99 research outputs found
Fast Discovery of Reliable k-terminal Subgraphs
Peer reviewe
Crossing-Free Acyclic Hamiltonian Path Completion for Planar st-Digraphs
In this paper we study the problem of existence of a crossing-free acyclic
hamiltonian path completion (for short, HP-completion) set for embedded upward
planar digraphs. In the context of book embeddings, this question becomes:
given an embedded upward planar digraph , determine whether there exists an
upward 2-page book embedding of preserving the given planar embedding.
Given an embedded -digraph which has a crossing-free HP-completion
set, we show that there always exists a crossing-free HP-completion set with at
most two edges per face of . For an embedded -free upward planar digraph
, we show that there always exists a crossing-free acyclic HP-completion set
for which, moreover, can be computed in linear time. For a width-
embedded planar -digraph , we show that we can be efficiently test
whether admits a crossing-free acyclic HP-completion set.Comment: Accepted to ISAAC200
An Algorithm for constructing Hjelmslev planes
Projective Hjelmslev planes and Affine Hjelmselv planes are generalisations
of projective planes and affine planes. We present an algorithm for
constructing a projective Hjelmslev planes and affine Hjelsmelv planes using
projective planes, affine planes and orthogonal arrays. We show that all
2-uniform projective Hjelmslev planes, and all 2-uniform affine Hjelsmelv
planes can be constructed in this way. As a corollary it is shown that all
2-uniform Affine Hjelmselv planes are sub-geometries of 2-uniform projective
Hjelmselv planes.Comment: 15 pages. Algebraic Design Theory and Hadamard matrices, 2014,
Springer Proceedings in Mathematics & Statistics 13
Scheduling Jobs in Flowshops with the Introduction of Additional Machines in the Future
This is the author's peer-reviewed final manuscript, as accepted by the publisher. The published article is copyrighted by Elsevier and can be found at: http://www.journals.elsevier.com/expert-systems-with-applications/.The problem of scheduling jobs to minimize total weighted tardiness in flowshops,\ud
with the possibility of evolving into hybrid flowshops in the future, is investigated in\ud
this paper. As this research is guided by a real problem in industry, the flowshop\ud
considered has considerable flexibility, which stimulated the development of an\ud
innovative methodology for this research. Each stage of the flowshop currently has\ud
one or several identical machines. However, the manufacturing company is planning\ud
to introduce additional machines with different capabilities in different stages in the\ud
near future. Thus, the algorithm proposed and developed for the problem is not only\ud
capable of solving the current flow line configuration but also the potential new\ud
configurations that may result in the future. A meta-heuristic search algorithm based\ud
on Tabu search is developed to solve this NP-hard, industry-guided problem. Six\ud
different initial solution finding mechanisms are proposed. A carefully planned\ud
nested split-plot design is performed to test the significance of different factors and\ud
their impact on the performance of the different algorithms. To the best of our\ud
knowledge, this research is the first of its kind that attempts to solve an industry-guided\ud
problem with the concern for future developments
A Pair of Disjoint 3-GDDs of type g^t u^1
Pairwise disjoint 3-GDDs can be used to construct some optimal
constant-weight codes. We study the existence of a pair of disjoint 3-GDDs of
type and establish that its necessary conditions are also sufficient.Comment: Designs, Codes and Cryptography (to appear
The multicovering radius problem for some types of discrete structures
The covering radius problem is a question in coding theory concerned with
finding the minimum radius such that, given a code that is a subset of an
underlying metric space, balls of radius over its code words cover the
entire metric space. Klapper introduced a code parameter, called the
multicovering radius, which is a generalization of the covering radius. In this
paper, we introduce an analogue of the multicovering radius for permutation
codes (cf. Keevash and Ku, 2006) and for codes of perfect matchings (cf. Aw and
Ku, 2012). We apply probabilistic tools to give some lower bounds on the
multicovering radii of these codes. In the process of obtaining these results,
we also correct an error in the proof of the lower bound of the covering radius
that appeared in Keevash and Ku (2006). We conclude with a discussion of the
multicovering radius problem in an even more general context, which offers room
for further research.Comment: To appear in Designs, Codes and Cryptography (2012
An Efficient Implementation of a Quasi-polynomial Algorithm for Generating Hypergraph Transversals
Given a finite set V, and a hypergraph , the hypergraph transversal problem calls for enumerating all minimal hitting sets (transversals) for . This problem plays an important role in practical applications as many other problems were shown to be polynomially equivalent to it. Fredman and Khachiyan (1996) gave an incremental quasi-polynomial time algorithm for solving the hypergraph transversal problem [9]. In this paper, we present an efficient implementation of this algorithm. While we show that our implementation achieves the same bound on the running time as in [9], practical experience with this implementation shows that it can be substantially faster. We also show that a slight modification of the algorithm in [9] can be used to give a stronger bound on the running time
Beyond interviews and focus groups: a framework for integrating innovative qualitative methods into randomised controlled trials of complex public health interventions
Background
Randomised controlled trials (RCTs) are widely used for establishing evidence of the effectiveness of interventions, yet public health interventions are often complex, posing specific challenges for RCTs. Although there is increasing recognition that qualitative methods can and should be integrated into RCTs, few frameworks and practical guidance highlight which qualitative methods should be integrated and for what purposes. As a result, qualitative methods are often poorly or haphazardly integrated into existing trials, and researchers rely heavily on interviews and focus group discussions. To improve current practice, we propose a framework for innovative qualitative research methods that can help address the challenges of RCTs for complex public health interventions.
