全文获取类型
收费全文 | 4628篇 |
免费 | 262篇 |
国内免费 | 400篇 |
专业分类
化学 | 1690篇 |
晶体学 | 13篇 |
力学 | 294篇 |
综合类 | 64篇 |
数学 | 2490篇 |
物理学 | 739篇 |
出版年
2023年 | 65篇 |
2022年 | 84篇 |
2021年 | 143篇 |
2020年 | 139篇 |
2019年 | 91篇 |
2018年 | 86篇 |
2017年 | 94篇 |
2016年 | 133篇 |
2015年 | 131篇 |
2014年 | 178篇 |
2013年 | 342篇 |
2012年 | 236篇 |
2011年 | 268篇 |
2010年 | 221篇 |
2009年 | 312篇 |
2008年 | 312篇 |
2007年 | 297篇 |
2006年 | 269篇 |
2005年 | 213篇 |
2004年 | 169篇 |
2003年 | 174篇 |
2002年 | 124篇 |
2001年 | 119篇 |
2000年 | 94篇 |
1999年 | 110篇 |
1998年 | 106篇 |
1997年 | 104篇 |
1996年 | 65篇 |
1995年 | 67篇 |
1994年 | 65篇 |
1993年 | 54篇 |
1992年 | 45篇 |
1991年 | 31篇 |
1990年 | 38篇 |
1989年 | 29篇 |
1988年 | 26篇 |
1987年 | 19篇 |
1986年 | 32篇 |
1985年 | 34篇 |
1984年 | 16篇 |
1983年 | 9篇 |
1982年 | 22篇 |
1981年 | 19篇 |
1980年 | 26篇 |
1979年 | 21篇 |
1978年 | 17篇 |
1977年 | 15篇 |
1976年 | 11篇 |
1973年 | 4篇 |
1972年 | 4篇 |
排序方式: 共有5290条查询结果,搜索用时 509 毫秒
1.
2.
3.
《Arabian Journal of Chemistry》2021,14(11):103421
Processing of Carapa guianensis seeds to obtain oil on an industrial scale generates a significant amount of by-product, approximately 66% w/w, which is called cake and is a potential source of biomolecules, including simple phenolic structures. For this reason, studies were carried out on the chemical profiles of hydrolyzed extract from this agro-industrial by-product through High Performance Thin-Layer Chromatography (HPTLC) and Gas Chromatography coupled to Mass Spectrometry (GC–MS). These techniques were used to detect metabolic classes and/or groups, and to identify, for the first time, thirteen simple phenolic acids in this by-product. The sample antioxidant capacity was determined by methods of 2,2-diphenyl-1-picrylhydrazyl (DPPH)and 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt (ABTS+) radicals direct sequestration. The hydrolyzed fraction showed a total of 63.47% in the relative abundance of the total of compounds, standing out: p-hydroxybenzoic acid (39.19%) and protocatechuic acid (3,4-dihydroxybenzoic acid) (5.62%), both from hydroxybenzoic acids and 3-(3,4-dihydroxyphenyl)lactic acid, (7.76%) hydroxycinnamic acids derivatives. In these results, the fraction rich in simple phenolic acids was obtained, attributing the prominent behavior of this matrix antioxidant activity, expressed by (IC50: of 16.42 µg/mL and 6.52 µg/mL for DPPH and ABTS+ radicals, respectively). The research demonstrated an alternative to applicability that involves sustainability from agro-industrial. These techniques were used to detect metabolic classes and/or groups, and to identify, for the first time, thirteen simple phenolic acids in this by-product, generating a process capable of converting biomass into a bioproduct, consisting of bioactive compounds, in addition to adding value to the industrial chain. 相似文献
4.
We investigate optimal harvesting control in a predator–prey model in which the prey population is represented by a first-order
partial differential equation with age-structure and the predator population is represented by an ordinary differential equation
in time. The controls are the proportions of the populations to be harvested, and the objective functional represents the
profit from harvesting. The existence and uniqueness of the optimal control pair are established. 相似文献
5.
Geng Sheng WANG 《数学学报(英文版)》2005,21(5):1005-1014
This work is concerned with Pontryagin's maximum principle of optimal control problems governed by some non-well-posed semilinear heat equations. A type of approach to the non-well-posed optimal control problem is given. 相似文献
6.
Parabolic Capacity and Soft Measures for Nonlinear Equations 总被引:2,自引:0,他引:2
We first introduce, using a functional approach, the notion of capacity related to the parabolic p-Laplace operator. Then we prove a decomposition theorem for measures (in space and time) that do not charge the sets of null capacity. We apply this result to prove existence and uniqueness of renormalized solutions for nonlinear parabolic initial boundary-value problems with such measures as right-hand side. 相似文献
7.
We consider an optimal growth (multi-sector) model with nonconvex technology. Using the Clarke results on generalized gradients, we prove that the value function has left and right derivatives with respect to the initial capital stock, without requiring supermodularity assumptions. 相似文献
8.
Efficient Sequential Quadratic Programming Implementations for Equality-Constrained Discrete-Time Optimal Control 总被引:1,自引:0,他引:1
Efficient sequential quadratic programming (SQP) implementations are presented for equality-constrained, discrete-time, optimal control problems. The algorithm developed calculates the search direction for the equality-based variant of SQP and is applicable to problems with either fixed or free final time. Problem solutions are obtained by solving iteratively a series of constrained quadratic programs. The number of mathematical operations required for each iteration is proportional to the number of discrete times N. This is contrasted by conventional methods in which this number is proportional to N
3. The algorithm results in quadratic convergence of the iterates under the same conditions as those for SQP and simplifies to an existing dynamic programming approach when there are no constraints and the final time is fixed. A simple test problem and two application problems are presented. The application examples include a satellite dynamics problem and a set of brachistochrone problems involving viscous friction. 相似文献
9.
一类经典”秘书问题”的推广 总被引:2,自引:0,他引:2
”秘书问题”在最优停时理论的发展中曾起过重要作用 ,实际中的一类问题与”秘书问题”有类似之处 ,但比”秘书问题”更复杂 .本文将经典”秘书问题”进行推广 ,建立了一类比经典”秘书问题”更有实际意义的模型 ,并给出了该类模型的解 . 相似文献
10.
Summary We consider the problem of maximizing the discounted net profit of a firm which purchases a quantity of some product at a
given time and afterwards advertises and sells the product progressively. We distinguish among the three possibilities of
assuming the final time to be either fixed, or bounded, or free. In all cases, after stating the problem in the optimal control
theory framework, we prove the existence of an optimal solution and characterize it using the Maximum Principle necessary
conditions. Furthermore, we prove that the convexity of the purchase cost function is a sufficient condition for the uniqueness
of the optimal solution.
Partially supported by MURST. 相似文献