全文获取类型
收费全文 | 14811篇 |
免费 | 1484篇 |
国内免费 | 1310篇 |
专业分类
化学 | 1291篇 |
晶体学 | 95篇 |
力学 | 1931篇 |
综合类 | 300篇 |
数学 | 11179篇 |
物理学 | 2809篇 |
出版年
2024年 | 22篇 |
2023年 | 142篇 |
2022年 | 170篇 |
2021年 | 207篇 |
2020年 | 352篇 |
2019年 | 376篇 |
2018年 | 382篇 |
2017年 | 467篇 |
2016年 | 467篇 |
2015年 | 363篇 |
2014年 | 697篇 |
2013年 | 1194篇 |
2012年 | 630篇 |
2011年 | 922篇 |
2010年 | 745篇 |
2009年 | 862篇 |
2008年 | 978篇 |
2007年 | 948篇 |
2006年 | 840篇 |
2005年 | 749篇 |
2004年 | 643篇 |
2003年 | 663篇 |
2002年 | 679篇 |
2001年 | 538篇 |
2000年 | 508篇 |
1999年 | 481篇 |
1998年 | 459篇 |
1997年 | 406篇 |
1996年 | 288篇 |
1995年 | 203篇 |
1994年 | 170篇 |
1993年 | 135篇 |
1992年 | 115篇 |
1991年 | 101篇 |
1990年 | 80篇 |
1989年 | 63篇 |
1988年 | 62篇 |
1987年 | 61篇 |
1986年 | 46篇 |
1985年 | 58篇 |
1984年 | 60篇 |
1983年 | 30篇 |
1982年 | 50篇 |
1981年 | 38篇 |
1980年 | 28篇 |
1979年 | 28篇 |
1978年 | 28篇 |
1977年 | 14篇 |
1976年 | 19篇 |
1973年 | 9篇 |
排序方式: 共有10000条查询结果,搜索用时 23 毫秒
101.
研究了基于级联二阶非线性的铌酸锂波导全光波长变换器的特性.首先从耦合模方程出发,比较了数值分析结果与小信号近似分析的结果.其次在数值分析基础上,分析了铌酸锂晶体的温度变化导致相应基频光波波长与极化反转光栅周期的变化关系.最后分析了在不同相互作用长度下,转换的光波功率与有效基频光波波长带宽、温度调谐带宽、极化反转光栅周期带宽等关系,以对全光波长变换器件进行优化设计
关键词:
级联二阶非线性
波长变换
准位相匹配
铌酸锂光波导 相似文献
102.
This paper proposes the Rice condition numbers for invariant subspace, singular sub-spaces of a matrix and deflating subspaces of a regular matrix pair. The first-order perturbation estimations for these subspaces are derived by applying perturbation expansions of orthogonal projection operators. 相似文献
103.
Kai-li Xiang 《计算数学(英文版)》2002,(2)
1. IntroductionWe consider a class of direct hybrid methods proposed in [11 for solving the second orderinitial value problemy" = f(t,y), y(0),y'(0) given (1.1)The basic method has the formandHere t. = nh and we define t.l.. = t. I aih, i = 1, 2 and n=0,1… 相似文献
104.
By an elementary calculation we obtain the exact mean values of Minkowksi functionals for a standard model of percolating sets. In particular, a recurrence theorem for the mean Euler characteristic recently put forward is shown to be incorrect. Related previous mathematical work is mentioned. We also conjecture bounds for the threshold density of continuum percolation, which are associated with the Euler characteristic. 相似文献
105.
Daniel FRANCO Gennaro INFANTE Donal O'REGAN 《数学学报(英文版)》2006,22(6):1745-1750
We establish new criteria for the existence of either positive or nonzero solutions of the Urysohn integral equation. We also discuss the existence of an interval of positive eigenvalues and sufficient conditions for the existence of at least a positive eigenvalue with a nonzero or positive eigenfunction for the Urysohn integral operator. Among others, we employ techniques based on fixed point index theory for compact maps, which are new for this type of equation. 相似文献
106.
Summary We study the integration theory for general integral metrics when restricted to upper integrals q, finding improvements in the relation between the classes of the q-integrable and the ql-integrable functions. We give new results and notions which lead to the desirable characterizations of q-integrable functions as ql-integrable f with q(|f|) < ∞, and of ql-integrable functions via the integrability of their upper truncations, under natural conditions which are fulfilled in most
finitely additive integration theories. 相似文献
107.
Youssef Randjiou 《Journal of Theoretical Probability》2006,19(3):701-720
Let μ+(t) and μ−(t) be the locations of the maximum and minimum, respectively, of a standard Brownian motion in the interval [0,t]. We establish a joint integral test for the lower functions of μ+(t) and μ−(t), in the sense of Paul Lévy. In particular, it yields the law of the iterated logarithm for max(μ+(t),μ−(t)) as a straightforward consequence. Our result is in agreement with well-known theorems of Chung and Erdős [(1952) Trans. Amer. Math. Soc. 72, 179–186.], and Csáki, F?ldes and Révész [(1987) Prob. Theory Relat. Fields
76, 477–497].
相似文献
108.
R. Minami A. Kasugai K. Takahashi N. Kobayashi Y. Mitsunaka K. Sakamoto 《International Journal of Infrared and Millimeter Waves》2006,27(1):13-24
A high efficiency launcher of quasi-optical (QO) mode converters for high power gyrotrons have been designed and tested. A
helical cut launcher radiates the RF power via its straight cut onto the first phase correcting mirror. The launchers have
been optimized for the TE31.8 mode at 170 GHz and TE22.6 mode at 110 GHz by numerically optimizing a launcher surface. The helical cut of the launcher has been optimized by taking
the taper angle into account. Further more, the amplitude of the surface perturbation have been optimized for improved focusing
in order to reduce the diffraction losses at the helical cut. Low power measurement shows a good agreement with the design.
High efficiency characteristics of the design have also been calculated on the assumption of frequency downshift due to the
thermal expansion of the cavity and stepwise frequency tuning by changing the operating mode. Besides, the possibility of
high efficiency launcher for higher mode is discussed, and these results give the prospect to high efficiency long pulse gyrotrons. 相似文献
109.
High even order generalizations of the traditional upwind method are introduced to solve second order ODE-BVPs without recasting
the problem as a first order system. Both theoretical analysis and numerical comparison with central difference schemes of
the same order show that these new methods may avoid typical oscillations and achieve high accuracy. Singular perturbation
problems are taken into account to emphasize the main features of the proposed methods.
AMS subject classification (2000) 65L10, 65L12, 65L50 相似文献
110.
In this paper, we study the consistency of a variant of fractionalstep Runge–Kutta methods. These methods are designed tointegrate efficiently semi-linear multidimensional parabolicproblems by means of linearly implicit time integration processes.Such time discretization procedures are also related to a splittingof the space differential operator (or the spatial discretizationof it) as a sum of simpler linear differentialoperators and a nonlinear term. 相似文献