23. 5. 2013 (čtvrtek [Thursday], 12:20, MFF UK Malá Strana S1)
Yonatan Bilu, Amit Daniely, Nati Linial and Mike Saks: On
the practically interesting instances of MAXCUT. STACS
2013. Also http://arxiv.org/abs/1205.4893
(referuje Dušan Knop)
Abstract. For many optimization problems, the instances of
practical interest often occupy just a tiny part of the
algorithm's space of instances. Following (Y. Bilu and N. Linial,
2010), we apply this perspective to MAXCUT, viewed as a clustering
problem. Using a variety of techniques, we investigate practically
interesting instances of this problem. Specifically, we show how
to solve in polynomial time distinguished, metric, expanding and
dense instances of MAXCUT under mild stability assumptions. In
particular, (1 + epsilon)-stability (which is optimal) suffices
for metric and dense MAXCUT. We also show how to solve in
polynomial time Omega(sqrt(n))-stable instances of MAXCUT,
substantially improving the best previously known result.
2. 5. 2013 (čtvrtek [Thursday], 12:20, MFF UK Malá Strana S1)
Martin Koutecký: Solving Hard Problems on Neighborhood
Diversity
25. 4. 2013 (čtvrtek [Thursday], 12:20, MFF UK Malá Strana S1)
Nikhil Bansal, Nitish Korula, Viswanath Nagarajan, and Aravind
Srinivasan: Solving
Packing Integer Programs via Randomized Rounding with
Alterations. Theory of Computing 8:533-565, 2012.
(referuje Martin Böhm)
Abstract. We give new approximation algorithms for packing
integer programs (PIPs) by employing the method of randomized
rounding combined with alterations. Our first result is a
simpler approximation algorithm for general PIPs which matches the
best known bounds, and which admits an efficient parallel
implementation. We also extend these results to a multi-criteria
version of PIPs. Our second result is for the class of packing
integer programs (PIPs) that are column sparse, i.e.,
where there is a specified upper bound
Manisha Bansal, Naveen Garg, and Neelima Gupta: A
5-Approximation for Capacitated Facility Location. ESA
2012.
(referuje Jakub Velkoborský)
Abstract. In this paper, we propose and analyze a local search algorithm for the capacitated facility location problem. Our algorithm is a modification of the algorithm proposed by Zhang et al. and improves the approximation ratio from 5.83 to 5. We achieve this by modifying the close, open and multi operations. The idea of taking linear combinations of inequalities used in Aggarwal et al. is crucial in achieving this result. The example proposed by Zhang et al. also shows that our analysis is tight.
28. 3., 4. 4. 2013 (čtvrtek [Thursday], 12:20, MFF UK Malá Strana S1)
Leah Epstein, Asaf Levin, Danny Segev and Oren Weimann: Improved
bounds for online preemptive matching. STACS 2013.
Also http://arxiv.org/abs/1207.1788
(referuje Pavel Veselý)
Abstract: When designing a preemptive
online algorithm for the maximum matching problem, we wish to
maintain a valid matching M while edges of the underlying graph
are presented one after the other. When presented with an edge e,
the algorithm should decide whether to augment the matching M by
adding e (in which case e may be removed later on) or to keep M in
its current form without adding e (in which case e is lost for
good). The objective is to eventually hold a matching M with
maximum weight.
The main contribution of this paper is to establish new lower and
upper bounds on the competitive ratio achievable by preemptive
online algorithms:
1. We provide a lower bound of 1+ln 2~1.693 on the competitive
ratio of any randomized algorithm for the maximum cardinality
matching problem, thus improving on the currently best known bound
of e/(e-1)~1.581 due to Karp, Vazirani, and Vazirani [STOC'90].
2. We devise a randomized algorithm that achieves an expected
competitive ratio of 5.356 for maximum weight matching. This
finding demonstrates the power of randomization in this context,
showing how to beat the tight bound of 3 +2\sqrt{2}~5.828 for
deterministic algorithms, obtained by combining the 5.828 upper
bound of McGregor [APPROX'05] and the recent 5.828 lower bound of
Varadaraja [ICALP'11].
21. 3. 2013 (čtvrtek [Thursday], 12:20, MFF UK Malá Strana S1)
Z. Nutov: Approximating
minimum-cost connectivity problems via uncrossable bifamilies.
ACM Trans. Alg. 9(1), Article no. 1, 2012.
(referuje Tomáš Masařík)
7. 3. 2013 (čtvrtek [Thursday], 12:20, MFF UK Malá Strana S1)
Dušan Knop: Parametrizovana slozitost delkou omezenych rezu a multi-rezu
Abstract: Predvedu dukaz, ze minimalni delkou omezeny rez je mozne najit FPT algoritmem v case f(L,t)n, kde L je parametr delky cest a t je stromova sirka grafu ve kterem rez hledame a f() je spocitatelna funkce. Pokud nam vyjde cas, tak predvedu i dusledky (napr. algoritmus pro vicekomoditni omezene rezy).
28. 2. 2013 (čtvrtek [Thursday], 12:20, MFF UK Malá Strana S1)
Petr Kolman: L-omezene rezy a nova linearni relaxace21. 2. 2013 (čtvrtek [Thursday], 12:20, MFF UK Malá Strana S1)
Jiří Sgall: Multiprocessor jobs, preemptive schedules, and
one-competitive online algorithms
(joint work with Gerhard J. Woeginger)
Abstract: We investigate online preemptive scheduling of parallel jobs with release times on m parallel machines. The jobs arrive over time, the length (the processing time) and the width (the number of machines needed) of each job are known at the release time of that job. Our goal is to construct a preemptive schedule with the minimal length of schedule (makespan). We study the case where the set of possible widths of jobs is known and fixed in advance. We characterize all the sets of widths for which it is possible to generate optimal or near-optimal schedules online.