全文获取类型
收费全文 | 6160篇 |
免费 | 375篇 |
国内免费 | 260篇 |
专业分类
化学 | 1059篇 |
晶体学 | 9篇 |
力学 | 1302篇 |
综合类 | 24篇 |
数学 | 3680篇 |
物理学 | 721篇 |
出版年
2024年 | 2篇 |
2023年 | 17篇 |
2022年 | 44篇 |
2021年 | 81篇 |
2020年 | 73篇 |
2019年 | 223篇 |
2018年 | 239篇 |
2017年 | 112篇 |
2016年 | 165篇 |
2015年 | 233篇 |
2014年 | 294篇 |
2013年 | 342篇 |
2012年 | 326篇 |
2011年 | 561篇 |
2010年 | 432篇 |
2009年 | 544篇 |
2008年 | 508篇 |
2007年 | 544篇 |
2006年 | 448篇 |
2005年 | 284篇 |
2004年 | 259篇 |
2003年 | 238篇 |
2002年 | 108篇 |
2001年 | 55篇 |
2000年 | 66篇 |
1999年 | 69篇 |
1998年 | 59篇 |
1997年 | 46篇 |
1996年 | 54篇 |
1995年 | 42篇 |
1994年 | 53篇 |
1993年 | 52篇 |
1992年 | 36篇 |
1991年 | 30篇 |
1990年 | 16篇 |
1989年 | 19篇 |
1988年 | 24篇 |
1987年 | 20篇 |
1986年 | 16篇 |
1985年 | 19篇 |
1984年 | 10篇 |
1983年 | 6篇 |
1982年 | 4篇 |
1981年 | 2篇 |
1980年 | 6篇 |
1979年 | 6篇 |
1978年 | 2篇 |
1977年 | 2篇 |
1974年 | 1篇 |
1957年 | 1篇 |
排序方式: 共有6795条查询结果,搜索用时 15 毫秒
1.
Maria Eduarda B. S. de Oliveira Adilson Sartoratto Jean Carlos Cardoso 《Molecules (Basel, Switzerland)》2020,25(24)
The efficient production of plant-derived medicinal compounds (PDMCs) from in vitro plants requires improvements in knowledge about control of plant or organ development and factors affecting the biosynthesis pathway of specific PDMCs under in vitro conditions, leading to a realistic large-scale tool for in vitro secondary metabolite production. Thus, this study aimed to develop an in vitro technique, through the induction and proliferation of calli, for production of plant fresh weight, and to compare the PDMC profile obtained from the plants versus in vitro calli of Phyllanthus amarus. It was successfully possible to obtain and proliferate two types of calli, one with a beige color and a friable appearance, obtained in the dark using Murashige and Skoog (MS) medium plus 2,4-dichlorophenoxyacetic acid (2,4-D), and a second type with a green color, rigid consistency, and nonfriable appearance obtained under light conditions and MS medium plus 6-benzyladenine (6-BA). In vitro micropropagated plants that gave rise to calli were also acclimatized in a greenhouse and cultivated until obtaining the mass for PDMC analysis and used as a control. While the micropropagated-derived plants concentrated the lignans niranthin, nirtetralin, and phyllanthin, the Phyllanthus amarus calli proliferated in vitro concentrated a completely different biochemical profile and synthesis of compounds, such as betulone, squalene, stigmasterol, and β-sitosterol, in addition to others not identified by GC-MS database. These results demonstrate the possibility of applying the calli in vitro from Phyllanthus amarus for production of important PDMCs unlike those obtained in cultures of differentiated tissues from field plants. 相似文献
2.
3.
The present paper is concerned with the asymptotic behaviors of radially symmetric solutions for the multi-dimensional Burgers equation on the exterior domain in , where the boundary and far field conditions are prescribed. We show that in some case where the corresponding 1-D Riemann problem for the non-viscous part admits a shock wave, the solution tends toward a linear superposition of stationary and rarefaction waves as time goes to infinity, and also show the decay rate estimates. Furthermore, we improve the results on the asymptotic stability of the stationary waves which are treated in the previous papers [2], [3]. Finally, for the case of , we give the complete classification of the asymptotic behaviors, which includes even a linear superposition of stationary and viscous shock waves. 相似文献
4.
