首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   923篇
  免费   62篇
  国内免费   109篇
化学   51篇
力学   61篇
综合类   21篇
数学   803篇
物理学   158篇
  2023年   8篇
  2022年   12篇
  2021年   12篇
  2020年   18篇
  2019年   22篇
  2018年   28篇
  2017年   26篇
  2016年   24篇
  2015年   16篇
  2014年   28篇
  2013年   110篇
  2012年   30篇
  2011年   50篇
  2010年   36篇
  2009年   51篇
  2008年   46篇
  2007年   48篇
  2006年   62篇
  2005年   50篇
  2004年   56篇
  2003年   57篇
  2002年   43篇
  2001年   38篇
  2000年   33篇
  1999年   34篇
  1998年   33篇
  1997年   25篇
  1996年   20篇
  1995年   6篇
  1994年   14篇
  1993年   11篇
  1992年   4篇
  1991年   3篇
  1990年   2篇
  1989年   4篇
  1988年   4篇
  1987年   3篇
  1986年   4篇
  1985年   3篇
  1984年   1篇
  1983年   1篇
  1982年   3篇
  1981年   1篇
  1979年   4篇
  1978年   3篇
  1977年   1篇
  1975年   1篇
  1974年   2篇
  1972年   1篇
  1959年   1篇
排序方式: 共有1094条查询结果,搜索用时 281 毫秒
91.
It has been recognized for some time that when cost-benefit analysis is applied to irreversible environmental decisions, such as that of developing or preserving wilderness land, there can be an option value associated with the preservation decision, which arises when there is future uncertainty with respect to the benefits of development or preservation. In this paper the provenance of option value is examined and it is shown that an important cause is a special kind of uncertainty, viz. the possibility of reversals in direction of the relative valuations of wilderness land and developed land, a property we refer to as ditonicity. It is shown that the more ditonic the relative valuation process the greater the deviance between the certainty-equivalence development policy and the stochastically optimal one, and thus by implication the greater the option value. In the two cases with zero ditonicity, when relative wilderness valuations always increase or always decrease (even though in a stochastic fashion), there is zero option value. The model used assumes that service flows from wilderness and developed land are size-dependent, with future relative values known only in terms of a stochastic process, which can take jumps up or down of the same proportional size, at random times. Development can be partial or total and can occur in impulses at any time over an infinite time horizon.  相似文献   
92.
We apply ideas from commutative algebra, and Morita theory to algebraic topology using ring spectra. This allows us to prove new duality results in algebra and topology, and to view (1) Poincaré duality for manifolds, (2) Gorenstein duality for commutative rings, (3) Benson–Carlson duality for cohomology rings of finite groups, (4) Poincaré duality for groups and (5) Gross–Hopkins duality in chromatic stable homotopy theory as examples of a single phenomenon.  相似文献   
93.
If κ < λ are such that κ is indestructibly supercompact and λ is 2λ supercompact, it is known from [4] that
  • {δ < κ | δ is a measurable cardinal which is not a limit of measurable cardinals and δ violates level by level equivalence between strong compactness and supercompactness}
must be unbounded in κ. On the other hand, using a variant of the argument used to establish this fact, it is possible to prove that if κ < λ are such that κ is indestructibly supercompact and λ is measurable, then
  • {δ < κ | δ is a measurable cardinal which is not a limit of measurable cardinals and δ satisfies level by level equivalence between strong compactness and supercompactness}
must be unbounded in κ. The two aforementioned phenomena, however, need not occur in a universe with an indestructibly supercompact cardinal and sufficiently few large cardinals. In particular, we show how to construct a model with an indestructibly supercompact cardinal κ in which if δ < κ is a measurable cardinal which is not a limit of measurable cardinals, then δ must satisfy level by level equivalence between strong compactness and supercompactness. We also, however, show how to construct a model with an indestructibly supercompact cardinal κ in which if δ < κ is a measurable cardinal which is not a limit of measurable cardinals, then δ must violate level by level equivalence between strong compactness and supercompactness. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   
94.
