全文获取类型
收费全文 | 7157篇 |
免费 | 846篇 |
国内免费 | 382篇 |
专业分类
化学 | 494篇 |
晶体学 | 2篇 |
力学 | 716篇 |
综合类 | 154篇 |
数学 | 4791篇 |
物理学 | 2228篇 |
出版年
2024年 | 6篇 |
2023年 | 91篇 |
2022年 | 93篇 |
2021年 | 152篇 |
2020年 | 187篇 |
2019年 | 152篇 |
2018年 | 143篇 |
2017年 | 266篇 |
2016年 | 299篇 |
2015年 | 169篇 |
2014年 | 442篇 |
2013年 | 582篇 |
2012年 | 398篇 |
2011年 | 403篇 |
2010年 | 328篇 |
2009年 | 426篇 |
2008年 | 456篇 |
2007年 | 454篇 |
2006年 | 390篇 |
2005年 | 377篇 |
2004年 | 287篇 |
2003年 | 287篇 |
2002年 | 263篇 |
2001年 | 253篇 |
2000年 | 205篇 |
1999年 | 192篇 |
1998年 | 173篇 |
1997年 | 130篇 |
1996年 | 133篇 |
1995年 | 123篇 |
1994年 | 58篇 |
1993年 | 59篇 |
1992年 | 51篇 |
1991年 | 39篇 |
1990年 | 43篇 |
1989年 | 24篇 |
1988年 | 17篇 |
1987年 | 34篇 |
1986年 | 19篇 |
1985年 | 46篇 |
1984年 | 20篇 |
1983年 | 15篇 |
1982年 | 15篇 |
1981年 | 11篇 |
1980年 | 13篇 |
1979年 | 23篇 |
1978年 | 7篇 |
1977年 | 4篇 |
1976年 | 7篇 |
1973年 | 4篇 |
排序方式: 共有8385条查询结果,搜索用时 140 毫秒
1.
In this article, we construct and analyze a residual-based a posteriori error estimator for a quadratic finite volume method (FVM) for solving nonlinear elliptic partial differential equations with homogeneous Dirichlet boundary conditions. We shall prove that the a posteriori error estimator yields the global upper and local lower bounds for the norm error of the FVM. So that the a posteriori error estimator is equivalent to the true error in a certain sense. Numerical experiments are performed to illustrate the theoretical results. 相似文献
2.
该文利用基于射线的盲解卷积方法,从直达声区的水面舰船噪声中提取出船和锚系于海底的垂直接收阵之间的时域信道响应,并利用直达波在不同阵元相对于参考阵元的到达时间差,通过序贯方法,利用射线模型和声速剖面信息,对水面舰船距接收阵的距离进行了估计。通过处理海深约为580 m的2016年美国圣巴巴拉海峡的实验数据,对1.6~3.5 km直达声区范围内Anna Maersk商船与垂直阵之间的距离进行了估计,验证了测距方法的有效性,并将结果与系统测量值和几何方法的估计值进行了比较。由于该方法不需要对海底参数进行估计,所以在海底参数未知时要优于传统匹配场方法;在声速剖面存在跃层且海底为多层分布的复杂信道条件下,该方法仍能对距离进行有效估计,且与测量值的相对误差在6%以内,小于几何方法的估计误差,测距结果精度较高。 相似文献
3.
We study the mean square of the error term of the mean value for binary Egyptian fractions.We get an asymptotic formula under the Riemann Hypothesis. 相似文献
4.
Time integration of Fourier pseudospectral DNS is usually performed using the classical fourth-order accurate Runge-Kutta method or other second- or third-order methods, with a fixed step size. We investigate the use of higher-order Runge-Kutta pairs and automatic step size control based on local error estimation. We find that the fifth-order accurate Runge-Kutta pair of Bogacki and Shampine gives much greater accuracy at a significantly reduced computational cost. Specifically, we demonstrate speedups of 2× to 10× for the same accuracy. Numerical tests (including the Taylor-Green vortex, Rayleigh-Taylor instability, and homogeneous isotropic turbulence) confirm the reliability and efficiency of the method. We also show that adaptive time stepping provides a significant computational advantage for some problems (like the development of a Rayleigh-Taylor instability) without compromising accuracy. 相似文献
5.
