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1.
Theminimum-degree greedy algorithm, or Greedy for short, is a simple and well-studied method for finding independent sets in graphs. We show that
it achieves a performance ratio of (Δ+2)/3 for approximating independent sets in graphs with degree bounded by Δ. The analysis
yields a precise characterization of the size of the independent sets found by the algorithm as a function of the independence
number, as well as a generalization of Turán’s bound. We also analyze the algorithm when run in combination with a known preprocessing
technique, and obtain an improved
performance ratio on graphs with average degree
, improving on the previous best
of Hochbaum. Finally, we present an efficient parallel and distributed algorithm attaining the performance guarantees of
Greedy.
Gordon Gekko [29].
A preliminary version of this paper appeared at the 26th ACM Symposium on Theory of Computing, 1994. This work was done while
both authors were at the Japan Advanced Institute of Science and Technology, Hokuriku. 相似文献
2.
This paper presents a method for the detection of fouling in a cross-flow heat exchanger. A numerical model is used to generate data when the heat exchanger is clean and corresponding data when fouling occurs. In a first step, the model is used to generate a long time series by simulating a clean heat exchanger. This allows the determination of a neural network model of the heat exchanger. Then, hundred sets of data are generated by simulating a fouled heat exchanger and it is checked that the simple Cusum test can be used to detect fouling without any false alarm, whatever the reference time series is. 相似文献
3.
In laser projection systems the observer in the far field of the image points on the screen will recognize serious speckle noise. There are many methods to reduce or eliminate speckles in the near field by reducing or eliminating temporal or spatial coherence of the laser. But for the far field it is hardly possible to change the coherence properties of laser sources so that speckles will disappear. We propose a new method for eliminating speckles in the far field by using a diffractive optical element. The intensity modulation depth in the far-field speckle pattern can be reduced to a few percent while good beam quality is preserved. 相似文献
4.
On spectrum sharing games 总被引:1,自引:0,他引:1
Magnús M. Halldórsson Joseph Y. Halpern Li Erran Li Vahab S. Mirrokni 《Distributed Computing》2010,22(4):235-248
Efficient spectrum-sharing mechanisms are crucial to alleviate the bandwidth limitation in wireless networks. In this paper, we consider the following question: can free spectrum be shared efficiently? We study this problem in the context of 802.11 or WiFi networks. Each access point (AP) in a WiFi network must be assigned a channel for it to service users. There are only finitely many possible channels that can be assigned. Moreover, neighboring access points must use different channels so as to avoid interference. Currently these channels are assigned by administrators who carefully consider channel conflicts and network loads. Channel conflicts among APs operated by different entities are currently resolved in an ad hoc manner (i.e., not in a coordinated way) or not resolved at all. We view the channel assignment problem as a game, where the players are the service providers and APs are acquired sequentially. We consider the price of anarchy of this game, which is the ratio between the total coverage of the APs in the worst Nash equilibrium of the game and what the total coverage of the APs would be if the channel assignment were done optimally by a central authority. We provide bounds on the price of anarchy depending on assumptions on the underlying network and the type of bargaining allowed between service providers. The key tool in the analysis is the identification of the Nash equilibria with the solutions to a maximal coloring problem in an appropriate graph. We relate the price of anarchy of these games to the approximation factor of local optimization algorithms for the maximum k-colorable subgraph problem. We also study the speed of convergence in these games. 相似文献
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8.
We consider the sum coloring and sum multicoloring problems on several
fundamental classes of graphs, including the classes of interval and
k-claw free graphs. We give an algorithm that approximates sum coloring
within a factor of 1.796, for any graph in which the maximum k-colorable
subgraph problem is polynomially solvable. In particular, this improves on
the previous best known ratio of 2 for interval graphs. We introduce a new
measure of coloring, robust throughput}, that indicates how quickly
the graph is colored, and show that our algorithm approximates this measure
within a factor of 1.4575.
In addition, we study the contiguous (or non-preemptive) sum multicoloring
problem on k-claw free graphs.
This models, for example,
the scheduling of dependent jobs on multiple dedicated machines, where each job
requires the exclusive use of at most k machines.
Assuming that k is a fixed constant,
we obtain the first constant factor approximation for the problem. 相似文献
9.
We study the wireless scheduling problem in the SINR model. More specifically, given a set of \(n\) links, each a sender–receiver pair, we wish to partition (or schedule) the links into the minimum number of slots, each satisfying interference constraints allowing simultaneous transmission. In the basic problem, all senders transmit with the same uniform power. We analyze a randomized distributed scheduling algorithm proposed by Kesselheim and Vöcking, and show that it achieves \(O(\log n)\)-approximation, an improvement of a logarithmic factor. This matches the best ratio known for centralized algorithms and holds in arbitrary metric space and for every length-monotone and sublinear power assignment. We also show that every distributed algorithm uses \(\varOmega (\log n)\) slots to schedule certain instances that require only two slots, which implies that the best possible absolute performance guarantee is logarithmic. 相似文献
10.
Thomas RD Schmidt HT Andler G Björkhage M Blom M Brännholm L Bäckström E Danared H Das S Haag N Halldén P Hellberg F Holm AI Johansson HA Källberg A Källersjö G Larsson M Leontein S Liljeby L Löfgren P Malm B Mannervik S Masuda M Misra D Orbán A Paál A Reinhed P Rensfelt KG Rosén S Schmidt K Seitz F Simonsson A Weimer J Zettergren H Cederquist H 《The Review of scientific instruments》2011,82(6):065112
We describe the design of a novel type of storage device currently under construction at Stockholm University, Sweden, using purely electrostatic focussing and deflection elements, in which ion beams of opposite charges are confined under extreme high vacuum cryogenic conditions in separate "rings" and merged over a common straight section. The construction of this double electrostatic ion ring experiment uniquely allows for studies of interactions between cations and anions at low and well-defined internal temperatures and centre-of-mass collision energies down to about 10 K and 10 meV, respectively. Position sensitive multi-hit detector systems have been extensively tested and proven to work in cryogenic environments and these will be used to measure correlations between reaction products in, for example, electron-transfer processes. The technical advantages of using purely electrostatic ion storage devices over magnetic ones are many, but the most relevant are: electrostatic elements which are more compact and easier to construct; remanent fields, hysteresis, and eddy-currents, which are of concern in magnetic devices, are no longer relevant; and electrical fields required to control the orbit of the ions are not only much easier to create and control than the corresponding magnetic fields, they also set no upper mass limit on the ions that can be stored. These technical differences are a boon to new areas of fundamental experimental research, not only in atomic and molecular physics but also in the boundaries of these fields with chemistry and biology. For examples, studies of interactions with internally cold molecular ions will be particular useful for applications in astrophysics, while studies of solvated ionic clusters will be of relevance to aeronomy and biology. 相似文献