首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   358848篇
  免费   13308篇
  国内免费   8249篇
化学   189669篇
晶体学   4535篇
力学   19553篇
综合类   575篇
数学   62916篇
物理学   103157篇
  2022年   3336篇
  2021年   3846篇
  2020年   4234篇
  2019年   4289篇
  2018年   14222篇
  2017年   13925篇
  2016年   12122篇
  2015年   6198篇
  2014年   7387篇
  2013年   13867篇
  2012年   16730篇
  2011年   24790篇
  2010年   15679篇
  2009年   15831篇
  2008年   18812篇
  2007年   20598篇
  2006年   11828篇
  2005年   11754篇
  2004年   10584篇
  2003年   9656篇
  2002年   8476篇
  2001年   8265篇
  2000年   6652篇
  1999年   5289篇
  1998年   4494篇
  1997年   4240篇
  1996年   4377篇
  1995年   3770篇
  1994年   3758篇
  1993年   3518篇
  1992年   3611篇
  1991年   3597篇
  1990年   3292篇
  1989年   3134篇
  1988年   2967篇
  1987年   2890篇
  1986年   2867篇
  1985年   3633篇
  1984年   3588篇
  1983年   3003篇
  1982年   3216篇
  1981年   2948篇
  1980年   2720篇
  1979年   2879篇
  1978年   3084篇
  1977年   3099篇
  1976年   3120篇
  1975年   2861篇
  1974年   2963篇
  1973年   3030篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
101.
The acoustic and perceptive characteristics of discourse units, including clauses, simple sentences, compound sentences and paragraphs, were studied. The acoustic characteristics were based on the analysis of a speech corpus consisted of ten monologues. The Perceived Boundary Strength (PBS) of the monologues was labeled on a five-point scale by 25 participants. The main results are as follows: (1) There is a significant difference in the PBS of clauses (corresponding to intonational phrase), sentences (including simple sentences and compound sentences) and paragraphs. But the PBS of simple sentences and compound sentences is the same, and the pitch reset and pause are also not significantly different between these two kinds of boundaries. (2) The pitch-based cue used to distinguish clauses, sentences and paragraphs appears to be the difference between pre- and post-boundary syllables, but not simply the pitch value of either syllable alone. (3) Declination mainly occurs within intonational phrases, whereas there is no consistent global declination in sentences or in paragraphs. (4) Pauses are longer and more varied at the boundary of larger discourse units. Moreover, there is a significant positive correlation between pause duration and degree of pitch reset at clause boundaries, but not at sentence or paragraph boundaries.  相似文献   
102.
A test for a function to be a solution of an elliptic PDE is given in terms of extensions, as solutions, from the boundaries inside the domains belonging to an isotopic family. It generalizes a result of Ehrenpreis for spheres moved along a straight line.

  相似文献   

103.
We prove that the holomorphic unipotent Jacobian conjecture is valid when n = 3.  相似文献   
104.
Data Envelopment Analysis (DEA) offers a piece-wise linear approximation of the production frontier. The approximation tends to be poor if the true frontier is not concave, eg in case of economies of scale or of specialisation. To improve the flexibility of the DEA frontier and to gain in empirical fit, we propose to extend DEA towards a more general piece-wise quadratic approximation, called Quadratic Data Envelopment Analysis (QDEA). We show that QDEA gives statistically consistent estimates for all production frontiers with bounded Hessian eigenvalues. Our Monte-Carlo simulations suggest that QDEA can substantially improve efficiency estimation in finite samples relative to standard DEA models.  相似文献   
105.
Hilbert重级数定理的一个改进   总被引:15,自引:3,他引:12       下载免费PDF全文
The object of this note is to prove the followingTheorem Let{a_n}and{b_n}be sequences of real numbers such that0<∑∑a_n~2<+∞and0<∑b_n~2<+∞.Then we have the inequalitysum from m=1 to∞sum from n=1 to∞a_mb_n/m+n<{sum from n=1 to∞(π-θ/n~(1/2)a_n~2}~1/2{sum from n=1 to∞(π-θ/n~(1/2)b_n~2}~1/2 (1)whereθ=3/2~(1/2)-1=1.121320343.  相似文献   
106.
107.
108.
Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 54, No. 3, pp. 438–444, March, 1991.  相似文献   
109.
We prove that a spread S over a locally compact nondlscrete field F defines a topological translation plane if and only if the spread is compact. For F=R, this is implicit in Breuning's thesis [Bre], cf. [B 2]. For the proof, we describe the point set of the projective translation plane as a quotient space of some projective space, with identifications taking place in one hyperplane. This is new even for F=R.  相似文献   
110.
An amphiphilic poly(ethylene oxide)‐block‐poly(dimethylsiloxane) (PEO–PDMS) diblock copolymer was used to template a bisphenol A type epoxy resin (ER); nanostructured thermoset blends of ER and PEO–PDMS were prepared with 4,4′‐methylenedianiline (MDA) as the curing agent. The phase behavior, crystallization, hydrogen‐bonding interactions, and nanoscale structures were investigated with differential scanning calorimetry, Fourier transform infrared spectroscopy, transmission electron microscopy, and small‐angle X‐ray scattering. The uncured ER was miscible with the poly(ethylene oxide) block of PEO–PDMS, and the uncured blends were not macroscopically phase‐separated. Macroscopic phase separation took place in the MDA‐cured ER/PEO–PDMS blends containing 60–80 wt % PEO–PDMS diblock copolymer. However, the composition‐dependent nanostructures were formed in the cured blends with 10–50 wt % PEO–PDMS, which did not show macroscopic phase separation. The poly(dimethylsiloxane) microdomains with sizes of 10–20 nm were dispersed in a continuous ER‐rich phase; the average distance between the neighboring microdomains was in the range of 20–50 nm. The miscibility between the cured ER and the poly(ethylene oxide) block of PEO–PDMS was ascribed to the favorable hydrogen‐bonding interaction. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3042–3052, 2006  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号