全文获取类型
收费全文 | 182篇 |
免费 | 14篇 |
国内免费 | 6篇 |
专业分类
化学 | 80篇 |
力学 | 1篇 |
数学 | 84篇 |
物理学 | 37篇 |
出版年
2023年 | 1篇 |
2022年 | 4篇 |
2021年 | 3篇 |
2020年 | 4篇 |
2019年 | 4篇 |
2018年 | 8篇 |
2017年 | 2篇 |
2016年 | 4篇 |
2015年 | 11篇 |
2014年 | 9篇 |
2013年 | 12篇 |
2012年 | 10篇 |
2011年 | 19篇 |
2010年 | 16篇 |
2009年 | 11篇 |
2008年 | 18篇 |
2007年 | 11篇 |
2006年 | 14篇 |
2005年 | 11篇 |
2004年 | 9篇 |
2003年 | 2篇 |
2002年 | 2篇 |
2001年 | 2篇 |
2000年 | 3篇 |
1998年 | 2篇 |
1997年 | 1篇 |
1995年 | 1篇 |
1994年 | 1篇 |
1993年 | 1篇 |
1991年 | 2篇 |
1985年 | 1篇 |
1980年 | 1篇 |
1940年 | 2篇 |
排序方式: 共有202条查询结果,搜索用时 828 毫秒
81.
A long-standing conjecture of Erd?s and Simonovits is that ex(n,C2k), the maximum number of edges in an n-vertex graph without a 2k-gon is asymptotically as n tends to infinity. This was known almost 40 years ago in the case of quadrilaterals. In this paper, we construct a counterexample to the conjecture in the case of hexagons. For infinitely many n, we prove that
82.
Noga Alon Konstantin Makarychev Yury Makarychev Assaf Naor 《Inventiones Mathematicae》2006,163(3):499-522
We introduce a new graph parameter, called the Grothendieck constant of a graph G=(V,E), which is defined as the least constant K such that for every A:E→ℝ,
The classical Grothendieck inequality corresponds to the case of bipartite graphs, but the case of general graphs is shown to have various algorithmic applications. Indeed, our work is motivated by
the algorithmic problem of maximizing the quadratic form ∑{u,v}∈EA(u,v)ϕ(u)ϕ(v) over all ϕ:V→{-1,1}, which arises in the study of correlation clustering and in the investigation of the spin glass model. We give upper
and lower estimates for the integrality gap of this program. We show that the integrality gap is
, where
is the Lovász Theta Function of the complement of G, which is always smaller than the chromatic number ofG. This yields an efficient constant factor approximation algorithm for the above maximization problem for a wide range of
graphs G. We also show that the maximum possible integrality gap is always at least Ω(log ω(G)), where ω(G) is the clique number of G.
In particular it follows that the maximum possible integrality gap for the complete graph on n vertices with no loops is Θ(logn). More generally, the maximum possible integrality gap for any perfect graph with chromatic number n is Θ(logn). The lower bound for the complete graph improves a result of Kashin and Szarek on Gram matrices of uniformly bounded functions,
and settles a problem of Megretski and of Charikar and Wirth. 相似文献
83.
Various new nonembeddability results (mainly into L1) are proved via Fourier analysis. In particular, it is shown that the Edit Distance on {0,1}d has L1 distortion We also give new lower bounds on the L1 distortion of flat tori, quotients of the discrete hypercube under group actions, and the transportation cost (Earthmover)
metric. 相似文献
84.
85.
Assaf Rinot 《Topology and its Applications》2007,155(3):135-140
We introduce a weakening of the generalized continuum hypothesis, which we will refer to as the prevalent singular cardinals hypothesis, and show it implies that every topological space of density and weight ℵω1 is not hereditarily Lindelöf.The assumption PSH is very weak, and in fact holds in all currently known models of ZFC. 相似文献
86.
87.
Assaf Libman 《Mathematische Zeitschrift》2007,255(3):515-548
We give a new proof of the Minami–Webb formula for classifying spaces of finite groups by exploiting Symonds’s resolution
of Webb’s conjecture. The methods are applicable to obtain a stable decomposition of Minami’s type for the classifying spaces
of the three exotic p-local finite groups which were introduced by Ruiz and Viruel at the prime 7. We obtain a similar decomposition for the classifying
spaces of a family of exotic p-local finite groups which were constructed by Broto, Levi and Oliver.
