212 research outputs found
From correlation to causation: the cruciality of a collectivity in the context of collective action
This paper discusses a longitudinal field study on collective action which aims to move beyond student samples and enhance mundane realism. First we provide a historical overview of the literature on the what (i.e., antecedents of collective action) and the how (i.e., the methods employed) of the social psychology of protest. This historical overview is substantiated with meta-analytical evidence on how these antecedents and methods changed over time. After the historical overview, we provide an empirical illustration of a longitudinal field study in a natural setting―a newly-built Dutch neighbourhood. We assessed changes in informal embeddedness, efficacy, identification, emotions, and grievances over time. Between t0 and t1 the residents protested against the plan to allow a mosque to carrying out their services in a community building in the neighbourhood. We examined the antecedents of protest before [t0] and after [t1] the protests, and whether residents participated or not. We show how a larger social network functions as a catalyst in steering protest participation. Our longitudinal field study replicates basic findings from experimental and survey research. However, it also shows that one antecedent in particular, which is hard to manipulate in the lab (i.e., the size of someone’s social network), proved to be of great importance. We suggest that in overcoming our most pertinent challenge―causality―we should not only remain in our laboratories but also go out and examine real-life situations with people situated in real-life social networks
Complications in Esophageal Surgery
This thesis describes randomized controlled trials regarding surgical techniques after esophagectomy and the use of the Comprehensive Complication Index
Faster space-efficient algorithms for Subset Sum, k -Sum, and related problems
We present randomized algorithms that solve subset sum and knapsack instances with n items in O∗ (20.86n) time, where the O∗ (∙ ) notation suppresses factors polynomial in the input size, and polynomial space, assuming random read-only access to exponentially many random bits. These results can be extended to solve binary integer programming on n variables with few constraints in a similar running time. We also show that for any constant k ≥ 2, random instances of k-Sum can be solved using O(nk -0.5polylog(n)) time and O(log n) space, without the assumption of random access to random bits.Underlying these results is an algorithm that determines whether two given lists of length n with integers bounded by a polynomial in n share a common value. Assuming random read-only access to random bits, we show that this problem can be solved using O(log n) space significantly faster than the trivial O(n2) time algorithm if no value occurs too often in the same list.</p
Connecting Terminals and 2-Disjoint Connected Subgraphs
Given a graph and a set of terminal vertices we say that a
superset of is -connecting if induces a connected graph, and
is minimal if no strict subset of is -connecting. In this paper we prove
that there are at most minimal -connecting sets when and that
these can be enumerated within a polynomial factor of this bound. This
generalizes the algorithm for enumerating all induced paths between a pair of
vertices, corresponding to the case . We apply our enumeration algorithm
to solve the {\sc 2-Disjoint Connected Subgraphs} problem in time
, improving on the recent algorithm of Cygan et
al. 2012 LATIN paper.Comment: 13 pages, 1 figur
Spotting Trees with Few Leaves
We show two results related to the Hamiltonicity and -Path algorithms in
undirected graphs by Bj\"orklund [FOCS'10], and Bj\"orklund et al., [arXiv'10].
First, we demonstrate that the technique used can be generalized to finding
some -vertex tree with leaves in an -vertex undirected graph in
time. It can be applied as a subroutine to solve the
-Internal Spanning Tree (-IST) problem in
time using polynomial space, improving upon previous algorithms for this
problem. In particular, for the first time we break the natural barrier of
. Second, we show that the iterated random bipartition employed by
the algorithm can be improved whenever the host graph admits a vertex coloring
with few colors; it can be an ordinary proper vertex coloring, a fractional
vertex coloring, or a vector coloring. In effect, we show improved bounds for
-Path and Hamiltonicity in any graph of maximum degree
or with vector chromatic number at most 8
New Tools and Connections for Exponential-Time Approximation
In this paper, we develop new tools and connections for exponential time approximation. In this setting, we are given a problem instance and an integer r>1, and the goal is to design an approximation algorithm with the fastest possible running time. We give randomized algorithms that establish an approximation ratio of
1.
r for maximum independent set in O∗(exp(O~(n/rlog2r+rlog2r)))
time,
2.
r for chromatic number in O∗(exp(O~(n/rlogr+rlog2r)))
time,
3.
(2−1/r)
for minimum vertex cover in O∗(exp(n/rΩ(r)))
time, and
4.
(k−1/r)
for minimum k-hypergraph vertex cover in O∗(exp(n/(kr)Ω(kr)))
time.