The construction of multipartite unextendible product bases(UPBs) is a basic problem in quantum information.We respectively construct two families of 2 × 2× 4 and 2 × 2 × 2 × 4 UPBs of size eight by using the existing four-qubit and five-qubit UPBs.As an application,we construct novel families of multipartite positive-partial-transpose entangled states,as well as their entanglement properties in terms of the geometric measure of entanglement.  相似文献   
95.
Bioactive equivalent combinatorial components play a critical role in herbal medicines. However, how to discover and enrich them efficiently is a question for herbal pharmaceuticals researchers. In our work, a novel two‐dimensional reversed‐phase/hydrophilic interaction high‐performance liquid chromatography method was established to perform real‐time components trapping and combining for preparation and isolation of coeluting components. Arenaria kansuensis was taken as an example, and solid‐phase extraction coupled with liquid–liquid extraction as a simple and efficient method for enriching trace components, reversed phase column coupled with hydrophilic interaction liquid chromatography XAmide column as two‐dimensional chromatography technology for isolation and preparation of coeluting constituents, enzyme‐linked immune‐sorbent assay as bio‐guided assay, and anti‐inflammatory bioactivity evaluation for bioactive constituents. A combination of 12 β‐carboline alkaloids was identified as anti‐inflammatory bioactive equivalent combinatorial components from A. kansuensis , which accounts for 1.9% w/w of original A. kansuensis . This work answers the key question of which are real anti‐inflammatory components from A. kansuensis and provides a fast and efficient approach for discovering and enriching trace β‐carboline alkaloids from herbal medicines for the first time. More importantly, the discovery of bioactive equivalent combinatorial components could improve the quality control of herbal products and inspire a herbal medicine based on combinatorial therapeutics.  相似文献   
96.
The equivalence group is determined for systems of linear ordinary differential equations in both the standard form and the normal form. It is then shown that the normal form of linear systems reducible by an invertible point transformation to the canonical form y (n)=0 consists of copies of the same iterative scalar equation. It is also shown that contrary to the scalar case, an iterative vector equation need not be reducible to the canonical form by an invertible point transformation. Other properties of iterative linear systems are also derived, as well as a simple algebraic formula for their general solution. Copyright © 2017 John Wiley & Sons, Ltd.  相似文献   
97.
In the first part of this contribution, the Lie-symmetries of the principle of least action associated to the constitutive equations of the DNLR formalism of relaxation have been presented. We examine in this second part the continuous symmetries corresponding to the simple case of stress relaxation under isothermal conditions. The well-known principle of time/temperature equivalence is discussed in terms of variational symmetry for the Jacobi’s action functional, and connected to the Onsager’s relation near the thermodynamic equilibrium.  相似文献   
98.
This paper presents a unified framework from which emerge the Lagrange equations, the Gibbs-Appell Equations and the Generalized Inverse Equations for describing the motion of constrained mechanical systems. The unified approach extends the applicability of the first two approaches to systems where the constraints are non-linear functions of the generalized velocities and are not necessarily independent. Furthermore, the approach leads to the Explicit Gibbs-Appell Equations.  相似文献   
99.
We consider the problem of locally linearizing a control system via topological transformations. According to [2,3], there is no naive generalization of the classical Grobman–Hartman theorem for ODEs to control systems: a generic control system, when viewed as a set of under-determined differential equations parametrized by the control, cannot be linearized using pointwise transformations on the state and the control values. However, if we allow the transformations to depend on the control at a functional level (open loop transformations), we are able to prove a version of the Grobman–Hartman theorem for control systems.  相似文献   
100.
Recently Zheng & Hwang established a series of independence theorems concerning with planar effective elastic properties. It is manifested that the estimation of the effective elastic properties of microcracked solids through the generalized self-consistent method (GSCM) contradicts with these independence theorems. In this paper it is shown that such contradiction is actually caused by the approximate algorithm adopted, while the exact solution of GSCM is consistent with these rigorously established independence theorems. Since only an approximate algorithm in GCSM is available in dealing with problems involving non-circular inclusions or holes, an intrinsic GSCM is proposed, which can be performed based on an approximate algorithm and the corresponding estimations are consistent with the independence theorems.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号