28 research outputs found
Total Domishold Graphs: a Generalization of Threshold Graphs, with Connections to Threshold Hypergraphs
A total dominating set in a graph is a set of vertices such that every vertex
of the graph has a neighbor in the set. We introduce and study graphs that
admit non-negative real weights associated to their vertices such that a set of
vertices is a total dominating set if and only if the sum of the corresponding
weights exceeds a certain threshold. We show that these graphs, which we call
total domishold graphs, form a non-hereditary class of graphs properly
containing the classes of threshold graphs and the complements of domishold
graphs, and are closely related to threshold Boolean functions and threshold
hypergraphs. We present a polynomial time recognition algorithm of total
domishold graphs, and characterize graphs in which the above property holds in
a hereditary sense. Our characterization is obtained by studying a new family
of hypergraphs, defined similarly as the Sperner hypergraphs, which may be of
independent interest.Comment: 19 pages, 1 figur
Fair allocation of indivisible goods under conflict constraints
We consider the fair allocation of indivisible items to several agents and
add a graph theoretical perspective to this classical problem. Thereby we
introduce an incompatibility relation between pairs of items described in terms
of a conflict graph. Every subset of items assigned to one agent has to form an
independent set in this graph. Thus, the allocation of items to the agents
corresponds to a partial coloring of the conflict graph. Every agent has its
own profit valuation for every item. Aiming at a fair allocation, our goal is
the maximization of the lowest total profit of items allocated to any one of
the agents. The resulting optimization problem contains, as special cases, both
{\sc Partition} and {\sc Independent Set}. In our contribution we derive
complexity and algorithmic results depending on the properties of the given
graph. We can show that the problem is strongly NP-hard for bipartite graphs
and their line graphs, and solvable in pseudo-polynomial time for the classes
of chordal graphs, cocomparability graphs, biconvex bipartite graphs, and
graphs of bounded treewidth. Each of the pseudo-polynomial algorithms can also
be turned into a fully polynomial approximation scheme (FPTAS).Comment: A preliminary version containing some of the results presented here
appeared in the proceedings of IWOCA 2020. Version 3 contains an appendix
with a remark about biconvex bipartite graph
Minimizing Maximum Dissatisfaction in the Allocation of Indivisible Items under a Common Preference Graph
We consider the task of allocating indivisible items to agents, when the
agents' preferences over the items are identical. The preferences are captured
by means of a directed acyclic graph, with vertices representing items and an
edge , meaning that each of the agents prefers item over item .
The dissatisfaction of an agent is measured by the number of items that the
agent does not receive and for which it also does not receive any more
preferred item. The aim is to allocate the items to the agents in a fair way,
i.e., to minimize the maximum dissatisfaction among the agents. We study the
status of computational complexity of that problem and establish the following
dichotomy: the problem is NP-hard for the case of at least three agents, even
on fairly restricted graphs, but polynomially solvable for two agents. We also
provide several polynomial-time results with respect to different underlying
graph structures, such as graphs of width at most two and tree-like structures
such as stars and matchings. These findings are complemented with fixed
parameter tractability results related to path modules and independent set
modules. Techniques employed in the paper include bottleneck assignment
problem, greedy algorithm, dynamic programming, maximum network flow, and
integer linear programming.Comment: 26 pages, 2 figure
Linearna separacija povezanih dominantnih množic v grafih
Povezana dominantna množica v grafu je dominantna množica točk, ki inducira povezan podgraf. Po zgledu sorodnih raziskav v literaturi o neodvisnih množicah, dominantnih množicah in totalno dominantnih množicah v tem članku raziskujemo razred grafov, v katerem lahko povezane dominantne množice točk ločimo od ostalih podmnožic točk z linearno utežno funkcijo. Natančneje, pravimo, da je graf povezano dominantno pragoven, če lahko njegovi množici točk priredimo take nenegativne realne uteži, da je množica točk povezana dominantna množica natanko tedaj, ko vsota uteži njenih elementov preseže določen prag. Grafe tega nehereditarnega razreda karakteriziramo s pomočjo množice minimalnih prerezov grafa. Podamo tudi več karakterizacij za hereditarni primer, tj. ko se za vsak povezan induciran podgraf zahteva, da je povezano dominantno pragoven. Karakterizacija s prepovedanimi induciranimi podgrafi implicira, da je ta razred grafov prava posplošitev dobro znanih razredov tetivnih grafov, bločnih grafov in trivialno popolnih grafov. Preučujemo tudi določene algoritmične vidike povezano dominantno pragovnih grafov. Na podlagi povezav z minimalnimi prerezi in lastnostmi izpeljanih hipergrafov in Boolovih funkcij pokažemo, da naš pristop vodi k novim polinomsko rešljivim primerom problema utežene povezane dominantne množice
On matching extendability of lexicographic products
A graph G of even order is ℓ-extendable if it is of order at least 2ℓ + 2, contains a matching of size ℓ, and if every such matching is contained in a perfect matching of G. In this paper, we study the extendability of lexicographic products of graphs. We characterize graphs G and H such that their lexicographic product is not 1-extendable. We also provide several conditions on the graphs G and H under which their lexicographic product is 2-extendable