6,319 research outputs found
Counting Shortest Two Disjoint Paths in Cubic Planar Graphs with an NC Algorithm
Given an undirected graph and two disjoint vertex pairs and
, the Shortest two disjoint paths problem (S2DP) asks for the minimum
total length of two vertex disjoint paths connecting with , and
with , respectively.
We show that for cubic planar graphs there are NC algorithms, uniform
circuits of polynomial size and polylogarithmic depth, that compute the S2DP
and moreover also output the number of such minimum length path pairs.
Previously, to the best of our knowledge, no deterministic polynomial time
algorithm was known for S2DP in cubic planar graphs with arbitrary placement of
the terminals. In contrast, the randomized polynomial time algorithm by
Bj\"orklund and Husfeldt, ICALP 2014, for general graphs is much slower, is
serial in nature, and cannot count the solutions.
Our results are built on an approach by Hirai and Namba, Algorithmica 2017,
for a generalisation of S2DP, and fast algorithms for counting perfect
matchings in planar graphs
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A Structural Theorem For Shortest Vertex-Disjoint Paths Computation in Planar Graphs
Given k terminal pairs (s₁,t₁),(s₂,t₂),..., (s[subscript k],t[subscript k]) in an edge-weighted graph G, the k Shortest Vertex-Disjoint Paths problem is to find a collection P₁, P₂,..., P[subscript k] of vertex-disjoint paths with minimum total length, where P[subscript i] is an s[subscript i]-to-t[subscript i] path. As a special case of the multi-commodity flow problem, computing vertex disjoint paths has found several applications, for example in VLSI design, or network routing. In this thesis we describe a Structural Theorem for a special case of the Shortest Vertex-Disjoint Paths problem in undirected planar graphs where the terminal vertices are on the boundary of the outer face. At a high level, our Structural Theorem guarantees that the i[superscript th] path of the k Shortest Vertex-Disjoint paths does not cross j[superscript th] (j ≠ i) path of the k-1 Vertex-Disjoint Paths problem
Shortest k-Disjoint Paths via Determinants
The well-known -disjoint path problem (-DPP) asks for pairwise
vertex-disjoint paths between specified pairs of vertices in a
given graph, if they exist. The decision version of the shortest -DPP asks
for the length of the shortest (in terms of total length) such paths. Similarly
the search and counting versions ask for one such and the number of such
shortest set of paths, respectively.
We restrict attention to the shortest -DPP instances on undirected planar
graphs where all sources and sinks lie on a single face or on a pair of faces.
We provide efficient sequential and parallel algorithms for the search versions
of the problem answering one of the main open questions raised by Colin de
Verdiere and Schrijver for the general one-face problem. We do so by providing
a randomised algorithm along with an time randomised
sequential algorithm. We also obtain deterministic algorithms with similar
resource bounds for the counting and search versions.
In contrast, previously, only the sequential complexity of decision and
search versions of the "well-ordered" case has been studied. For the one-face
case, sequential versions of our routines have better running times for
constantly many terminals. In addition, the earlier best known sequential
algorithms (e.g. Borradaile et al.) were randomised while ours are also
deterministic.
The algorithms are based on a bijection between a shortest -tuple of
disjoint paths in the given graph and cycle covers in a related digraph. This
allows us to non-trivially modify established techniques relating counting
cycle covers to the determinant. We further need to do a controlled
inclusion-exclusion to produce a polynomial sum of determinants such that all
"bad" cycle covers cancel out in the sum allowing us to count "good" cycle
covers.Comment: 17 pages, 6 figure
Walking Through Waypoints
We initiate the study of a fundamental combinatorial problem: Given a
capacitated graph , find a shortest walk ("route") from a source to a destination that includes all vertices specified by a set
: the \emph{waypoints}. This waypoint routing problem
finds immediate applications in the context of modern networked distributed
systems. Our main contribution is an exact polynomial-time algorithm for graphs
of bounded treewidth. We also show that if the number of waypoints is
logarithmically bounded, exact polynomial-time algorithms exist even for
general graphs. Our two algorithms provide an almost complete characterization
of what can be solved exactly in polynomial-time: we show that more general
problems (e.g., on grid graphs of maximum degree 3, with slightly more
waypoints) are computationally intractable
Charting the Algorithmic Complexity of Waypoint Routing
Modern computer networks support interesting new routing models in which traffic flows from a source sto a destination t can be flexibly steered through a sequence of waypoints, such as (hardware) middleboxes or (virtualized) network functions (VNFs), to create innovative network services like service chains or segment routing. While the benefits and technological challenges of providing such routing models have been articulated and studied intensively over the last years, less is known about the underlying algorithmic traffic routing problems.
The goal of this paper is to provide the network community with an overview of algorithmic techniques for waypoint routing and also inform about limitations due to computational hardness. In particular, we put the waypoint routing problem into perspective with respect to classic graph theoretical problems. For example, we find that while computing a shortest path from a source s to a destination t is simple (e.g., using Dijkstra's algorithm), the problem of finding a shortest route from s to t via a single waypoint already features a deep combinatorial structure.</jats:p
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