全文获取类型
收费全文 | 111329篇 |
免费 | 10043篇 |
国内免费 | 8859篇 |
专业分类
化学 | 46303篇 |
晶体学 | 1422篇 |
力学 | 13382篇 |
综合类 | 1647篇 |
数学 | 30807篇 |
物理学 | 36670篇 |
出版年
2023年 | 982篇 |
2022年 | 1628篇 |
2021年 | 2902篇 |
2020年 | 2737篇 |
2019年 | 2584篇 |
2018年 | 2390篇 |
2017年 | 2964篇 |
2016年 | 3477篇 |
2015年 | 2889篇 |
2014年 | 4491篇 |
2013年 | 7481篇 |
2012年 | 5381篇 |
2011年 | 6134篇 |
2010年 | 5235篇 |
2009年 | 6702篇 |
2008年 | 6910篇 |
2007年 | 7325篇 |
2006年 | 6160篇 |
2005年 | 5134篇 |
2004年 | 4419篇 |
2003年 | 4427篇 |
2002年 | 6681篇 |
2001年 | 3525篇 |
2000年 | 3235篇 |
1999年 | 2717篇 |
1998年 | 2645篇 |
1997年 | 1819篇 |
1996年 | 1731篇 |
1995年 | 1577篇 |
1994年 | 1599篇 |
1993年 | 1346篇 |
1992年 | 1396篇 |
1991年 | 975篇 |
1990年 | 857篇 |
1989年 | 674篇 |
1988年 | 629篇 |
1987年 | 545篇 |
1986年 | 490篇 |
1985年 | 585篇 |
1984年 | 525篇 |
1983年 | 304篇 |
1982年 | 484篇 |
1981年 | 653篇 |
1980年 | 519篇 |
1979年 | 580篇 |
1978年 | 466篇 |
1977年 | 387篇 |
1976年 | 284篇 |
1974年 | 110篇 |
1973年 | 185篇 |
排序方式: 共有10000条查询结果,搜索用时 62 毫秒
991.
992.
We design and analyze an unconditionally convergent nonstandard finite-difference method to study transmission dynamics of a mathematical model of HIV-TB co-infection. The dynamics of this model are studied using the qualitative theory of dynamical systems. These qualitative features of the continuous model are preserved by the numerical method that we propose in this paper. This method also preserves positivity of the solution which is one of the essential requirements when modelling epidemic diseases. Furthermore, we show that the numerical method is unconditionally stable. Competitive numerical results confirming theoretical investigations are provided. Comparisons are also made with other conventional approaches that are routinely used to solve these types of problems. 相似文献
993.
994.
In many atmospheric and earth sciences, it is of interest to identify dominant spatial patterns of variation based on data observed at p locations and n time points with the possibility that p > n. While principal component analysis (PCA) is commonly applied to find the dominant patterns, the eigenimages produced from PCA may exhibit patterns that are too noisy to be physically meaningful when p is large relative to n. To obtain more precise estimates of eigenimages, we propose a regularization approach incorporating smoothness and sparseness of eigenimages, while accounting for their orthogonality. Our method allows data taken at irregularly spaced or sparse locations. In addition, the resulting optimization problem can be solved using the alternating direction method of multipliers, which is easy to implement, and applicable to a large spatial dataset. Furthermore, the estimated eigenfunctions provide a natural basis for representing the underlying spatial process in a spatial random-effects model, from which spatial covariance function estimation and spatial prediction can be efficiently performed using a regularized fixed-rank kriging method. Finally, the effectiveness of the proposed method is demonstrated by several numerical examples. 相似文献
995.
Recently, there is a growing interest in the spectral approximation by the Prolate Spheroidal Wave Functions (PSWFs) . This is due to the promising new contributions of these functions in various classical as well as emerging applications from Signal Processing, Geophysics, Numerical Analysis, etc. The PSWFs form a basis with remarkable properties not only for the space of band-limited functions with bandwidth c, but also for the Sobolev space . The quality of the spectral approximation and the choice of the parameter c when approximating a function in by its truncated PSWFs series expansion, are the main issues. By considering a function as the restriction to of an almost time-limited and band-limited function, we try to give satisfactory answers to these two issues. Also, we illustrate the different results of this work by some numerical examples. 相似文献
996.
The existence problem of a ‐cycle frame of type is now solved for any quadruple . 相似文献
997.
998.
It is shown that the matrix sequence generated by Euler's method starting from the identity matrix converges to the principal pth root of a square matrix, if all the eigenvalues of the matrix are in a region including the one for Newton's method given by Guo in 2010. The convergence is cubic if the matrix is invertible. A modification version of Euler's method using the Schur decomposition is developed. Numerical experiments show that the modified algorithm has the overall good numerical behavior. 相似文献
999.
On the two classes of high‐order convergent methods of approximate inverse preconditioners for solving linear systems 下载免费PDF全文
Suzan C. Buranay Dervis Subasi Ovgu C. Iyikal 《Numerical Linear Algebra with Applications》2017,24(6)
Two classes of methods for approximate matrix inversion with convergence orders p =3?2k +1 (Class 1) and p =5?2k ?1 (Class 2), k ≥1 an integer, are given based on matrix multiplication and matrix addition. These methods perform less number of matrix multiplications compared to the known hyperpower method or p th‐order method for the same orders and can be used to construct approximate inverse preconditioners for solving linear systems. Convergence, error, and stability analyses of the proposed classes of methods are provided. Theoretical results are justified with numerical results obtained by using the proposed methods of orders p =7,13 from Class 1 and the methods with orders p =9,19 from Class 2 to obtain polynomial preconditioners for preconditioning the biconjugate gradient (BICG) method for solving well‐ and ill‐posed problems. From the literature, methods with orders p =8,16 belonging to a family developed by the effective representation of the p th‐order method for orders p =2k , k is integer k ≥1, and other recently given high‐order convergent methods of orders p =6,7,8,12 for approximate matrix inversion are also used to construct polynomial preconditioners for preconditioning the BICG method to solve the considered problems. Numerical comparisons are given to show the applicability, stability, and computational complexity of the proposed methods by paying attention to the asymptotic convergence rates. It is shown that the BICG method converges very quickly when applied to solve the preconditioned system. Therefore, the cost of constructing these preconditioners is amortized if the preconditioner is to be reused over several systems of same coefficient matrix with different right sides. 相似文献
1000.
Danjie Zhang Ming-Hui Chen Joseph G. Ibrahim Mark E. Boye Wei Shen 《Journal of computational and graphical statistics》2017,26(1):121-133
Joint models for longitudinal and survival data are routinely used in clinical trials or other studies to assess a treatment effect while accounting for longitudinal measures such as patient-reported outcomes. In the Bayesian framework, the deviance information criterion (DIC) and the logarithm of the pseudo-marginal likelihood (LPML) are two well-known Bayesian criteria for comparing joint models. However, these criteria do not provide separate assessments of each component of the joint model. In this article, we develop a novel decomposition of DIC and LPML to assess the fit of the longitudinal and survival components of the joint model, separately. Based on this decomposition, we then propose new Bayesian model assessment criteria, namely, ΔDIC and ΔLPML, to determine the importance and contribution of the longitudinal (survival) data to the model fit of the survival (longitudinal) data. Moreover, we develop an efficient Monte Carlo method for computing the conditional predictive ordinate statistics in the joint modeling setting. A simulation study is conducted to examine the empirical performance of the proposed criteria and the proposed methodology is further applied to a case study in mesothelioma. Supplementary materials for this article are available online. 相似文献