We consider the question whether, given a countable family of lattices in a locally compact abelian group G, there exist functions such that the resulting generalized shift-invariant system is a tight frame of . This paper develops a new approach to the study of generalized shift-invariant system via almost periodic functions, based on a novel unconditional convergence property. From this theory, we derive characterizing relations for tight and dual frame generators, we introduce the system bandwidth as a measure of the total bandwidth a generalized shift-invariant system can carry, and we show that the so-called Calderón sum is uniformly bounded from below for generalized shift-invariant frames. Without the unconditional convergence property, we show, counter intuitively, that even orthonormal bases can have arbitrary small system bandwidth. Our results show that the question of existence of frame generators for a general lattice system is rather subtle and depends on analytical and algebraic properties of the lattice system. 相似文献
5.
Motivated by the quasi-local mass problem in general relativity, we study the rigidity of isometric immersions with the same mean curvature into a warped product space. As a corollary of our main result, two star-shaped hypersurfaces in a spatial Schwarzschild or AdS-Schwarzschild manifold with nonzero mass differ only by a rotation if they are isometric and have the same mean curvature. We also prove similar results if the mean curvature condition is replaced by an -curvature condition. 相似文献
6.
We study thin obstacle problems involving the energy functional with -growth. We prove higher integrability and Hölder regularity for the gradient of minimizers of the thin obstacle problems under the assumption that the variable exponent is Hölder continuous. 相似文献
7.
Liang Zhao 《Journal of Differential Equations》2019,266(9):5615-5624
In this paper, let be n-dimensional noncompact metric measure space which satisfies Poincaré inequality with some Ricci curvature condition. We obtain a Liouville theorem for positive weak solutions to weighted p-Lichnerowicz equation where are real constants. 相似文献
8.
Antonio Kumpera 《Journal of Differential Equations》2019,266(2-3):911-935
We first discuss the problems in the theory of ordinary differential equations that gave rise to the concept of a flag system and illustrate these with the Cartan criterion for Monge equations (1st order) as well as the Cartan statement concerning the local equivalence of Monge–Ampère type equations (2nd order). Next, we describe a prolongation functor operating on the infinitesimal symmetries (automorphisms) of the Darboux flag and extending these, isomorphically, to all the symmetries of any other flag. Hence, flag systems cannot be distinguished by their symmetry algebras and the local classification of these objects is approached by considering higher order isotropies of these algebras as well as the groupoids of k-th order formal equivalences since the differential equations defining the latter provide precious information for the application of flag systems to differential equations (e.g., Cartan's criterion for non-linear Monge equations). In examining the behavior of the isotropy algebras, that can either diminish or remain the same, when passing from a derived system to the consecutive system , we obtain a full set of numerical invariants for the elementary flag systems that moreover specifies the local models. 相似文献
9.
Mark Prendergast Cormac Breen Aibhin Bray Fiona Faulkner Brian Carroll Dominic Quinn 《International Journal of Mathematical Education in Science & Technology》2018,49(8):1203-1218
Many studies over the past 30 years have highlighted the important role of students’ beliefs for successful problem-solving in mathematics. Given the recent emphasis afforded to problem-solving on the reformed Irish secondary school mathematics curriculum, the main aim of this study was to identify Irish students’ (n = 975) beliefs about the field. A quantitative measure of these beliefs was attained through the use of the Indiana Mathematical Belief Scale, an existing 30-item (five-scale) self-report questionnaire. A statistical analysis of the data revealed that students who were further through their secondary education had a stronger belief that not all problems could be solved by applying routine procedures. In contrast, the same students held less positive beliefs than their younger counterparts that they could solve time-consuming problems and that conceptual understanding was important. The analysis also indicated that gender had a significant impact on three of the five belief scales. 相似文献
10.
Tomasz Przebinda 《Journal of Functional Analysis》2018,274(5):1284-1305
We relate the distribution characters and the wave front sets of unitary representation for real reductive dual pairs of type I in the stable range. 相似文献