Methods
We used a stepped approach to develop a practical framework for researchers. This consisted of (1) a systematic review of the innovative qualitative methods mentioned in the health literature, (2) in-depth interviews with 23 academics from different methodological backgrounds working on RCTs of public health interventions in 11 different countries, and (3) a framework development and group consensus-building process.
Results
The findings are presented in accordance with the CONSORT (Consolidated Standards of Reporting Trials) Statement categories for ease of use. We identify the main challenges of RCTs for public health interventions alongside each of the CONSORT categories, and potential innovative qualitative methods that overcome each challenge are listed as part of a Framework for the Integration of Innovative Qualitative Methods into RCTs of Complex Health Interventions. Innovative qualitative methods described in the interviews include rapid ethnographic appraisals, document analysis, diary methods, interactive voice responses and short message service, community mapping, spiral walks, pair interviews and visual participatory analysis.
Conclusions
The findings of this study point to the usefulness of observational and participatory methods for trials of complex public health interventions, offering a novel contribution to the broader literature about the need for mixed methods approaches. Integrating a diverse toolkit of qualitative methods can enable appropriate adjustments to the intervention or process (or both) of data collection during RCTs, which in turn can create more sustainable and effective interventions. However, such integration will require a cultural shift towards the adoption of method-neutral research approaches, transdisciplinary collaborations, and publishing regimes
Spanning forests and the q-state Potts model in the limit q \to 0
We study the q-state Potts model with nearest-neighbor coupling v=e^{\beta
J}-1 in the limit q,v \to 0 with the ratio w = v/q held fixed. Combinatorially,
this limit gives rise to the generating polynomial of spanning forests;
physically, it provides information about the Potts-model phase diagram in the
neighborhood of (q,v) = (0,0). We have studied this model on the square and
triangular lattices, using a transfer-matrix approach at both real and complex
values of w. For both lattices, we have computed the symbolic transfer matrices
for cylindrical strips of widths 2 \le L \le 10, as well as the limiting curves
of partition-function zeros in the complex w-plane. For real w, we find two
distinct phases separated by a transition point w=w_0, where w_0 = -1/4 (resp.
w_0 = -0.1753 \pm 0.0002) for the square (resp. triangular) lattice. For w >
w_0 we find a non-critical disordered phase, while for w < w_0 our results are
compatible with a massless Berker-Kadanoff phase with conformal charge c = -2
and leading thermal scaling dimension x_{T,1} = 2 (marginal operator). At w =
w_0 we find a "first-order critical point": the first derivative of the free
energy is discontinuous at w_0, while the correlation length diverges as w
\downarrow w_0 (and is infinite at w = w_0). The critical behavior at w = w_0
seems to be the same for both lattices and it differs from that of the
Berker-Kadanoff phase: our results suggest that the conformal charge is c = -1,
the leading thermal scaling dimension is x_{T,1} = 0, and the critical
exponents are \nu = 1/d = 1/2 and \alpha = 1.Comment: 131 pages (LaTeX2e). Includes tex file, three sty files, and 65
Postscript figures. Also included are Mathematica files forests_sq_2-9P.m and
forests_tri_2-9P.m. Final journal versio
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