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   8676篇
  免费   636篇
  国内免费   176篇
化学   6018篇
晶体学   42篇
力学   328篇
综合类   12篇
数学   1222篇
物理学   1866篇
  2023年   116篇
  2022年   142篇
  2021年   197篇
  2020年   211篇
  2019年   190篇
  2018年   176篇
  2017年   163篇
  2016年   398篇
  2015年   351篇
  2014年   362篇
  2013年   540篇
  2012年   590篇
  2011年   611篇
  2010年   396篇
  2009年   377篇
  2008年   478篇
  2007年   443篇
  2006年   423篇
  2005年   377篇
  2004年   297篇
  2003年   229篇
  2002年   250篇
  2001年   150篇
  2000年   138篇
  1999年   133篇
  1998年   110篇
  1997年   130篇
  1996年   126篇
  1995年   108篇
  1994年   79篇
  1993年   79篇
  1992年   50篇
  1991年   63篇
  1990年   64篇
  1989年   51篇
  1988年   38篇
  1987年   44篇
  1986年   40篇
  1985年   32篇
  1983年   23篇
  1982年   27篇
  1981年   34篇
  1980年   27篇
  1979年   27篇
  1978年   30篇
  1977年   35篇
  1976年   24篇
  1975年   22篇
  1974年   26篇
  1973年   28篇
排序方式: 共有9488条查询结果,搜索用时 15 毫秒
81.
COMPOSITE-STEP LIKE FILTER METHODS FOR EQUALITY CONSTRAINT PROBLEMS   总被引:2,自引:0,他引:2  
In a composite-step approach, a step sk is computed as the sum of two components vk and hk. The normal component vk, which is called the vertical step, aims to improve the linearized feasibility, while the tangential component hk, which is also called horizontal step, concentrates on reducing a model of the merit functions. As a filter method, it reduces both the infeasibility and the objective function. This is the same property of these two methods. In this paper, one concerns the composite-step like filter approach. That is, a step is tangential component hk if the infeasibility is reduced. Or else, sk is a compositestep composed of normal component Vk and tangential component hk.  相似文献   
82.
Analogue to the definition $K + L := \bigcup_{x\in K}(x + L)$ of the Minkowski addition in the euclidean geometry it is proposed to define the (noncommutative) addition $K \vdash L := \bigcup_{0\, \leqsl\, \rho\,\leqsl\, a(\varphi),0\,\leqsl\,\varphi\,<\, 2\pi}T_{\rho}^{(\varphi)}(L)$ for compact, convex and smoothly bounded sets K and L in the hyperbolic plane $\Omega$ (Kleins model). Here $\rho = a(\varphi)$ is the representation of the boundary $\partial$ K in geodesic polar coordinates and $T_{\rho}^{(\varphi)}$ is the hyperbolic translation of $\Omega$ of length $\rho$ along the line through the origin o of direction $\varphi$. In general this addition does not preserve convexity but nevertheless we may prove as main results: (1) $o \in$ int $K, o \in$ int L and K,L horocyclic convex imply the strict convexity of $K \vdash L$, and (2) in this case there exists a hyperbolic mixed volume $V_h(K,L)$ of K and L which has a representation by a suitable integral over the unit circle.  相似文献   
83.
本文研究了T型管状接头在面内和面外弯曲循环载荷作用下的疲劳强度问题,通过将材料常数和初始裂纹尺寸作为随机变量,采用MonCarlo模拟方法对疲劳寿命进行了统计分析.同时,对计算结果进行了线性回归,并与实验结果作了比较。  相似文献   
84.
The differential cross sections of tritons from the (d, t) reaction on9Be,10B and13C targets have been measured in the angular range of 5° LAB110° with relatively small errors, 5%. The experimental data were analysed in terms of the standard DWBA using both zero-range and exact finite-range approaches.  相似文献   
85.
The isotope254No was produced in the fusion reaction48Ca +208Pb. Using the velocity filter SHIP and radiochemical techniques it was found that the nuclide254No with a half-life of 55 s decays byα, EC, and spontaneous-fission. Deduced partial half-lives are (61±2) s forα-decay, (550 ?160 +370 ) s for EC and [2.2 ?1.0 +2.0 ]×104 s for spontaneous fission.  相似文献   
86.
The application of a semiconductor-laser amplifier with reduced facet reflectivity as all-optical switch is described. An optical data signal is switched on and off with an extinction ratio of 10 dB when an optical control signal is turned on and off. The two optical signals are polarized perpendicularly to each other. The device was operated up to 100 MHz.Nanjing Institute of Posts and Telecommunication Nanjing, PR China  相似文献   
87.
The elastic scattering was investigated especially in the Coulomb sensitive scattering regions. Angular distributions of the differential cross section and of the analyzing powerA y, with emphasis on small scattering angles, were measured at 10.0 and 14.1 MeV. For extreme backward angles up toJ c.m.=179° the differential cross section was measured at 12.0, 14.1 and 16.5 MeV. The data have been compared with recent Faddeev calculations based on the realistic meson-exchange Paris potential and including an approximate Coulomb correction. There are discrepancies between the data and the calculations especially for the analyzing power. This indicates that the approximate treatment of Coulomb effects and possibly also the purely nuclear part of the calculations need to be improved.  相似文献   
88.
An inhomogeneous steady state pattern of nonlinear reaction-diffusion equations with no-flux boundary conditions is usually computed by solving the corresponding time-dependent reaction-diffusion equations using temporal schemes. Nonlinear solvers (e.g., Newton's method) take less CPU time in direct computation for the steady state; however, their convergence is sensitive to the initial guess, often leading to divergence or convergence to spatially homogeneous solution. Systematically numerical exploration of spatial patterns of reaction-diffusion equations under different parameter regimes requires that the numerical method be efficient and robust to initial condition or initial guess, with better likelihood of convergence to an inhomogeneous pattern. Here, a new approach that combines the advantages of temporal schemes in robustness and Newton's method in fast convergence in solving steady states of reaction-diffusion equations is proposed. In particular, an adaptive implicit Euler with inexact solver (AIIE) method is found to be much more efficient than temporal schemes and more robust in convergence than typical nonlinear solvers (e.g., Newton's method) in finding the inhomogeneous pattern. Application of this new approach to two reaction-diffusion equations in one, two, and three spatial dimensions, along with direct comparisons to several other existing methods, demonstrates that AIIE is a more desirable method for searching inhomogeneous spatial patterns of reaction-diffusion equations in a large parameter space.  相似文献   
89.
90.
We present a ring-down absorption spectrometer based on a continuous-wave CO laser in the mid-infrared spectral region near lambda = 5 microm. Using a linear ring-down cavity (length: 0.5 m) with high reflective mirrors (R = 99.988 %), we observed a noise-equivalent absorption coefficient of 3 x 10(-10) cm(-1)Hz(-1/2). This corresponds to a noise-equivalent concentration of 800 parts per trillion (ppt) for (14)NO and 40 ppt for (15)NO in 1 s averaging time. We achieve a time resolution of 1 s which allows time resolved simultaneous detection of the two N isotopes. The delta(15)N value was obtained with a precision of +/-1.2 per thousand in a sample with a NO fraction of 11 ppm. The simultaneous detection enables the use of (15)NO as a tracer molecule for endogenous biomedical processes.  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