132 research outputs found
An efficient algorithm for finding maximum cycle packings in reducible flow graphs
Reducible flow graphs occur naturally in connection with flow-charts of computer programs and are used extensively for code optimization and global data flow analysis. In this paper we present an O(n2m log (n 2/m)) algorithm for finding a maximum cycle packing in any weighted reducible flow graph with n vertices and m arcs. © Springer-Verlag 2004.postprin
Exact Localisations of Feedback Sets
The feedback arc (vertex) set problem, shortened FASP (FVSP), is to transform
a given multi digraph into an acyclic graph by deleting as few arcs
(vertices) as possible. Due to the results of Richard M. Karp in 1972 it is one
of the classic NP-complete problems. An important contribution of this paper is
that the subgraphs , of all elementary
cycles or simple cycles running through some arc , can be computed in
and , respectively. We use
this fact and introduce the notion of the essential minor and isolated cycles,
which yield a priori problem size reductions and in the special case of so
called resolvable graphs an exact solution in . We show
that weighted versions of the FASP and FVSP possess a Bellman decomposition,
which yields exact solutions using a dynamic programming technique in times
and
, where , , respectively. The parameters can
be computed in , ,
respectively and denote the maximal dimension of the cycle space of all
appearing meta graphs, decoding the intersection behavior of the cycles.
Consequently, equal zero if all meta graphs are trees. Moreover, we
deliver several heuristics and discuss how to control their variation from the
optimum. Summarizing, the presented results allow us to suggest a strategy for
an implementation of a fast and accurate FASP/FVSP-SOLVER
Minimum Forcing Sets for Miura Folding Patterns
We introduce the study of forcing sets in mathematical origami. The origami
material folds flat along straight line segments called creases, each of which
is assigned a folding direction of mountain or valley. A subset of creases
is forcing if the global folding mountain/valley assignment can be deduced from
its restriction to . In this paper we focus on one particular class of
foldable patterns called Miura-ori, which divide the plane into congruent
parallelograms using horizontal lines and zig-zag vertical lines. We develop
efficient algorithms for constructing a minimum forcing set of a Miura-ori map,
and for deciding whether a given set of creases is forcing or not. We also
provide tight bounds on the size of a forcing set, establishing that the
standard mountain-valley assignment for the Miura-ori is the one that requires
the most creases in its forcing sets. Additionally, given a partial
mountain/valley assignment to a subset of creases of a Miura-ori map, we
determine whether the assignment domain can be extended to a locally
flat-foldable pattern on all the creases. At the heart of our results is a
novel correspondence between flat-foldable Miura-ori maps and -colorings of
grid graphs.Comment: 20 pages, 16 figures. To appear at the ACM/SIAM Symp. on Discrete
Algorithms (SODA 2015
New Exact and Approximation Algorithms for the Star Packing Problem in Undirected Graphs
By a T-star we mean a complete bipartite graph K_{1,t} for some t <= T. For an undirected graph G, a T-star packing is a collection of node-disjoint T-stars in G.
For example, we get ordinary matchings for and packings of paths of length 1 and 2 for . Hereinafter we assume that T >= 2.
Hell and Kirkpatrick devised an ad-hoc augmenting algorithm that finds a T-star packing covering the maximum number of nodes. The latter algorithm also yields a min-max formula.
We show that T-star packings are reducible to network flows, hence the above problem is solvable in time (hereinafter n denotes the number of nodes in G, and m --- the number of edges).
For the edge-weighted case (in which weights may be assumed positive) finding a maximum -packing is NP-hard. A novel 9/4 T/(T + 1)-factor approximation algorithm is presented.
For non-negative node weights the problem reduces to a special case of a max-cost flow. We develop a divide-and-conquer approach that solves it in O(m sqrt(n) log(n)) time. The node-weighted problem with arbitrary weights is more difficult. We prove that it is NP-hard for T >= 3 and is solvable in strongly-polynomial time for T = 2
Tight Localizations of Feedback Sets
The classical NP-hard feedback arc set problem (FASP) and feedback vertex set
problem (FVSP) ask for a minimum set of arcs or
vertices whose removal , makes a given multi-digraph acyclic, respectively. Though both
problems are known to be APX-hard, approximation algorithms or proofs of
inapproximability are unknown. We propose a new
-heuristic for the directed FASP. While a ratio of is known to be a lower bound for the APX-hardness, at least by
empirical validation we achieve an approximation of . The most
relevant applications, such as circuit testing, ask for solving the FASP on
large sparse graphs, which can be done efficiently within tight error bounds
due to our approach.Comment: manuscript submitted to AC
Fast and Processor-Efficient Parallel Algorithms for Reducible Flow Graphs
Coordinated Science Laboratory was formerly known as Control Systems LaboratoryJoint Services Electronics Program / N00014-84-C-014
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