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   42442篇
  免费   2947篇
  国内免费   2207篇
化学   22605篇
晶体学   267篇
力学   4137篇
综合类   580篇
数学   9675篇
物理学   10332篇
  2024年   61篇
  2023年   381篇
  2022年   758篇
  2021年   1119篇
  2020年   1078篇
  2019年   1055篇
  2018年   956篇
  2017年   1207篇
  2016年   1609篇
  2015年   1187篇
  2014年   1749篇
  2013年   3088篇
  2012年   2413篇
  2011年   2188篇
  2010年   1913篇
  2009年   2430篇
  2008年   2438篇
  2007年   2719篇
  2006年   2268篇
  2005年   2084篇
  2004年   1999篇
  2003年   1669篇
  2002年   1313篇
  2001年   1079篇
  2000年   1052篇
  1999年   944篇
  1998年   884篇
  1997年   777篇
  1996年   727篇
  1995年   679篇
  1994年   598篇
  1993年   484篇
  1992年   454篇
  1991年   379篇
  1990年   345篇
  1989年   239篇
  1988年   239篇
  1987年   147篇
  1986年   128篇
  1985年   151篇
  1984年   122篇
  1983年   61篇
  1982年   104篇
  1981年   63篇
  1980年   47篇
  1979年   77篇
  1978年   38篇
  1977年   15篇
  1976年   25篇
  1975年   10篇
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
151.
本文证明了环面上具有间断梯度的势函数的模拟退火过程:dXt=-VU(Xt)dt √2dWt概率收敛到势函数的全局极小集附近。  相似文献   
152.
The reactions of ethylene glycol mononitrate and glycerol 1,3-dinitrate with methylphosphonic dichloride afforded new nitroxyalkyl methylphosphonates.  相似文献   
153.
我国医疗费用增长与医疗设备投入的相关性研究   总被引:3,自引:0,他引:3  
近年来我国卫生总费用占GDP的比例增长很快,而在卫生费用结构上则发生政府投入下降和个人支出的上升。本文采集了大量的统计数据,采用统计相关性研究的方法,论证了医疗设备规模的不断扩大是导致医疗费用上升的主要因素之一;医疗设备的快速增长与医院维修费用投入的增长直接相关,而引起医疗设备维修大量投入的主要原因则是故障期内医疗设备的非正常闲置。本文通过对个人卫生费用的上升、医疗设备规模的扩大及其医疗设备维修投入费用增加相关性的分析,说明了在这种关系下,十分有必要研究医院医疗设备维修体系,以在卫生投入和患者权益上达到平衡。  相似文献   
154.
运用中学生男性性别角色期望调查表和因子分析方法对贵阳市中学生进行男性性别角色期望调查和定性与定量分析,为中学生树立适宜的性别角色观,促进其心理健康发展提供量化依据.分析显示:传统的男性角色并不被现代中学生完全接受,无论是男中学生还是女中学生都在力图探求一种新的男性性别角色模式,且两性中学生对这一新的男性性别角色模式的期望和把握是有一定差异的.  相似文献   
155.
设$p\geq 7$素数,$A$为模$p$的Steenrod代数. 我们利用Adams谱序列证明了球面稳定同伦群$\pi_{\ast}S$中,存在由$b_1g_0\tilde{\gamma}_{s}\in Ext_A^{s+4,(s+1)p^2q+spq+sq+s-3}(Z_p,Z_p)$所表示的新的非平凡元素族,其中$q=2(p-1)$, $3\leq s相似文献   
156.
Numerical solutions to three systems of integrable evolutionary equations from the Toda lattice hierarchy are analyzed. These are the classical Toda lattice, the second local dispersive flow, and the second extended dispersive flow. Special attention is given to the properties of soliton solutions. For the equations of the second local flow, two types of solitons interacting in a special manner are found. Solutions corresponding to various initial data are qualitatively outlined.  相似文献   
157.
