全文获取类型
收费全文 | 365615篇 |
免费 | 3407篇 |
国内免费 | 1569篇 |
专业分类
化学 | 191737篇 |
晶体学 | 5289篇 |
力学 | 17336篇 |
综合类 | 66篇 |
数学 | 43751篇 |
物理学 | 112412篇 |
出版年
2020年 | 3313篇 |
2019年 | 3796篇 |
2018年 | 5294篇 |
2017年 | 5364篇 |
2016年 | 7099篇 |
2015年 | 3927篇 |
2014年 | 6496篇 |
2013年 | 14779篇 |
2012年 | 12219篇 |
2011年 | 14707篇 |
2010年 | 11102篇 |
2009年 | 11020篇 |
2008年 | 13991篇 |
2007年 | 13974篇 |
2006年 | 12879篇 |
2005年 | 11680篇 |
2004年 | 10748篇 |
2003年 | 9508篇 |
2002年 | 9560篇 |
2001年 | 10677篇 |
2000年 | 8013篇 |
1999年 | 5987篇 |
1998年 | 5186篇 |
1997年 | 5036篇 |
1996年 | 4865篇 |
1995年 | 4289篇 |
1994年 | 4361篇 |
1993年 | 4263篇 |
1992年 | 4531篇 |
1991年 | 4861篇 |
1990年 | 4568篇 |
1989年 | 4538篇 |
1988年 | 4388篇 |
1987年 | 4338篇 |
1986年 | 4192篇 |
1985年 | 5448篇 |
1984年 | 5500篇 |
1983年 | 4648篇 |
1982年 | 4921篇 |
1981年 | 4622篇 |
1980年 | 4254篇 |
1979年 | 4677篇 |
1978年 | 5006篇 |
1977年 | 5099篇 |
1976年 | 5251篇 |
1975年 | 4856篇 |
1974年 | 4878篇 |
1973年 | 5018篇 |
1972年 | 3976篇 |
1971年 | 3380篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
In this paper, we examine a class of convex problems of Bolza type, involving a time delay in the state. It encompasses a variety of time-delay problems arising in the calculus of variations and optimal control. A duality analysis is carried out which, among other things, leads to a characterization of minimizers in terms of the Euler-Lagrange inclusion. The results obtained improve in significant respects on what is achievable by techniques previously employed, based on elimination of the time delay by introduction of an infinite-dimensional state space or on the method of steps. 相似文献
992.
LetC(X,E) andC(Y,F) denote the spaces of continuous functions on the Tihonov spacesX andY, taking values in the Banach spacesE andF, respectively. A linear mapH:C(X,E)→C(Y,F) isseparating iff(x)g(x)=0 for allx inX impliesHf(y)Hg(y)=0 for ally inY. Some automatic continuity properties and Banach-Stone type theorems (i.e., asserting that isometries must be of a certain
form) for separating mapsH between spaces of real- and complex-valued functions have already been developed. The extension of such results to spaces
of vector-valued functions is the general subject of this paper. We prove in Theorem 4.1, for example, for compactX andY, that a linear isometryH betweenC(X,E) andC(Y,F) is a “Banach-Stone” map if and only ifH is “biseparating (i.e,H andH
−1 are separating). The Banach-Stone theorems of Jerison and Lau for vector-valued functions are then deduced in Corollaries
4.3 and 4.4 for the cases whenE andF or their topological duals, respectively, are strictly convex.
Research supported by the Fundació Caixa Castelló, MI/25.043/92 相似文献
993.
E. Hernández L. Rodríguez-Marín 《Journal of Optimization Theory and Applications》2007,134(1):119-134
In this paper, we study optimization problems where the objective function and the binding constraints are set-valued maps
and the solutions are defined by means of set-relations among all the images sets (Kuroiwa, D. in Takahashi, W., Tanaka, T.
(eds.) Nonlinear analysis and convex analysis, pp. 221–228, 1999). We introduce a new dual problem, establish some duality theorems and obtain a Lagrangian multiplier rule of nonlinear type
under convexity assumptions. A necessary condition and a sufficient condition for the existence of saddle points are given.
The authors thank the two referees for valuable comments and suggestions on early versions of the paper. The research of the
first author was partially supported by Ministerio de Educación y Ciencia (Spain) Project MTM2006-02629 and by Junta de Castilla
y León (Spain) Project VA027B06. 相似文献
994.
In this paper, we prove that every automorphism of the semigroup of invertible matrices with nonnegative elements over a linearly
ordered associative ring on some specially defined subgroup coincides with the composition of an inner automorphism of the
semigroup, an order-preserving automorphism of the ring, and a central homothety.
__________
Translated from Fundamentalnaya i Prikladnaya Matematika, Vol. 11, No. 2, pp. 3–23, 2005. 相似文献
995.
996.
997.
We consider a nonlinear optimal control problem with an integral functional in which the integrand is the characteristic function
of a closed set in the phase space. An approximation method is applied to prove the necessary conditions of optimality in
the form of a Pontryagin maximum principle without any prior assumptions on the behavior of the optimal trajectory. Similarly
to phase-constrained problems, we derive conditions of nondegeneracy and pointwise nontriviality of the maximum principle.
__________
Translated from Nelineinaya Dinamika i Upravlenie, No. 4, pp. 179–204, 2004. 相似文献
998.
V. E. Zubov A. D. Kudakov N. L. Levshin 《Bulletin of the Russian Academy of Sciences: Physics》2007,71(11):1548-1549
It is established that reversible adsorption of water and methyl alcohol molecules, occurring via formation of hydrogen bonds, changes the dynamic properties of domain walls in the surface region of soft ferromagnets, as well as their initial static magnetic susceptibility. A mechanism is proposed for the effects revealed. 相似文献
999.
The difference between the classical treatment offlexible body impact and the treatment of impact in flexiblemultibody dynamics is due to several fundamental reasons. Inthe classical impact theory, simple structures such as beamsand plates are used. Infinite dimensional models can bedeveloped for these simple structural elements to study theimpact dynamics and the wave propagation problem. Flexiblemultibody impact problems, on the other hand, involve bodieswith complex geometry that cannot be modeled using infinitenumber of degrees of freedom. Furthermore, the classicalimpact theory has been mainly concerned with the impactbetween a rigid mass that moves without constraints beforeit impacts a simple flexible structure. This is not amultibody simulation scenario in which the impact occursbetween kinematically constrained bodies that are subjectedto impulsive constraint forces in addition to the impactforces. These constraint forces can influence the motion ofthe two bodies immediately after impact, and as aconsequence, the simple classical theory scenario of impactdoes not apply. It is the objective of this paper to discussthe use of the restitution condition in flexible multibodyimpact problems and demonstrate that the use of thisapproach does not exclude the classical formulation.Nonetheless, the impulse momentum balance approach can serveas an effective and efficient procedure for solving theimpact problem in finite dimensional models that do not obeythe classical wave theory. Energy results of simplestructural elements are presented in order to demonstratethe consistency of using the impulse momentum balanceapproach in solving impact problems in finite dimensionalflexible body applications. 相似文献
1000.
This paper presents an application in the automotive industry where a combination of electronic speckle-pattern interferometry and laser doppler velocimetry were used at a critical stage in the design process of an internal combustion engine. Combined deformation and surface relief measurements were used to study the phase and amplitude of deformation of a vibrating engine. The relief data was combined with the interferometer geometry and used to geometrically correct the deformation data, in an effort to improve accuracy. The measurements allowed rapid identification and quantification of design weaknesses, particularly those causing undesirable resonances. This led to a significant reduction in the design time and lowering of costs, when compared with existing design optimisation methods. 相似文献