2,174 research outputs found
Half-Duplex Routing is NP-hard
Routing is a widespread approach to transfer information from a source node
to a destination node in many deployed wireless ad-hoc networks. Today's
implemented routing algorithms seek to efficiently find the path/route with the
largest Full-Duplex (FD) capacity, which is given by the minimum among the
point-to-point link capacities in the path. Such an approach may be suboptimal
if then the nodes in the selected path are operated in Half-Duplex (HD) mode.
Recently, the capacity (up to a constant gap that only depends on the number of
nodes in the path) of an HD line network i.e., a path) has been shown to be
equal to half of the minimum of the harmonic means of the capacities of two
consecutive links in the path. This paper asks the questions of whether it is
possible to design a polynomial-time algorithm that efficiently finds the path
with the largest HD capacity in a relay network. This problem of finding that
path is shown to be NP-hard in general. However, if the number of cycles in the
network is polynomial in the number of nodes, then a polynomial-time algorithm
can indeed be designed
Blocking optimal arborescences
The problem of covering minimum cost common bases of two matroids is
NP-complete, even if the two matroids coincide, and the costs are all equal to
1. In this paper we show that the following special case is solvable in
polynomial time: given a digraph with a designated root node
and arc-costs , find a minimum cardinality subset of the
arc set such that intersects every minimum -cost -arborescence.
By an -arborescence we mean a spanning arborescence of root . The
algorithm we give solves a weighted version as well, in which a nonnegative
weight function (unrelated to ) is also given, and we
want to find a subset of the arc set such that intersects every minimum
-cost -arborescence, and is minimum. The running
time of the algorithm is , where and denote the number of
nodes and arcs of the input digraph, and is the time needed for a
minimum cut computation in this digraph. A polyhedral description is not
given, and seems rather challenging
Blocking unions of arborescences
Given a digraph and a positive integer , a subset
is called a \textbf{-union-arborescence}, if it is the disjoint union of
spanning arborescences. When also arc-costs are given,
minimizing the cost of a -union-arborescence is well-known to be tractable.
In this paper we take on the following problem: what is the minimum cardinality
of a set of arcs the removal of which destroys every minimum -cost
-union-arborescence. Actually, the more general weighted problem is also
considered, that is, arc weights (unrelated to ) are
also given, and the goal is to find a minimum weight set of arcs the removal of
which destroys every minimum -cost -union-arborescence. An equivalent
version of this problem is where the roots of the arborescences are fixed in
advance. In an earlier paper [A. Bern\'ath and Gy. Pap, \emph{Blocking optimal
arborescences}, Integer Programming and Combinatorial Optimization, Springer,
2013] we solved this problem for . This work reports on other partial
results on the problem. We solve the case when both and are uniform --
that is, find a minimum size set of arcs that covers all
-union-arbosercences. Our algorithm runs in polynomial time for this
problem. The solution uses a result of [M. B\'ar\'asz, J. Becker, and A. Frank,
\emph{An algorithm for source location in directed graphs}, Oper. Res. Lett.
\textbf{33} (2005)] saying that the family of so-called insolid sets (sets with
the property that every proper subset has a larger in-degree) satisfies the
Helly-property, and thus can be (efficiently) represented as a subtree
hypergraph. We also give an algorithm for the case when only is uniform but
is not. This algorithm is only polynomial if is not part of the input
Parameterized Algorithms for Directed Maximum Leaf Problems
We prove that finding a rooted subtree with at least leaves in a digraph
is a fixed parameter tractable problem. A similar result holds for finding
rooted spanning trees with many leaves in digraphs from a wide family
that includes all strong and acyclic digraphs. This settles completely an open
question of Fellows and solves another one for digraphs in . Our
algorithms are based on the following combinatorial result which can be viewed
as a generalization of many results for a `spanning tree with many leaves' in
the undirected case, and which is interesting on its own: If a digraph of order with minimum in-degree at least 3 contains a rooted
spanning tree, then contains one with at least leaves
Min (A)cyclic Feedback Vertex Sets and Min Ones Monotone 3-SAT
In directed graphs, we investigate the problems of finding: 1) a minimum
feedback vertex set (also called the Feedback Vertex Set problem, or MFVS), 2)
a feedback vertex set inducing an acyclic graph (also called the Vertex
2-Coloring without Monochromatic Cycles problem, or Acyclic FVS) and 3) a
minimum feedback vertex set inducing an acyclic graph (Acyclic MFVS).
We show that these problems are strongly related to (variants of) Monotone
3-SAT and Monotone NAE 3-SAT, where monotone means that all literals are in
positive form. As a consequence, we deduce several NP-completeness results on
restricted versions of these problems. In particular, we define the 2-Choice
version of an optimization problem to be its restriction where the optimum
value is known to be either D or D+1 for some integer D, and the problem is
reduced to decide which of D or D+1 is the optimum value. We show that the
2-Choice versions of MFVS, Acyclic MFVS, Min Ones Monotone 3-SAT and Min Ones
Monotone NAE 3-SAT are NP-complete. The two latter problems are the variants of
Monotone 3-SAT and respectively Monotone NAE 3-SAT requiring that the truth
assignment minimize the number of variables set to true.
