全文获取类型
收费全文 | 15438篇 |
免费 | 1757篇 |
国内免费 | 1316篇 |
专业分类
化学 | 5376篇 |
晶体学 | 185篇 |
力学 | 1287篇 |
综合类 | 200篇 |
数学 | 5904篇 |
物理学 | 5559篇 |
出版年
2024年 | 10篇 |
2023年 | 187篇 |
2022年 | 224篇 |
2021年 | 260篇 |
2020年 | 383篇 |
2019年 | 406篇 |
2018年 | 356篇 |
2017年 | 461篇 |
2016年 | 634篇 |
2015年 | 511篇 |
2014年 | 804篇 |
2013年 | 1321篇 |
2012年 | 807篇 |
2011年 | 1087篇 |
2010年 | 865篇 |
2009年 | 981篇 |
2008年 | 952篇 |
2007年 | 1039篇 |
2006年 | 924篇 |
2005年 | 811篇 |
2004年 | 669篇 |
2003年 | 609篇 |
2002年 | 569篇 |
2001年 | 468篇 |
2000年 | 460篇 |
1999年 | 377篇 |
1998年 | 333篇 |
1997年 | 308篇 |
1996年 | 247篇 |
1995年 | 170篇 |
1994年 | 176篇 |
1993年 | 113篇 |
1992年 | 92篇 |
1991年 | 104篇 |
1990年 | 87篇 |
1989年 | 96篇 |
1988年 | 103篇 |
1987年 | 74篇 |
1986年 | 46篇 |
1985年 | 58篇 |
1984年 | 64篇 |
1983年 | 30篇 |
1982年 | 46篇 |
1981年 | 39篇 |
1980年 | 29篇 |
1979年 | 32篇 |
1978年 | 19篇 |
1977年 | 16篇 |
1976年 | 14篇 |
1973年 | 12篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
941.
We consider a production system with finite buffers and arbitrary topology where service time is subject to interruptions in one of three ways, viz. machine breakdown, machine vacations or Npolicy. We develop a unified approximation (analytical) methodology to calculate the throughput of the system using queueing networks together with decomposition, isolation and expansion techniques. The methodology is rigorously tested covering a large experimental region. Orthogonal arrays are used to design the experiments in order to keep the number of experiments manageable. The results obtained using the approximation methodology are compared to the simulation results. The ttests carried out to investigate the differences between the two results show that they are statistically insignificant. Finally, we test the methodology by applying it to several arbitrary topology networks. The results show that the performance of the approximation methodology is consistent, robust and produces excellent results in a variety of experimental conditions. 相似文献
942.
Random orders on
invariant under permutations are called exchangeable. The compact and convex set of all random total orders is shown to be a Bauer simplex whose set of extreme points, the socalled totally ordered paintbox processes, is homeomorphically parametrized by almost uniform distributions on the unit interval, i.e. by probability measures w on [0, 1] whose distribution functions are w-almost surely the identity. 相似文献
943.
The evolution Boussinesq equations describe the evolution of the temperature and velocity fields of viscous incompressible Newtonian fluids. Very often, they are a reasonable model to render relevant phenomena of flows in which the thermal effects play an essential role. In the paper we prescribe non-Dirichlet boundary conditions on a part of the boundary and prove the existence and uniqueness of solutions to the Boussinesq equations on a (short) time interval. The length of the time interval depends only on certain norms of the given data. In the proof we use a fixed point theorem method in Sobolev spaces with non-integer order derivatives. The proof is performed for Lipschitz domains and a wide class of data. 相似文献
944.
利用单调迭代方法,在Banach空间研究了更为一般脉冲微分方程的初值问题的最小最大拟解的存在性及迭代逼近程序。 相似文献
945.
P. G. Walsh. 《Mathematics of Computation》2000,69(231):1183-1191
The purpose of this paper is to describe a method to determine whether a bivariate polynomial with rational coefficients is irreducible when regarded as an element in , the ring of polynomials with coefficients from the field of Laurent series in with rational coefficients. This is achieved by computing certain associated Puiseux expansions, and as a result, a polynomial-time complexity bound for the number of bit operations required to perform this irreducibility test is computed.
946.
Higher-order corrections of the Einstein–Hilbert action of general relativity can be recovered by imposing the existence of a Noether symmetry to a class of theories of gravity where the Ricci scalar R and its d'Alembertian R are present. In several cases, it is possible to get exact cosmological solutions or, at least, to simplify the dynamics by recovering constants of motion. The main result is that a Noether vector seems to rule the presence of higher-order corrections of gravity. 相似文献
947.
Beghin Luisa Knopova Viktoria P. Leonenko Nikolai N. Orsingher Enzo 《Journal of statistical physics》2000,99(3-4):769-781
We analyze the asymptotic behavior of the rescaled solution to the linear Korteweg–de Vries equation when the initial conditions are supposed to be random and weakly dependent. By means of the method of moments we prove the Gaussianity of the limiting process and we present its correlation function. The same technique is applied to the analysis of another third-order heat-type equation. 相似文献
948.
Ajoy G Ramakrishna J Bahçeli S Klinowski J 《Solid state nuclear magnetic resonance》2000,16(4):63-338
We derive a complete analytical solution for the powder magic angle spinning (MAS) nuclear magnetic resonance (NMR) lineshape in the presence of second-order quadrupole interaction, considering a radiofrequency (rf) pulse of finite width, a finite MAS frequency, and a non-zero asymmetry parameter. Ix is calculated using two approaches. The first applies time-dependent perturbation theory in the presence of the rf pulse and stationary perturbation theory (SPT) in its absence. The second is based on the Magnus expansion of the density matrix in the interaction representation during the pulse and SPT in its absence. We solve the problem in the laboratory frame using the properties of the Fourier transform and spin operators. Diagonalisation is not required. Both approaches agree well with each other under all conditions and also with the transition probability approach for the central transition. The Magnus expansion exists at all times and the effect of the non-secular terms is negligible. We describe an analytical method of averaging Ix over the Euler angles and simulate the 11B MAS NMR lineshapes for crystalline and vitreous B2O3. A critical analysis is given of all earlier calculations of the MAS NMR lineshape. 相似文献
949.
950.
用电子束蒸发沉积方法在X切LBO(X-LBO)晶体上镀制了两种不同膜系结构的1 064和532 nm倍频增透膜,其中一种膜系结构为基底/ZrO2/Y2O3/Al2O3/SiO2/空气,另一种为基底/0.5Al2O3/ZrO2/Y2O3/Al2O3/SiO2/空气,两种膜系结构的主要差别在于有无氧化铝过渡层。测量了薄膜的反射率光谱曲线,发现两种增透膜在1 064和532 nm处的反射率均小于0.5%,实际镀制结果与理论设计曲线的差异主要是由材料折射率的变化引起的。且对样品在空气环境中进行了温度为473 K的退火处理,结果发现两种膜系结构均表现了较优异的光学性能,氧化铝过渡层的加入使薄膜具有强的热应力性能。 相似文献