全文获取类型
收费全文 | 2527篇 |
免费 | 77篇 |
国内免费 | 21篇 |
专业分类
化学 | 1480篇 |
晶体学 | 28篇 |
力学 | 60篇 |
数学 | 456篇 |
物理学 | 601篇 |
出版年
2022年 | 25篇 |
2021年 | 22篇 |
2020年 | 42篇 |
2019年 | 56篇 |
2018年 | 32篇 |
2017年 | 25篇 |
2016年 | 71篇 |
2015年 | 42篇 |
2014年 | 51篇 |
2013年 | 128篇 |
2012年 | 129篇 |
2011年 | 135篇 |
2010年 | 81篇 |
2009年 | 64篇 |
2008年 | 151篇 |
2007年 | 125篇 |
2006年 | 136篇 |
2005年 | 134篇 |
2004年 | 96篇 |
2003年 | 89篇 |
2002年 | 82篇 |
2001年 | 29篇 |
2000年 | 35篇 |
1999年 | 23篇 |
1998年 | 30篇 |
1997年 | 36篇 |
1996年 | 37篇 |
1995年 | 35篇 |
1994年 | 36篇 |
1993年 | 28篇 |
1992年 | 23篇 |
1991年 | 29篇 |
1990年 | 22篇 |
1989年 | 29篇 |
1988年 | 25篇 |
1987年 | 22篇 |
1985年 | 34篇 |
1984年 | 39篇 |
1983年 | 21篇 |
1982年 | 31篇 |
1981年 | 33篇 |
1980年 | 22篇 |
1979年 | 31篇 |
1978年 | 27篇 |
1977年 | 27篇 |
1976年 | 21篇 |
1975年 | 24篇 |
1974年 | 31篇 |
1973年 | 18篇 |
1972年 | 15篇 |
排序方式: 共有2625条查询结果,搜索用时 15 毫秒
111.
112.
In mobile edge computing systems, the edge server placement problem is mainly tackled as a multi-objective optimization problem and solved with mixed integer programming, heuristic or meta-heuristic algorithms, etc. These methods, however, have profound defect implications such as poor scalability, local optimal solutions, and parameter tuning difficulties. To overcome these defects, we propose a novel edge server placement algorithm based on deep q-network and reinforcement learning, dubbed DQN-ESPA, which can achieve optimal placements without relying on previous placement experience. In DQN-ESPA, the edge server placement problem is modeled as a Markov decision process, which is formalized with the state space, action space and reward function, and it is subsequently solved using a reinforcement learning algorithm. Experimental results using real datasets from Shanghai Telecom show that DQN-ESPA outperforms state-of-the-art algorithms such as simulated annealing placement algorithm (SAPA), Top-K placement algorithm (TKPA), K-Means placement algorithm (KMPA), and random placement algorithm (RPA). In particular, with a comprehensive consideration of access delay and workload balance, DQN-ESPA achieves up to 13.40% and 15.54% better placement performance for 100 and 300 edge servers respectively. 相似文献
113.
One reason that the solid-state imaging field is exciting is because of its potential to provide spatially resolved chemical information from deep inside a solid sample. To achieve this goal, methods are needed which exploit the many NMR parameters to generate image contrast. We describe two strategies for employing the chemical-shift anisotropy as a contrast mechanism in images of abundant nuclei in solid polymers. We illustrate these approaches with images which map spatial variations of molecular orientation in samples of polytetrafluoroethylene. 相似文献
114.
Nonlinear Dynamics - This paper addresses the problem of trajectory tracking control of an underactuated surface vessel moving in a two-dimensional space in the presence of unknown disturbances. In... 相似文献
115.
Michael G. Neubauer William Watkins Joel Zeitlin 《Linear algebra and its applications》1998,280(2-3):109-127
The purpose of this paper is to exhibit new infinite families of D-optimal (0, 1)-matrices. We show that Hadamard designs lead to D-optimal matrices of size (j, mj) and (j − 1, mj), for certain integers j ≡ 3 (mod 4) and all positive integers m. For j a power of a prime and j ≡ 1 (mod 4), supplementary difference sets lead to D-optimal matrices of size (j, 2mj) and (j − 1, 2mj), for all positive integers m. We also show that for a given j and d sufficiently large, about half of the entries in each column of a D-optimal matrix are ones. This leads to a new relationship between D-optimality for (0, 1)-matrices and for (±1)-matrices. Known results about D-optimal (±1)-matrices are then used to obtain new D-optimal (0, 1)-matrices. 相似文献
116.
117.
118.
Dr. Jingjun Hao Prof. Arnold L. Rheingold Marzieh Kavand Dr. Kipp J. van Schooten Prof. Christoph Boehme Dr. Marçal Capdevila‐Cortada Prof. Juan J. Novoa Eva Wöss Prof. Günther Knör Prof. Joel S. Miller 《Chemistry (Weinheim an der Bergstrasse, Germany)》2016,22(35):12312-12315
The reaction of 2,3,5,6‐tetracyanopyridine (TCNPy) and Cr(C6H6)2 forms diamagnetic σ‐[TCNPy]22? possessing a 1.572(3) Å intrafragment sp3–sp3 bond. This is in contrast to the structurally related 1,2,4,5‐tetracyanobenzene and 1,2,4,5‐tetracyanopyrazine that form π‐dimer dianions possessing long, multicenter bonds. 相似文献
119.
Maja Ponikvar-Svet Diana N. Zeiger Loryn R. Keating Joel F. Liebman 《Structural chemistry》2012,23(6):2019-2037
In the current review of the journal Structural Chemistry, the contents of the issues 1–3 for the calendar year 2012 are related to thermochemistry. A brief thermochemical commentary is added to the summary of each article. 相似文献
120.
A transition metal-catalyzed/Curtius rearrangement sequence toward the development of conformationally constrained alpha-Boc-aminophosphonates 2-6 is described. An approach using the versatile tert-butylphosphonoacetate moieties 1a and 1b to derive an array of mono- and bicyclic alpha-Boc-aminophosphonate systems is presented. Conformational constraint is incorporated using either the ring-closing metathesis reaction catalyzed by the first generation Grubbs catalyst or intramolecular cyclopropanation mediated by Rh2(OAc)4. Using the tert-butyl ester functionality in 1a or 1b as a potential amino group, the Curtius rearrangement provides an efficient route toward the target alpha-Boc-aminophosphonates. 相似文献