Finally, we propose two classes of directed graphs for which Acyclic FVS is
polynomially solvable, namely flow reducible graphs (for which MFVS is already
known to be polynomially solvable) and C1P-digraphs (defined by an adjacency
matrix with the Consecutive Ones Property)
Disimplicial arcs, transitive vertices, and disimplicial eliminations
In this article we deal with the problems of finding the disimplicial arcs of
a digraph and recognizing some interesting graph classes defined by their
existence. A diclique of a digraph is a pair of sets of vertices such
that is an arc for every and . An arc is
disimplicial when is a diclique. We show that the problem
of finding the disimplicial arcs is equivalent, in terms of time and space
complexity, to that of locating the transitive vertices. As a result, an
efficient algorithm to find the bisimplicial edges of bipartite graphs is
obtained. Then, we develop simple algorithms to build disimplicial elimination
schemes, which can be used to generate bisimplicial elimination schemes for
bipartite graphs. Finally, we study two classes related to perfect disimplicial
elimination digraphs, namely weakly diclique irreducible digraphs and diclique
irreducible digraphs. The former class is associated to finite posets, while
the latter corresponds to dedekind complete finite posets.Comment: 17 pags., 3 fig
Resilience of Complex Networks
This article determines and characterizes the minimal number of actuators
needed to ensure structural controllability of a linear system under structural
alterations that can severe the connection between any two states. We assume
that initially the system is structurally controllable with respect to a given
set of controls, and propose an efficient system-synthesis mechanism to find
the minimal number of additional actuators required for resilience of the
system w.r.t such structural changes. The effectiveness of this approach is
demonstrated by using standard IEEE power networks
A Maximum Linear Arrangement Problem on Directed Graphs
We propose a new arrangement problem on directed graphs, Maximum Directed
Linear Arrangement (MaxDLA). This is a directed variant of a similar problem
for undirected graphs, in which however one seeks maximum and not minimum; this
problem known as the Minimum Linear Arrangement Problem (MinLA) has been much
studied in the literature. We establish a number of theorems illustrating the
behavior and complexity of MaxDLA. First, we relate MaxDLA to Maximum Directed
Cut (MaxDiCut) by proving that every simple digraph on vertices
satisfies .
Next, we prove that MaxDiCut is NP-Hard for planar digraphs (even with the
added restriction of maximum degree 15); it follows from the above bounds that
MaxDLA is also NP-Hard for planar digraphs. In contrast, Hadlock (1975) and
Dorfman and Orlova (1972) showed that the undirected Maximum Cut problem is
solvable in polynomial time on planar graphs.
On the positive side, we present a polynomial-time algorithm for solving
MaxDLA on orientations of trees with degree bounded by a constant, which
translates to a polynomial-time algorithm for solving MinLA on the complements
of those trees. This pairs with results by Goldberg and Klipker (1976),
Shiloach (1979) and Chung (1984) solving MinLA in polynomial time on trees.
Finally, analogues of Harper's famous isoperimetric inequality for the
hypercube, in the setting of MaxDLA, are shown for tournaments, orientations of
graphs with degree at most two, and transitive acyclic digraphs.Comment: 12 page
Packing and domination parameters in digraphs
Given a digraph , a set is a packing set in if
there are no arcs joining vertices of and for any two vertices
the sets of in-neighbors of and are disjoint. The set is a
dominating set (an open dominating set) in if every vertex not in (in
) has an in-neighbor in . Moreover, a dominating set is called a
total dominating set if the subgraph induced by has no isolated vertices.
The packing sets of maximum cardinality and the (total, open) dominating sets
of minimum cardinality in digraphs are studied in this article. We prove that
the two optimal sets concerning packing and domination achieve the same value
for directed trees, and give some applications of it. We also show analogous
equalities for all connected contrafunctional digraphs, and characterize all
such digraphs for which such equalities are satisfied. Moreover, sharp
bounds on the maximum and the minimum cardinalities of packing and dominating
sets, respectively, are given for digraphs. Finally, we present solutions for
two open problems, concerning total and open dominating sets of minimum
cardinality, pointed out in [Australas. J. Combin. 39 (2007), 283--292]
Problem collection from the IML programme: Graphs, Hypergraphs, and Computing
This collection of problems and conjectures is based on a subset of the open
problems from the seminar series and the problem sessions of the Institut
Mitag-Leffler programme Graphs, Hypergraphs, and Computing. Each problem
contributor has provided a write up of their proposed problem and the
collection has been edited by Klas Markstr\"om.Comment: This problem collection is published as part of the IML preprint
series for the research programme and also available there
http://www.mittag-leffler.se/research-programs/preprint-series?course_id=4401.
arXiv admin note: text overlap with arXiv:1403.5975, arXiv:0706.4101 by other
author
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