971 research outputs found
Controlling instabilities along a 3DVar analysis cycle by assimilating in the unstable subspace: a comparison with the EnKF
A hybrid scheme obtained by combining 3DVar with the Assimilation in the
Unstable Subspace (3DVar-AUS) is tested in a QG model, under perfect model
conditions, with a fixed observational network, with and without observational
noise. The AUS scheme, originally formulated to assimilate adaptive
observations, is used here to assimilate the fixed observations that are found
in the region of local maxima of BDAS vectors (Bred vectors subject to
assimilation), while the remaining observations are assimilated by 3DVar.
The performance of the hybrid scheme is compared with that of 3DVar and of an
EnKF. The improvement gained by 3DVar-AUS and the EnKF with respect to 3DVar
alone is similar in the present model and observational configuration, while
3DVar-AUS outperforms the EnKF during the forecast stage. The 3DVar-AUS
algorithm is easy to implement and the results obtained in the idealized
conditions of this study encourage further investigation toward an
implementation in more realistic contexts
On the Approximability of Digraph Ordering
Given an n-vertex digraph D = (V, A) the Max-k-Ordering problem is to compute
a labeling maximizing the number of forward edges, i.e.
edges (u,v) such that (u) < (v). For different values of k, this
reduces to Maximum Acyclic Subgraph (k=n), and Max-Dicut (k=2). This work
studies the approximability of Max-k-Ordering and its generalizations,
motivated by their applications to job scheduling with soft precedence
constraints. We give an LP rounding based 2-approximation algorithm for
Max-k-Ordering for any k={2,..., n}, improving on the known
2k/(k-1)-approximation obtained via random assignment. The tightness of this
rounding is shown by proving that for any k={2,..., n} and constant
, Max-k-Ordering has an LP integrality gap of 2 -
for rounds of the
Sherali-Adams hierarchy.
A further generalization of Max-k-Ordering is the restricted maximum acyclic
subgraph problem or RMAS, where each vertex v has a finite set of allowable
labels . We prove an LP rounding based
approximation for it, improving on the
approximation recently given by Grandoni et al.
(Information Processing Letters, Vol. 115(2), Pages 182-185, 2015). In fact,
our approximation algorithm also works for a general version where the
objective counts the edges which go forward by at least a positive offset
specific to each edge.
The minimization formulation of digraph ordering is DAG edge deletion or
DED(k), which requires deleting the minimum number of edges from an n-vertex
directed acyclic graph (DAG) to remove all paths of length k. We show that
both, the LP relaxation and a local ratio approach for DED(k) yield
k-approximation for any .Comment: 21 pages, Conference version to appear in ESA 201
Analytical modeling of multipass welding process with distributed heat source
In the welding process, the most interesting regions for heat transfer analysis are the fusion zone (FZ) and the heat affected zone (HAZ), where high temperatures are reached. These high temperature levels cause phase transformations and alterations in the mechanical properties of the welded metal. The calculations to estimate the temperature distribution in multiple pass welding is more complex than in the single pass processes, due to superimposed thermal effects of one pass over the previous passes. In the present work, a comparison is made between thermal cycles obtained from analytical models regarding point (concentrated) and Gaussian (distributed) heat sources. The use of distributed heat source prevents infinite temperatures values near the fusion zone. The comparison shows that the thermal cycles obtained from the distributed heat source model are more reliable than those obtained from the concentrated heat source model.302305FundaĆ§Ć£o de Amparo Ć Pesquisa do Estado de SĆ£o Paulo (FAPESP
Lagarta-dos-cafezais: o caso de RondƓnia.
Nesta publicaĆ§Ć£o verificou-se a existĆŖncia de 64 propriedades com 618 hectares afetados pelo ataque da lagarta-dos-cafezais no municĆpio de Cacoal-RO e as medidas de controle adotadas para conter a expansĆ£o de E. imperialis com a reduĆ§Ć£o significativa da Ć”rea atacada.bitstream/item/72102/1/0000004037-ct68-lagartadoscafezais.pd
High rate locally-correctable and locally-testable codes with sub-polynomial query complexity
In this work, we construct the first locally-correctable codes (LCCs), and
locally-testable codes (LTCs) with constant rate, constant relative distance,
and sub-polynomial query complexity. Specifically, we show that there exist
binary LCCs and LTCs with block length , constant rate (which can even be
taken arbitrarily close to 1), constant relative distance, and query complexity
. Previously such codes were known to exist
only with query complexity (for constant ), and
there were several, quite different, constructions known.
