260 research outputs found
The Stable Roommates problem with short lists
We consider two variants of the classical Stable Roommates problem with Incomplete (but strictly ordered) preference lists (sri) that are degree constrained, i.e., preference lists are of bounded length. The first variant, egald-sri, involves finding an egalitarian stable matching in solvable instances of sri with preference lists of length at most d. We show that this problem is NP-hard even if d = 3. On the positive side we give a 2d+372d+37-approximation algorithm for d ∈{3,4,5} which improves on the known bound of 2 for the unbounded preference list case. In the second variant of sri, called d-srti, preference lists can include ties and are of length at most d. We show that the problem of deciding whether an instance of d-srti admits a stable matching is NP-complete even if d = 3. We also consider the “most stable” version of this problem and prove a strong inapproximability bound for the d = 3 case. However for d = 2 we show that the latter problem can be solved in polynomial time
Matchings with lower quotas: Algorithms and complexity
We study a natural generalization of the maximum weight many-to-one matching problem. We are given an undirected bipartite graph G=(A∪˙P,E)G=(A∪˙P,E) with weights on the edges in E, and with lower and upper quotas on the vertices in P. We seek a maximum weight many-to-one matching satisfying two sets of constraints: vertices in A are incident to at most one matching edge, while vertices in P are either unmatched or they are incident to a number of matching edges between their lower and upper quota. This problem, which we call maximum weight many-to-one matching with lower and upper quotas (WMLQ), has applications to the assignment of students to projects within university courses, where there are constraints on the minimum and maximum numbers of students that must be assigned to each project. In this paper, we provide a comprehensive analysis of the complexity of WMLQ from the viewpoints of classical polynomial time algorithms, fixed-parameter tractability, as well as approximability. We draw the line between NPNP-hard and polynomially tractable instances in terms of degree and quota constraints and provide efficient algorithms to solve the tractable ones. We further show that the problem can be solved in polynomial time for instances with bounded treewidth; however, the corresponding runtime is exponential in the treewidth with the maximum upper quota umaxumax as basis, and we prove that this dependence is necessary unless FPT=W[1]FPT=W[1]. The approximability of WMLQ is also discussed: we present an approximation algorithm for the general case with performance guarantee umax+1umax+1, which is asymptotically best possible unless P=NPP=NP. Finally, we elaborate on how most of our positive results carry over to matchings in arbitrary graphs with lower quotas
On absolutely and simply popular rankings
Van Zuylen et al. introduced the notion of a popular ranking in a voting
context, where each voter submits a strictly-ordered list of all candidates. A
popular ranking of the candidates is at least as good as any other
ranking in the following sense: if we compare to , at
least half of all voters will always weakly prefer~. Whether a voter
prefers one ranking to another is calculated based on the Kendall distance.
A more traditional definition of popularity -- as applied to popular
matchings, a well-established topic in computational social choice -- is
stricter, because it requires at least half of the voters \emph{who are not
indifferent between and } to prefer~. In this paper, we
derive structural and algorithmic results in both settings, also improving upon
the results by van Zuylen et al. We also point out strong connections to the
famous open problem of finding a Kemeny consensus with 3 voters.Comment: full version of the AAMAS 2021 extended abstract 'On weakly and
strongly popular rankings
The Stable Roommates problem with short lists = A stabil szobatársprobléma rövid preferencialistákkal
Spectral State Transitions of the Ultraluminous X-ray Source IC 342 X-1
We observed the Ultraluminous X-ray Source IC 342 X-1 simultaneously in X-ray
and radio with Chandra and the JVLA to investigate previously reported
unresolved radio emission coincident with the ULX. The Chandra data reveal a
spectrum that is much softer than observed previously and is well modelled by a
thermal accretion disc spectrum. No significant radio emission above the rms
noise level was observed within the region of the ULX, consistent with the
interpretation as a thermal state though other states cannot be entirely ruled
out with the current data. We estimate the mass of the black hole using the
modelled inner disc temperature to be based on a Shakura-Sunyaev
disc model. Through a study of the hardness and high-energy curvature of
available X-ray observations, we find that the accretion state of X-1 is not
determined by luminosity alone.Comment: 10 pages, 5 Figures. MNRAS: Accepted 2014 July 2
Radio Detections During Two State Transitions of the Intermediate Mass Black Hole HLX-1
Relativistic jets are streams of plasma moving at appreciable fractions of
the speed of light. They have been observed from stellar mass black holes
(320 solar masses, M) as well as supermassive black holes
(1010 M) found in the centres of most galaxies. Jets
should also be produced by intermediate mass black holes (1010
M), although evidence for this third class of black hole has until
recently been weak. We report the detection of transient radio emission at the
location of the intermediate mass black hole candidate ESO 243-49 HLX-1, which
is consistent with a discrete jet ejection event. These observations also allow
us to refine the mass estimate of the black hole to be between 9
10 M and 9 10 M.Comment: 13 pages, includes supplementary online information. Published in
Science in August 201
Matchings with lower quotas: Algorithms and complexity = Alsó kvótás párosítások: algoritmusok és komplexitás
Matchings with lower quotas : algorithms and complexity
We study a natural generalization of the maximum weight many-to-one matching problem. We are given an undirected bipartite graph G=(A∪˙P,E) with weights on the edges in E, and with lower and upper quotas on the vertices in P. We seek a maximum weight many-to-one matching satisfying two sets of constraints: vertices in A are incident to at most one matching edge, while vertices in P are either unmatched or they are incident to a number of matching edges between their lower and upper quota. This problem, which we call maximum weight many-to-one matching with lower and upper quotas (WMLQ), has applications to the assignment of students to projects within university courses, where there are constraints on the minimum and maximum numbers of students that must be assigned to each project. In this paper, we provide a comprehensive analysis of the complexity of WMLQ from the viewpoints of classical polynomial time algorithms, fixed-parameter tractability, as well as approximability. We draw the line between NP-hard and polynomially tractable instances in terms of degree and quota constraints and provide efficient algorithms to solve the tractable ones. We further show that the problem can be solved in polynomial time for instances with bounded treewidth; however, the corresponding runtime is exponential in the treewidth with the maximum upper quota umax as basis, and we prove that this dependence is necessary unless FPT=W[1]. The approximability of WMLQ is also discussed: we present an approximation algorithm for the general case with performance guarantee umax+1, which is asymptotically best possible unless P=NP. Finally, we elaborate on how most of our positive results carry over to matchings in arbitrary graphs with lower quotas
On Weakly and Strongly Popular Rankings
Van Zuylen et al. introduced the notion of a popular ranking in a voting context, where each voter submits a strictly-ordered list of all candidates. A popular ranking pi of the candidates is at least as good as any other ranking sigma in the following sense: if we compare pi to sigma, at least half of all voters will always weakly prefer pi. Whether a voter prefers one ranking to another is calculated based on the Kendall distance.
A more traditional definition of popularity---as applied to popular matchings, a well-established topic in computational social choice---is stricter, because it requires at least half of the voters who are not indifferent between pi and sigma to prefer pi. In this paper, we derive structural and algorithmic results in both settings, also improving upon the results by van Zylen et al. We also point out connections to the famous open problem of finding a Kemeny consensus with 3 voters
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