognition, Informatics and Logic》2005,57(2):130-139
The string matching with mismatches problem requires finding the Hamming distance between a pattern P of length m and every length m substring of text T with length n. Fischer and Paterson's FFT-based algorithm solves the problem without error in O(σnlogm), where σ is the size of the alphabet Σ [SIAM–AMS Proc. 7 (1973) 113–125]. However, this in the worst case reduces to O(nmlogm). Atallah, Chyzak and Dumas used the idea of randomly mapping the letters of the alphabet to complex roots of unity to estimate the score vector in time O(nlogm) [Algorithmica 29 (2001) 468–486]. We show that the algorithm's score variance can be substantially lowered by using a bijective mapping, and specifically to zero in the case of binary and ternary alphabets. This result is extended via alphabet remappings to deterministically solve the string matching with mismatches problem with a constant factor of 2 improvement over Fischer–Paterson's method. 相似文献
16.
Heikki Hyyr 《Journal of Discrete Algorithms》2005,3(2-4):215-229
Using bit-parallelism has resulted in fast and practical algorithms for approximate string matching under Levenshtein edit distance, which permits a single edit operation to insert, delete or substitute a character. Depending on the parameters of the search, currently the fastest non-filtering algorithms in practice are the O(km/wn) algorithm of Wu and Manber, the O((k+2)(m−k)/wn) algorithm of Baeza-Yates and Navarro, and the O(m/wn) algorithm of Myers, where m is the pattern length, n is the text length, k is the error threshold and w is the computer word size. In this paper we discuss a uniform way of modifying each of these algorithms to permit also a fourth type of edit operation: transposing two adjacent characters in the pattern. This type of edit distance is also known as Damerau edit distance. In the end we also present an experimental comparison of the resulting algorithms. 相似文献
17.
O. Gravrand E. De Borniol S. Bisotto L. Mollard G. Destefanis 《Journal of Electronic Materials》2007,36(8):981-987
This paper aims at studying the feasibility of very long infrared wavelength (VLWIR) (12–18 μm) focal plane arrays using n-on-p planar ion-implanted technology. To explore and analyze the feasibility of such VLWIR detectors, a set of four Cd
x
Hg1−x
Te LPE layers with an 18 μ cutoff at 50 K has been processed at Defir (LETI/LIR–Sofradir joint laboratory), using both our “standard” n-on-p process and our improved low dark current process. Several 320 × 256 arrays, 30-μm pitch, have been hybridized on standard Sofradir readout circuits and tested. Small dimension test arrays characterization
is also presented. Measured photonic currents with a 20°C black body suggest an internal quantum efficiency above 50%. Typical
I(V) curves and thermal evolution of the saturation current are discussed, showing that standard photodiodes remain diffusion
limited at low biases for temperatures down to 30 K. Moreover, the dark current gain brought by the improved process is clearly
visible for temperatures higher than 40 K. Noise measurements are also discussed showing that a very large majority of detectors
appeared background limited under usual illumination and biases. In our opinion, such results demonstrate the feasibility
of high-performance complex focal plane arrays in the VLWIR range at medium term. 相似文献
18.
19.
本文提出的MMD算法用于提高模型区别错误信息和正确信息的能力.利用该算法在对模型的参数进行重估计时.涉及到复杂的目标函数的梯度运算.击运用矩阵运算使得梯度运算变得简单明了,因此本文给出了MMD算法下的HMM参数重估计的矩阵表示形式并给出了证明. 相似文献
20.
Semra Harmandar 《Journal of Quantitative Spectroscopy & Radiative Transfer》2004,84(4):395-407
A radiation code based on method of lines solution of discrete ordinates method for radiative heat transfer in axisymmetric cylindrical enclosures containing absorbing-emitting medium was developed and tested for predictive accuracy by applying it to (i) test problems with black and grey walls (ii) a gas turbine combustor simulator enclosing a non-homogeneous absorbing-emitting medium and benchmarking its steady-state predictions against exact solutions and measurements. Comparisons show that it provides accurate solutions for radiative heat fluxes and can be used with confidence in conjunction with CFD codes based on the same approach. 相似文献