(Throughout, O~
and O∗ omit polyloglog(r) and factors polynomial in the input size, respectively.) The best known time bounds for all problems were O∗(2n/r) (Bourgeois et al. i
New Tools and Connections for Exponential-Time Approximation
In this paper, we develop new tools and connections for exponential time approximation. In this setting, we are given a problem instance and an integer r>1, and the goal is to design an approximation algorithm with the fastest possible running time. We give randomized algorithms that establish an approximation ratio of
1.
r for maximum independent set in O∗(exp(O~(n/rlog2r+rlog2r)))
time,
2.
r for chromatic number in O∗(exp(O~(n/rlogr+rlog2r)))
time,
3.
(2−1/r)
for minimum vertex cover in O∗(exp(n/rΩ(r)))
time, and
4.
(k−1/r)
for minimum k-hypergraph vertex cover in O∗(exp(n/(kr)Ω(kr)))
time.
(Throughout, O~
and O∗ omit polyloglog(r) and factors polynomial in the input size, respectively.) The best known time bounds for all problems were O∗(2n/r) (Bourgeois et al. in Discret Appl Math 159(17):1954–1970, 2011; Cygan et al. in Exponential-time approximation of hard problems, 2008). For maximum independent set and chromatic number, these bounds were complemented by exp(n1−o(1)/r1+o(1)) lower bounds (under the Exponential Time Hypothesis (ETH)) (Chalermsook et al. in Foundations of computer science, FOCS, pp. 370–379, 2013; Laekhanukit in Inapproximability of combinatorial problems in subexponential-time. Ph.D. thesis, 2014). Our results show that the naturally-looking O∗(2n/r) bounds are not tight for all these problems. The key to these results is a sparsification procedure that reduces a problem to a bounded-degree variant, allowing the use of approximation algorithms for bounded-degree graphs. To obtain the first two results, we introduce a new randomized branching rule. Finally, we show a connection between PCP parameters and exponential-time approximation algorithms. This connection together with our independent set algorithm refute the possibility to overly reduce the size of Chan’s PCP (Chan in J. ACM 63(3):27:1–27:32, 2016). It also implies that a (significant) improvement over our result will refute the gap-ETH conjecture (Dinur in Electron Colloq Comput Complex (ECCC) 23:128, 2016; Manurangsi and Raghavendra in A birthday repetition theorem and complexity of approximating dense CSPs, 2016)
A -Vertex Kernel for Maximum Internal Spanning Tree
We consider the parameterized version of the maximum internal spanning tree
problem, which, given an -vertex graph and a parameter , asks for a
spanning tree with at least internal vertices. Fomin et al. [J. Comput.
System Sci., 79:1-6] crafted a very ingenious reduction rule, and showed that a
simple application of this rule is sufficient to yield a -vertex kernel.
Here we propose a novel way to use the same reduction rule, resulting in an
improved -vertex kernel. Our algorithm applies first a greedy procedure
consisting of a sequence of local exchange operations, which ends with a
local-optimal spanning tree, and then uses this special tree to find a
reducible structure. As a corollary of our kernel, we obtain a deterministic
algorithm for the problem running in time
Finding and counting vertex-colored subtrees
The problems studied in this article originate from the Graph Motif problem
introduced by Lacroix et al. in the context of biological networks. The problem
is to decide if a vertex-colored graph has a connected subgraph whose colors
equal a given multiset of colors . It is a graph pattern-matching problem
variant, where the structure of the occurrence of the pattern is not of
interest but the only requirement is the connectedness. Using an algebraic
framework recently introduced by Koutis et al., we obtain new FPT algorithms
for Graph Motif and variants, with improved running times. We also obtain
results on the counting versions of this problem, proving that the counting
problem is FPT if M is a set, but becomes W[1]-hard if M is a multiset with two
colors. Finally, we present an experimental evaluation of this approach on real
datasets, showing that its performance compares favorably with existing
software.Comment: Conference version in International Symposium on Mathematical
Foundations of Computer Science (MFCS), Brno : Czech Republic (2010) Journal
Version in Algorithmic
Does Routine Endoscopy or Contrast Swallow Study After Esophagectomy and Gastric Tube Reconstruction Change Patient Management?
Background: Anastomotic leakage is a severe complication after esophagectomy. The objective was to investigate the diagnostic and predictive value of routine contrast swallow study and endoscopy for the detection of anastomotic dehiscence in patients after esophagectomy. Methods: All patients who underwent contrast swallow and/or endoscopy within 7 days after oesophagectomy for cancer between January 2005 and December 2009 were selected from an institutional database. Results: Some 173 patients underwent endoscopy, and 184 patients underwent a contrast swallow study. The sensitivity of endoscopy for anastomotic leakage requiring intervention is 56 %, specificity 41 %, positive predictive value (PPV) 8 %, and negative predictive value (NPV) 95 %. The sensitivity of contrast swallow study for detecting leakage requiring intervention in patients without signs of leakage was 20 %, specificity 20 %, PPV 3 %, and NPV 97 %. Conclusions: In patients without clinical suspicion of leakage, there is no benefit to perform routine examinations
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