全文获取类型
收费全文 | 457238篇 |
免费 | 3852篇 |
国内免费 | 1537篇 |
专业分类
化学 | 225040篇 |
晶体学 | 6711篇 |
力学 | 23056篇 |
综合类 | 9篇 |
数学 | 74377篇 |
物理学 | 133434篇 |
出版年
2021年 | 3077篇 |
2020年 | 3402篇 |
2019年 | 3847篇 |
2018年 | 15208篇 |
2017年 | 14878篇 |
2016年 | 13000篇 |
2015年 | 5093篇 |
2014年 | 7052篇 |
2013年 | 17218篇 |
2012年 | 16339篇 |
2011年 | 25351篇 |
2010年 | 16327篇 |
2009年 | 16530篇 |
2008年 | 20773篇 |
2007年 | 22930篇 |
2006年 | 14145篇 |
2005年 | 13719篇 |
2004年 | 12848篇 |
2003年 | 12102篇 |
2002年 | 10976篇 |
2001年 | 11390篇 |
2000年 | 8948篇 |
1999年 | 6743篇 |
1998年 | 5719篇 |
1997年 | 5714篇 |
1996年 | 5327篇 |
1995年 | 4688篇 |
1994年 | 4583篇 |
1993年 | 4507篇 |
1992年 | 4886篇 |
1991年 | 5001篇 |
1990年 | 4799篇 |
1989年 | 4631篇 |
1988年 | 4598篇 |
1987年 | 4545篇 |
1986年 | 4301篇 |
1985年 | 5624篇 |
1984年 | 5985篇 |
1983年 | 4840篇 |
1982年 | 5221篇 |
1981年 | 5086篇 |
1980年 | 4826篇 |
1979年 | 5075篇 |
1978年 | 5297篇 |
1977年 | 5284篇 |
1976年 | 5298篇 |
1975年 | 4970篇 |
1974年 | 4940篇 |
1973年 | 5224篇 |
1972年 | 3400篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
921.
R Ehrhardt 《The Journal of the Operational Research Society》1997,48(10):1013-1021
We consider a firm that manages its internal manufacturing operations according to a just-in-time (JIT) system but maintains an inventory of finished goods as a buffer against random demands from external customers. We formulate a model in which finished goods are replenished by a small fixed quantity each time period. In the interest of schedule stability, the size of the replenishment quantity must remain fixed for a predetermined interval of time periods. We analyse the single-interval problem in depth, showing how to compute a cost-minimising value of the replenishment quantity for a given interval length, and characterising the optimal cost, inventory levels and service as functions of the interval length and initial inventory. The model displays significant cost and service penalties for schedule stability. A dynamic version of the problem is also formulated, and shown to be convex in nature with relatively easily computed optima. 相似文献
922.
We study the expected delay in cyclic polling models with general ‘branching-type’ service disciplines. For this class of
models, which contains models with exhaustive and gated service as special cases, we obtain closed-form expressions for the
expected delay under standard heavy-traffic scalings. We identify a single parameter associated with the service discipline
at each queue, which we call the ‘exhaustiveness’. We show that the scaled expected delay figures depend on the service policies
at the queues only through the exhaustiveness of each of the service disciplines. This implies that the influence of different
service disciplines, but with the same exhaustiveness, on the expected delays at the queues becomes the same when the system
reaches saturation. This observation leads to a new classification of the service disciplines. In addition, we show monotonicity
of the scaled expected delays with respect to the exhaustiveness of the service disciplines. This induces a complete ordering
in terms of efficiency of the service disciplines. The results also lead to new rules for optimization of the system performance
with respect to the service disciplines at the queues. Further, the exact asymptotic results suggest simple expected waiting-time
approximations for polling models in heavy traffic. Numerical experiments show that the accuracy of the approximations is
excellent for practical heavy-traffic scenarios.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
923.