We constructed a scanning near-field optical microscope (SNOM) on a commercially available atomic force microscopy (AFM) apparatus (SPM-9500J2; Shimadzu Corp.) to measure the stress distribution in ceramic composite materials. Features of our SNOM system are: (1) a compact SNOM head substituted for the original AFM head; (2) a wide scanning range (125 × 125 μm2) inherited from the original scanner; (3) use of conventional shear-force regulation; (4) an optical system for the illumination-collection (I-C) mode; (5) excitation by a 488 nm line of an Ar-ion laser, and (6) light detection by photon counting or a polychromator equipped with an electronically cooled charge coupled device (CCD). This SNOM system was used to measure the surface structure and stress distribution of an Al2O3/ZrO2 eutectic composite. We simultaneously measured topographic images and fluorescence spectra of an Al2O3/ZrO2 eutectic composite. We estimated its peak intensity, peak position, and peak width from the fluorescence spectrum during scanning, which respectively correspond to the abundance of Al2O3, stress in the grain, and the anisotropy of that stress. Mapping images showed that the stress and its anisotropy were weaker in the center of the Al2O3 grain than its boundary between Al2O3 and ZrO2. That observation suggests that Al2O3 underwent intense anisotropic stress induced by volume expansion in the phase transition of ZrO2 from the cubic phase to the monoclinic phase during preparation.  相似文献   
158.
Traceability in illuminance measurements at the National Metrology Institute of Turkey (TüBITAK-UME) was established in 2003 with a detector-based realization. The new measurement technique was developed for the determination of illuminance responsivity and upgrating of the illuminance scale. The unit of the illuminance responsivity, in A/lx, was measured with an expanded uncertainty of 0.2% (k = 2) by supplying using the developed scanning technique for the calculation of color correction factor. The surface of a radiometer was scanned using a double-monochromator facility upgraded with an x-y scanning system. The illuminance responsivity as a function of bandpass and temperature were also investigated in this study. To use a radiometer in the photometric applications of metrology, a light-sensitive device, a so-called trap detector, was characterized by measuring the absolute responsivity, the non-linearity, and spatial non-uniformity.  相似文献   
159.
U Camci  Z Can  Y Nutku  Y Sucu  D Yazici 《Pramana》2006,67(6):1043-1053
We present the explicit form of the symplectic structure of anti-self-dual Yang-Mills (ASDYM) equations in Yang’s J- and K-gauges in order to establish the bi-Hamiltonian structure of this completely integrable system. Dirac’s theory of constraints is applied to the degenerate Lagrangians that yield the ASDYM equations. The constraints are second class as in the case of all completely integrable systems which stands in sharp contrast to the situation in full Yang-Mills theory. We construct the Dirac brackets and the symplectic 2-forms for both J- and K-gauges. The covariant symplectic structure of ASDYM equations is obtained using the Witten-Zuckerman formalism. We show that the appropriate component of the Witten-Zuckerman closed and conserved 2-form vector density reduces to the symplectic 2-form obtained from Dirac’s theory. Finally, we present the Bäcklund transformation between the J- and K-gauges in order to apply Magri’s theorem to the respective two Hamiltonian structures.  相似文献   
160.
This paper re-assesses three independently developed approaches that are aimed at solving the problem of zero-weights or non-zero slacks in Data Envelopment Analysis (DEA). The methods are weights restricted, non-radial and extended facet DEA models. Weights restricted DEA models are dual to envelopment DEA models with restrictions on the dual variables (DEA weights) aimed at avoiding zero values for those weights; non-radial DEA models are envelopment models which avoid non-zero slacks in the input-output constraints. Finally, extended facet DEA models recognize that only projections on facets of full dimension correspond to well defined rates of substitution/transformation between all inputs/outputs which in turn correspond to non-zero weights in the multiplier version of the DEA model. We demonstrate how these methods are equivalent, not only in their aim but also in the solutions they yield. In addition, we show that the aforementioned methods modify the production frontier by extending existing facets or creating unobserved facets. Further we propose a new approach that uses weight restrictions to extend existing facets. This approach has some advantages in computational terms, because extended facet models normally make use of mixed integer programming models, which are computationally demanding.  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