71 research outputs found
On the Computational Complexity of Non-dictatorial Aggregation
We investigate when non-dictatorial aggregation is possible from an
algorithmic perspective, where non-dictatorial aggregation means that the votes
cast by the members of a society can be aggregated in such a way that the
collective outcome is not simply the choices made by a single member of the
society. We consider the setting in which the members of a society take a
position on a fixed collection of issues, where for each issue several
different alternatives are possible, but the combination of choices must belong
to a given set of allowable voting patterns. Such a set is called a
possibility domain if there is an aggregator that is non-dictatorial, operates
separately on each issue, and returns values among those cast by the society on
each issue. We design a polynomial-time algorithm that decides, given a set
of voting patterns, whether or not is a possibility domain. Furthermore, if
is a possibility domain, then the algorithm constructs in polynomial time
such a non-dictatorial aggregator for . We then show that the question of
whether a Boolean domain is a possibility domain is in NLOGSPACE. We also
design a polynomial-time algorithm that decides whether is a uniform
possibility domain, that is, whether admits an aggregator that is
non-dictatorial even when restricted to any two positions for each issue. As in
the case of possibility domains, the algorithm also constructs in polynomial
time a uniform non-dictatorial aggregator, if one exists. Then, we turn our
attention to the case where is given implicitly, either as the set of
assignments satisfying a propositional formula, or as a set of consistent
evaluations of an sequence of propositional formulas. In both cases, we provide
bounds to the complexity of deciding if is a (uniform) possibility domain.Comment: 21 page
Solving order constraints in logarithmic space.
We combine methods of order theory, finite model theory, and universal algebra to study, within the constraint satisfaction framework, the complexity of some well-known combinatorial problems connected with a finite poset. We identify some conditions on a poset which guarantee solvability of the problems in (deterministic, symmetric, or non-deterministic) logarithmic space. On the example of order constraints we study how a certain algebraic invariance property is related to solvability of a constraint satisfaction problem in non-deterministic logarithmic space
A Graph Based Backtracking Algorithm for Solving General CSPs
Many AI tasks can be formalized as constraint satisfaction problems (CSPs), which involve finding values for variables subject to constraints. While solving a CSP is an NP-complete task in general, tractable classes of CSPs have been identified based on the structure of the underlying constraint graphs. Much effort has been spent on exploiting structural properties of the constraint graph to improve the efficiency of finding a solution. These efforts contributed to development of a class of CSP solving algorithms called decomposition algorithms. The strength of CSP decomposition is that its worst-case complexity depends on the structural properties of the constraint graph and is usually better than the worst-case complexity of search methods. Its practical application is limited, however, since it cannot be applied if the CSP is not decomposable. In this paper, we propose a graph based backtracking algorithm called omega-CDBT, which shares merits and overcomes the weaknesses of both decomposition and search approaches
GRS 1915+105 : High-energy Insights with SPI/INTEGRAL
We report on results of two years of INTEGRAL/SPI monitoring of the Galactic
microquasar GRS 1915+105. From September 2004 to May 2006, the source has been
observed twenty times with long (approx 100 ks) exposures. We present an
analysis of the SPI data and focus on the description of the high-energy (> 20
keV) output of the source. We found that the 20 - 500 keV spectral emission of
GRS 1915+105 was bound between two states. It seems that these high-energy
states are not correlated with the temporal behavior of the source, suggesting
that there is no direct link between the macroscopic characteristics of the
coronal plasma and the the variability of the accretion flow. All spectra are
well fitted by a thermal comptonization component plus an extra high-energy
powerlaw. This confirms the presence of thermal and non-thermal electrons
around the black hole.Comment: 7 pages, 8 figures, 2 tables; accepted (09/11/2008) for publication
in A&
Minimum Energy Broadcast and Disk Cover in Grid Wireless Networks
Abstract. The Minimum Energy Broadcast problem consists in finding the minimum-energy range assignment for a given set S of n stations of an ad hoc wireless network that allows a source station to perform broadcast operations over S. We prove a nearly tight asymptotical bound on the optimal cost for the Minimum Energy Broadcast problem on square grids. We emphasize that finding tight bounds for this problem restriction is far to be easy: it involves the Gauss’s Circle problem and the Apollonian Circle Packing. We also derive near-tight bounds for the Bounded-Hop version of this problem. Our results imply that the best-known heuristic, the MST-based one, for the Minimum Energy Broadcast problem is far to achieve optimal solutions (even) on very regular, well-spread instances: its worst-case approximation ratio is about pi and it yields Ω( n) hops. As a by product, we get nearly tight bounds for the Minimum Disk Cover problem and for its restriction in which the allowed disks must have non-constant radius. Finally, we emphasize that our upper bounds are obtained via polynomial time constructions.
Functional dependencies of variables in wait-free programs
Suppose that we are given a waite-free protocol for the asynchronous, concurrent processes P1,P2,…,Pr, Q1,Q2,…,Qs, with r≥2, s≥0. For any run (or interleaving) ρ of the protocol and any initialization init of all the protocol variables let X[ρ, init] be the value of the variable X at the end of the run ρ. The variables X1,X2,…,Xr “belonging” to the processors P1,P2,…,Pr, respectively, are called functionally dependent for the initialization init, if for any runs ρ, ρ of the protocol, (∀i,j)(Xi[ρ,init]=Xi[σ,init]⇔Xj[ρ,init]=Xj[σ,init]). (∀i,j)(Xi[ρ,init]=Xi[σ,init]⇔Xj[ρ,init]=Xj[σ,init]). For any run ρ and any initialization init of the protocol define the evaluation mapping evalX1,X2,…,Xr(ρ, init)=(X1[ρ, init],X2[ρ, init],…,Xr[ρ, init]). We show that for any protocol as above, the variables X1,X2,…,Xr are functionally dependent for the initialization init if and only if the quantity evalX1,X2,…,Xr (ρ, init) is independent of ρ
On the Stability of Generalized Second Price Auctions with Budgets
The Generalized Second Price (GSP) auction used typically to model sponsored search auctions does not include the notion of budget constraints, which is present in practice. Motivated by this, we introduce the different variants of GSP auctions that take budgets into account in natural ways. We examine their stability by focusing on the existence of Nash equilibria and envy-free assignments. We highlight the differences between these mechanisms and find that only some of them exhibit both notions of stability. This shows the importance of carefully picking the right mechanism to ensure stable outcomes in the presence of budgets. © 2015, Springer Science+Business Media New York
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