介绍了一种条带束流位置监测器(BPM)的设计与仿真方法。在国家同步辐射实验室"太赫兹近场高通量材料物性测试系统"工程项目中,针对波荡器出口处真空室非正交对称性的问题,设计了矩形真空室和跑道形真空室下的两种非正交对称性条带BPM,并与传统的圆形真空室下条带BPM进行对比。基于边界元法,利用MATLAB软件分别对三种真空室下的条带BPM进行建模和仿真。仿真结果表明:相对于传统的圆形真空室下条带BPM,矩形和跑道形真空室下条带BPM灵敏度提高了30%,阻抗匹配误差相对降低了20%,束流位置拟合误差降低了80%。考虑加工精度,矩形真空室下的条带BPM更适用于该工程。 相似文献
6.
Murad B. Khorsheed Qasim M. Zainel Oday A. Hassen Saad M. Darwish 《Entropy (Basel, Switzerland)》2020,22(12)
This paper applies the entropy-based fractal indexing scheme that enables the grid environment for fast indexing and querying. It addresses the issue of fault tolerance and load balancing-based fractal management to make computational grids more effective and reliable. A fractal dimension of a cloud of points gives an estimate of the intrinsic dimensionality of the data in that space. The main drawback of this technique is the long computing time. The main contribution of the suggested work is to investigate the effect of fractal transform by adding R-tree index structure-based entropy to existing grid computing models to obtain a balanced infrastructure with minimal fault. In this regard, the presented work is going to extend the commonly scheduling algorithms that are built based on the physical grid structure to a reduced logical network. The objective of this logical network is to reduce the searching in the grid paths according to arrival time rate and path’s bandwidth with respect to load balance and fault tolerance, respectively. Furthermore, an optimization searching technique is utilized to enhance the grid performance by investigating the optimum number of nodes extracted from the logical grid. The experimental results indicated that the proposed model has better execution time, throughput, makespan, latency, load balancing, and success rate. 相似文献
7.
8.
9.
Fahimeh Salari Sehdaran Mohammad H. Zandi Alireza Bahrampour 《Physics letters. A》2019,383(35):126006
This work aims to compare results of dissipating and pure dephasing single-qubit probes in characterizing the cutoff frequency of a harmonic reservoir Ohmic spectral density. In particular, we proved that a dissipating single-qubit improves the estimation precision of the cutoff frequency estimator. The information backflow and outflow of the dissipating and the dephasing single-qubit are compared. The relation between the quantum Fisher information and the information outflow and backflow of the probe is obtained. The results show that difference between the values of information outflow and backflow of a dissipating single-qubit is larger than a dephasing single-qubit. So, a single-qubit probe extracts more information from an Ohmic reservoir by increasing the difference between the values of information outflow and backflow of the probe. 相似文献
10.
To accelerate head-related transfer functions (HRTFs) measurement, two or more independent sound sources are usually employed in the measurement system. However, the multiple scattering between adjacent sound
sources may influence the accuracy of measurement. On the other hand, the directivity of sound source could induce measurement error. Therefore, a model consisting of two spherical sound sources with approximate omni-directivity and a rigid-spherical head is proposed to evaluate the errors in HRTF measurement caused by multiple scattering between sources. An example of analysis using multipole re-expansion indicates that the error of ipsilateral HRTFs are within the bound of ± 1.0 dB below a frequency of 20 kHz, provided that the sound source radius does not exceed 0.025 m, the source distance relative to head center is not less than 0.5 m, and the angular interval between two adjacent
sources is not less than 20 degrees. Similar conclusions under different conditions can also be analyzed and discussed by using this calculation method. Furthermore, the results are verified by measurements of HRTFs for a rigid
sphere and a KEMAR artificial head. 相似文献