82 research outputs found
A Novel Algorithm for the All-Best-Swap-Edge Problem on Tree Spanners
Given a 2-edge connected, unweighted, and undirected graph with
vertices and edges, a -tree spanner is a spanning tree of
in which the ratio between the distance in of any pair of vertices and the
corresponding distance in is upper bounded by . The minimum value
of for which is a -tree spanner of is also called the
{\em stretch factor} of . We address the fault-tolerant scenario in which
each edge of a given tree spanner may temporarily fail and has to be
replaced by a {\em best swap edge}, i.e. an edge that reconnects at a
minimum stretch factor. More precisely, we design an time and space
algorithm that computes a best swap edge of every tree edge. Previously, an
time and space algorithm was known for
edge-weighted graphs [Bil\`o et al., ISAAC 2017]. Even if our improvements on
both the time and space complexities are of a polylogarithmic factor, we stress
the fact that the design of a time and space algorithm would be
considered a breakthrough.Comment: The paper has been accepted for publication at the 29th International
Symposium on Algorithms and Computation (ISAAC 2018). 12 pages, 3 figure
Linear Time Distributed Swap Edge Algorithms
In this paper, we consider the all best swap edges problem in
a distributed environment. We are given a 2-edge connected positively weighted network X, where all communication is routed through a rooted spanning tree T of X. If one tree edge e = {x, y} fails, the communication network will be disconnected. However, since X is 2-edge connected,
communication can be restored by replacing e by non-tree edge e′, called a swap edge of e, whose ends lie in different components of T − e. Of all possible swap edges of e, we would like to choose the best, as defined by the application. The all best swap edges problem is to identify the
best swap edge for every tree edge, so that in case of any edge failure, the best swap edge can be activated quickly. There are solutions to this problem for a number of cases in the literature. A major concern for all these solutions is to minimize the number of messages. However, especially in fault-transient environments, time is a crucial factor. In this
paper we present a novel technique that addresses this problem from a time perspective; in fact, we present a distributed solution that works in linear time with respect to the height h of T for a number of differentcriteria, while retaining the optimal number of messages. To the best of
our knowledge, all previous solutions solve the problem in O(h^2) time in the cases we consider
The Graph Lottery Ticket Hypothesis: Finding Sparse, Informative Graph Structure
Graph learning methods help utilize implicit relationships among data items,
thereby reducing training label requirements and improving task performance.
However, determining the optimal graph structure for a particular learning task
remains a challenging research problem.
In this work, we introduce the Graph Lottery Ticket (GLT) Hypothesis - that
there is an extremely sparse backbone for every graph, and that graph learning
algorithms attain comparable performance when trained on that subgraph as on
the full graph. We identify and systematically study 8 key metrics of interest
that directly influence the performance of graph learning algorithms.
Subsequently, we define the notion of a "winning ticket" for graph structure -
an extremely sparse subset of edges that can deliver a robust approximation of
the entire graph's performance. We propose a straightforward and efficient
algorithm for finding these GLTs in arbitrary graphs. Empirically, we observe
that performance of different graph learning algorithms can be matched or even
exceeded on graphs with the average degree as low as 5
Community-aware network sparsification
Network sparsification aims to reduce the number of edges of a network while maintaining its structural properties; such properties include shortest paths, cuts, spectral measures, or network modularity. Sparsification has multiple applications, such as, speeding up graph-mining algorithms, graph visualization, as well as identifying the important network edges.
In this paper we consider a novel formulation of the network-sparsification problem. In addition to the network, we also consider as input a set of communities. The goal is to sparsify the network so as to preserve the network structure with respect to the given communities. We introduce two variants of the community-aware sparsification problem, leading to sparsifiers that satisfy different connectedness community properties. From the technical point of view, we prove hardness results and devise effective approximation algorithms. Our experimental results on a large collection of datasets demonstrate the effectiveness of our algorithms.https://epubs.siam.org/doi/10.1137/1.9781611974973.48Accepted manuscrip
Fault Tolerant and Fully Dynamic DFS in Undirected Graphs: Simple Yet Efficient
We present an algorithm for a fault tolerant Depth First Search (DFS) Tree in an undirected graph. This algorithm is drastically simpler than the current state-of-the-art algorithms for this problem, uses optimal space and optimal preprocessing time, and still achieves better time complexity. This algorithm also leads to a better time complexity for maintaining a DFS tree in a fully dynamic environment
Improved Roundtrip Spanners, Emulators, and Directed Girth Approximation
Roundtrip spanners are the analog of spanners in directed graphs, where the
roundtrip metric is used as a notion of distance. Recent works have shown
existential results of roundtrip spanners nearly matching the undirected case,
but the time complexity for constructing roundtrip spanners is still widely
open.
This paper focuses on developing fast algorithms for roundtrip spanners and
related problems. For any -vertex directed graph with edges (with
non-negative edge weights), our results are as follows:
- 3-roundtrip spanner faster than APSP: We give an
-time algorithm that constructs a roundtrip spanner of
stretch and optimal size . Previous constructions of roundtrip
spanners of the same size either required time [Roditty, Thorup,
Zwick SODA'02; Cen, Duan, Gu ICALP'20], or had worse stretch [Chechik and
Lifshitz SODA'21].
- Optimal roundtrip emulator in dense graphs: For integer , we give
an -time algorithm that constructs a roundtrip \emph{emulator}
of stretch and size , which is optimal for constant
under Erd\H{o}s' girth conjecture. Previous work of [Thorup and Zwick STOC'01]
implied a roundtrip emulator of the same size and stretch, but it required
construction time. Our improved running time is near-optimal for
dense graphs.
- Faster girth approximation in sparse graphs: We give an
-time algorithm that -approximates the girth of a
directed graph. This can be compared with the previous -approximation
algorithm in time by [Chechik and Lifshitz
SODA'21]. In sparse graphs, our algorithm achieves better running time at the
cost of a larger approximation ratio.Comment: To appear in SODA 202
Faster Parallel Algorithm for Approximate Shortest Path
We present the first work, time
algorithm in the PRAM model that computes -approximate
single-source shortest paths on weighted, undirected graphs. This improves upon
the breakthrough result of Cohen~[JACM'00] that achieves
work and time. While most previous approaches, including
Cohen's, leveraged the power of hopsets, our algorithm builds upon the recent
developments in \emph{continuous optimization}, studying the shortest path
problem from the lens of the closely-related \emph{minimum transshipment}
problem. To obtain our algorithm, we demonstrate a series of near-linear work,
polylogarithmic-time reductions between the problems of approximate shortest
path, approximate transshipment, and -embeddings, and establish a
recursive algorithm that cycles through the three problems and reduces the
graph size on each cycle. As a consequence, we also obtain faster parallel
algorithms for approximate transshipment and -embeddings with
polylogarithmic distortion. The minimum transshipment algorithm in particular
improves upon the previous best work sequential algorithm of
Sherman~[SODA'17].
To improve readability, the paper is almost entirely self-contained, save for
several staple theorems in algorithms and combinatorics.Comment: 53 pages, STOC 202
LIPIcs, Volume 244, ESA 2022, Complete Volume
LIPIcs, Volume 244, ESA 2022, Complete Volum
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