Our codes are based on a general distance-amplification method of Alon and
Luby~\cite{AL96_codes}. We show that this method interacts well with local
correctors and testers, and obtain our main results by applying it to suitably
constructed LCCs and LTCs in the non-standard regime of \emph{sub-constant
relative distance}.
Along the way, we also construct LCCs and LTCs over large alphabets, with the
same query complexity , which additionally have
the property of approaching the Singleton bound: they have almost the
best-possible relationship between their rate and distance. This has the
surprising consequence that asking for a large alphabet error-correcting code
to further be an LCC or LTC with query
complexity does not require any sacrifice in terms of rate and distance! Such a
result was previously not known for any query complexity.
Our results on LCCs also immediately give locally-decodable codes (LDCs) with
the same parameters
New Notions and Constructions of Sparsification for Graphs and Hypergraphs
A sparsiļ¬er of a graph G (BenczuĀ“r and Karger; Spielman and Teng) is a sparse weighted subgraph Ė G that approximately retains the same cut structure of G. For general graphs, non-trivial sparsiļ¬cation is possible only by using weighted graphs in which diļ¬erent edges have diļ¬erent weights. Even for graphs that admit unweighted sparsiļ¬ers (that is, sparsiļ¬ers in which all the edge weights are equal to the same scaling factor), there are no known polynomial time algorithms that ļ¬nd such unweighted sparsiļ¬ers. We study a weaker notion of sparsiļ¬cation suggested by Oveis Gharan, in which the number of cut edges in each cut (S, ĀÆ S) is not approximated within a multiplicative factor (1 + Ē«), but is, instead, approximated up to an additive term bounded by Ē« times d Ā· |S| + vol(S), where d is the average
Genome-wide linkage analysis of age at onset of alcohol dependence: a comparison between microsatellites and single-nucleotide polymorphisms
BACKGROUND: Using the dataset provided for Genetic Analysis Workshop 14 by the Collaborative Study on the Genetics of Alcoholism, we performed genome-wide linkage analysis of age at onset of alcoholism to compare the utility of microsatellites and single-nucleotide polymorphisms (SNPs) in genetic linkage study. METHODS: A multipoint nonparametric variance component linkage analysis method was applied to the survival distribution function obtained from semiparametric proportional hazards model of the age at onset phenotype of alcoholism. Three separate linkage analyses were carried out using 315 microsatellites, 2,467 and 9,467 SNPs, spanning the 22 autosomal chromosomes. RESULTS: Heritability of age at onset was estimated to be approximately 12% (p < 0.001). We observed weak correlation, both in trend and strength, of genome-wide linkage signals between microsatellites and SNPs. Results from SNPs revealed more and stronger linkage signals across the genome compared with those from microsatellites. The only suggestive evidence of linkage from microsatellites was on chromosome 1 (LOD of 1.43). Differences in map densities between the two sets of SNPs used in this study did not appear to confer an advantage in terms of strength of linkage signals. CONCLUSION: Our study provided support for better performance of dense SNP maps compared with the sparse mirosatellite maps currently available for linkage analysis of quantitative traits. This better performance could be attributable to precise definition and high map resolutions achievable with dense SNP maps, thus resulting in increased power to detect possible loci affecting given trait or disease
Buffering Negative Impacts of Divorce on Children: Evaluating Impact of Divorce Education
Following the call for more stringent evaluation methodology and recently documented national Extension presence in the field of divorce education for parents and children, the study reported here describes a local multi-level evaluation to capture program impact of a stakeholder-accepted divorce education program. Using a post-then-pre retrospective scale and validated measures of inter-parental conflict and child adjustment, participants showed significant gains in key co-parenting and community resources knowledge and significant decreases in inter-parental conflict. Selected components of the evaluation design are applicable to Extension faculty working in divorce education programs
- ā¦