D. V. Peregoudov 《Theoretical and Mathematical Physics》1997,113(1):1331-1341
The effective potential method, used in quantum field theory to study spontaneous symmetry violation, is discussed from the
point of view of Bogoliubov’s quasi-averaging procedure. It is shown that the effective potential method is a disguised type
of this procedure. The catastrophe theory approach to the study of phase transitions is discussed and the existence of the
potentials used in that approach is proved from the statistical point of view. It is shown that in the case of broken symmetry,
the nonconvex effective potential is not a Legendre transform of the generating functional for connected Green’s functions.
Instead, it is a part of the potential used in catastrophe theory. The relationship between the effective potential and the
Legendre transform of the generating functional for connected Green’s functions is given by Maxwell’s rule. A rigorous rule
for evaluating quasi-averaged quantities within the framework of the effective potential method is established.
Translated from Teoreticheskaya i Matematicheskaya Fizika, Vol. 113, No. 1, pp. 149–161, October, 1997. 相似文献
924.
We characterize orbifolds in terms of their sheaves, and show that orbifolds correspond exactly to a specific class of smooth groupoids. As an application, we construct fibered products of orbifolds and prove a change-of-base formula for sheaf cohomology. 相似文献
925.
926.
M. Trunk J. Popp I. Hartmann M. Lankers E. Urlaub W. Kiefer 《Fresenius' Journal of Analytical Chemistry》1996,355(3-4):354-356
In situ measurements of gas-liquid surface reactions of single aerosol microdroplets are presented. By means of optical levitation in combination with elastic (Mie) and inelastic (Raman) light scattering it is possible to get information on the chemistry of e.g. acid/base reactions as well as the physical behavior of single microparticles. 相似文献
927.
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
928.
V. M. Ogenko A. V. Fesenko N. D. Konovalova V. I. Stepanenko 《Reaction Kinetics and Catalysis Letters》1993,50(1-2):169-173
It has been established that CO oxidation with excess O2 on Ni/Al2O3 forms an inhomogeneous NiO contact surface that smoothes critical effects in the sharp reactivity change. 相似文献
929.
One of the shortcomings of R&D evaluation is a lack of emphasis on analytical assessment of the value of an on-going R&D project. This paper addresses the problem concerning the distributive aspect of access to superior knowledge. Decision tree analysis and probability models appear to be appropriate tools for assessing the values of an intermediate result and patent reward of a firm's R&D decision in an environment of perfect information and oligopolistic competition. The assessed values are used as minimum prices acceptable to the firm when the knowledge is disseminated to the public. This paper will attempt to resolve this problem through the determination of the appropriate values of the reservation price of the first-stage invention in terms of the final reward and of a patent reward for which the inventor is willing to apply. 相似文献
930.
H. Tietze-Jaensch D. Sieger R. Geick W. Zulehner A. de Geyer 《Applied Physics A: Materials Science & Processing》1992,54(1):19-21
The formation of silicon oxide precipitates from Czochralski grown silicon depends on the time and temperature of the heat treatment as well as on the initial content of interstitially dissolved oxygen. Samples containing between 5×1017 Oi/cm3 and 13×1017 Oi/cm3 have been heated at 750° C for 96 h. SiO2 precipitates of various shape and size have been obtained and investigated by means of small angle neutron scattering (SANS) in the Q-range 0.05 Å–1<Q<0.2 Å–1. The obtained SANS patterns reveal a typical anisotropy of their intensity distribution, which splits into a central peak at Q<0.1 Å–1 due to the shape of the individual particles and a number of weak intensities for large Q-values, originating from a correlation between defects, possibly between the precipitates. While these correlation peaks in the SANS patterns are seen best for rather low values of about (5–7)×1017 Oi/cm3 oxygen content, the central peak anisotropy is most pronounced for higher values of ca 10×1017 Oi/cm3. The integrated intensity of the central peak increases with increasing initial oxygen content. For comparison, untreated samples of the same initial oxygen content do not reveal any anisotropic SAN scattering or a broadened central peak beam. 相似文献