The author was supported by the Nuffield Foundation Grant NAL/00735/G. 相似文献
88.
Motivated by applications in telephone call centers, we consider a service system model with m customer classes and r server pools. The model is one with doubly stochastic arrivals, which means that the m-vector λ of instantaneous arrival rates is allowed to vary both temporally and stochastically. Two levels of dynamic control
are considered: customers may be either blocked or accepted at the time of their arrival, and then accepted customers of each
class must be routed, either immediately upon acceptance or after some period of waiting, to a server pool that is qualified
to handle that class. Customers who are made to wait before commencement of their service are liable to defect. The objective
is to minimize the expected sum of blocking costs, waiting costs and defection costs over a fixed and finite planning horizon.
We consider an asymptotic parameter regime in which (i) the arrival rates, service rates and defection rates are uniformly
accelerated by a large factor κ, then (ii) arrival rates are increased by an additional factor g(κ), and the number of servers in each pool is increased by g(κ) as well. This produces a separation of time scales, justifying a pointwise stationary stochastic fluid approximation for
our original system model. In the stochastic fluid approximation, optimal admission control and routing decisions are determined
by a simple linear program that uses the current arrival rate vector λ as data. We explain how to implement the fluid model's
optimal control policy in our original service system context, and prove that the proposed implementation is asymptotically
optimal in the first-order sense.
AMS subject classification: 60K30, 90B15, 90B36 相似文献
89.
F. H. Assaf 《Journal of Solid State Electrochemistry》1998,3(1):39-47
The anodic polarization behaviour of Al-Sn alloy (5.6% Sn) was studied in aerated 1, 1.5 and 2 M H3PO4 acid solutions using potentiodynamic and potentiostatic techniques. Anodic behaviour of Al and Sn metals was investigated
for comparison. The results show that the alloy exhibits active-passive transition behaviour. The passivation of the examined
alloy is due to the formation of oxide film for both Al and Sn incorporated with their phosphates. In general, at constant
concentration of PO4
3− ion, the passive current density (I
pass) is decreased with increase of pH in the range 2.5–5. Further, the influence of Cl− ions on the passivity of the given alloy was studied. It was found that the aggressive effect of Cl− ions on the passive film is inhibited with increase in phosphate concentration and pH. However, the addition of organic compounds
(2- and 3-picoline and 2-aminopyridine) shows that only 2-aminopyridine inhibited the attack of Cl− ions.
Received: 24 October 1997 / Accepted: 5 February 1998 相似文献
90.
Barnes PR Anderson AY Juozapavicius M Liu L Li X Palomares E Forneli A O'Regan BC 《Physical chemistry chemical physics : PCCP》2011,13(8):3547-3558
A simple and powerful approach for assessing the recombination losses in dye sensitised solar cells (DSSCs) across the current voltage curve (j-V) as a function of TiO(2) electron concentration (n) is demonstrated. The total flux of electrons recombining with iodine species in the electrolyte and oxidised dye molecules can be thought of as a recombination current density, defined as j(rec) = j(inj)-j where j(inj) is the current of electrons injected from optically excited dye states and j is the current density collected at cell voltage (V). The electron concentration at any given operating conditions is determined by charge extraction. This allows comparison of factors influencing electron recombination rates at matched n. We show that j(rec) is typically 2-3 times higher under 1 sun equivalent illumination (j(inj) > 0) relative to dark (j(inj) = 0) conditions. This difference was increased by increasing light intensity, electrolyte iodine concentration and electrolyte solvent viscosity. The difference was reduced by increasing the electrolyte iodide concentration and increasing the temperature. These results allowed us to verify a numerical model of complete operational cells (Barnes et al., Phys. Chem. Chem. Phys., DOI: 10.1039/c0cp01554g) and to relate the differences in j(rec) to physical processes in the devices. The difference between j(rec) in the light and dark can be explained by two factors: (1) an increase in the concentration of electron acceptor species (I(3)(-) and/or I(2)) when current is flowing under illumination relative to dark conditions where the current is flowing in the opposite direction, and (2) a non-trivial contribution from electron recombination to oxidised dye molecules under light conditions. More generally, the technique helps to assign the observed relationship between the components, processing and performance of DSSCs to more fundamental physical processes. 相似文献