269 research outputs found
Low Dimensional Ordering on a Lattice Model
A simple d-dimensional lattice model is proposed, incorporating some degree
of frustration and thus capable of describing some aspects of molecular
orientation in covalently bound molecular solids. For d=2 the model is shown to
be equivalent to the standard two-dimensional Ising model, while for d>2 it
describes a peculiar transition from an isotropic high temperature phase to a
low-dimensional anisotropic low temperature state. A general mean field
analysis is presented and compared to some exact limit properties.Comment: 11 pages, 2 PostScript figures, to appear in Phys. Lett.
An Efficient Rigorous Approach for Identifying Statistically Significant Frequent Itemsets
As advances in technology allow for the collection, storage, and analysis of
vast amounts of data, the task of screening and assessing the significance of
discovered patterns is becoming a major challenge in data mining applications.
In this work, we address significance in the context of frequent itemset
mining. Specifically, we develop a novel methodology to identify a meaningful
support threshold s* for a dataset, such that the number of itemsets with
support at least s* represents a substantial deviation from what would be
expected in a random dataset with the same number of transactions and the same
individual item frequencies. These itemsets can then be flagged as
statistically significant with a small false discovery rate. We present
extensive experimental results to substantiate the effectiveness of our
methodology.Comment: A preliminary version of this work was presented in ACM PODS 2009. 20
pages, 0 figure
Short telomeres in embryonic stem cells affect stable differentiation
Murine embryonic stem cells (ESCs) are self-renewing, pluripotent cells able to
differentiate into cells of all three germ layers. Pluripotency and self-renewal are
maintained primarily by the core transcriptional factors Nanog, Oct4 and Sox2, but
require the cooperation of other factors and coregulators and an efficient telomere
maintenance mechanism. In mammals, telomere maintenance is achieved via a
telomerase reverse transcriptase (Tert) that acts together with an RNA component
(Terc). Maintenance of functional telomeres is essential to allow ESC proliferation,
nevertheless if and how it is involved in the achievement and preservation of cell
differentiation is still unknown. Here, we used Tert deficient mouse ESCs to
elucidate the role of telomere length in differentiation. We found that Tert-/- ESCs
with critically short telomeres are delayed, but still capable, to achieve differentiation
after leukemia inhibitory factor (LIF) withdrawal and all-trans retinoic acid (ATRA)
treatment, but failed to maintain it after LIF re-introduction to the growth medium.
Telomere shortening effect on differentiation was accompanied by pluripotency gene
dysregulation (e.g. Nanog overexpression), DNA hypomethylation and epigenetic
disorders. This phenotype of metastable differentiation could be rescued by telomere
lengthening via re-introduction of Tert, depletion of Nanog via short hairpin RNA, or
via enforced expression of the de novo DNA methyltransferase 3b. These results
reveal an unanticipated role of telomeres in the epigenetic regulation of gene
expression and cell fate determination during physiological or pathological
processes
Scalable Distributed Approximation of Internal Measures for Clustering Evaluation
The most widely used internal measure for clustering evaluation is the
silhouette coefficient, whose naive computation requires a quadratic number of
distance calculations, which is clearly unfeasible for massive datasets.
Surprisingly, there are no known general methods to efficiently approximate the
silhouette coefficient of a clustering with rigorously provable high accuracy.
In this paper, we present the first scalable algorithm to compute such a
rigorous approximation for the evaluation of clusterings based on any metric
distances. Our algorithm hinges on a Probability Proportional to Size (PPS)
sampling scheme, and, for any fixed , it
approximates the silhouette coefficient within a mere additive error
with probability , using a very small number of
distance calculations. We also prove that the algorithm can be adapted to
obtain rigorous approximations of other internal measures of clustering
quality, such as cohesion and separation. Importantly, we provide a distributed
implementation of the algorithm using the MapReduce model, which runs in
constant rounds and requires only sublinear local space at each worker, which
makes our estimation approach applicable to big data scenarios. We perform an
extensive experimental evaluation of our silhouette approximation algorithm,
comparing its performance to a number of baseline heuristics on real and
synthetic datasets. The experiments provide evidence that, unlike other
heuristics, our estimation strategy not only provides tight theoretical
guarantees but is also able to return highly accurate estimations while running
in a fraction of the time required by the exact computation, and that its
distributed implementation is highly scalable, thus enabling the computation of
internal measures for very large datasets for which the exact computation is
prohibitive.Comment: 16 pages, 4 tables, 1 figur
A new polystyrene-based ionomer/MWCNT nanocomposite for wearable skin temperature sensors
The present work outlines the fabrication and testing of a novel skin temperature sensor based on exfoliated
and undamaged multi-walled carbon nanotubes (MWCNTs) dispersed in a poly(vinylbenzyl chloride)
derivative with triethylamine (PVBC_Et3N). The dispersions were prepared by sonicating MWCNT/
PVBC_Et3N mixtures in dimethylformamide for 5 min and the quantification of the MWCNTs dispersed
was evaluated by UVâvis spectroscopy investigations and thermogravimetric analyses.
The investigations demonstrated the realization of MWCNT/PVBC_Et3N sensors with a resistance sensitivity
to temperature close to 0.004 K1, an absolute value that is comparable to the highest values
found in metals. The temperature dependence of the resistance was also found very reproducible in
the range 20â40 C, thus suggesting the possibility of using the MWCNT/PVBC_Et3N system for the fabrication
of small wearable temperature sensors for the monitoring of chronic wounds
Vapochromic features of new luminogens based on julolidine-containing styrene copolymers
We report on vapochromic films suitable for detecting volatile organic compounds (VOCs), based on new polystyrene copolymers containing julolidine fluorescent molecular rotors (JCAEM). Poly(styrene-co-hydroxyethylmethacrylate) copolymers functionalized with cyanovinyl-julolidine moieties of different compositions were prepared, (P(STY-co-JCAEM)(m) with m = 0.06-0.61). The sensing performance of the spin-coated copolymer films demonstrated significant vapochromism when exposed to VOCs characterized by high vapour pressure and a favourable interaction with the polymer matrix, such as Et2O and CH2Cl2. It is worth mentioning that the fluorescence decrease rate was 7 times faster than that of previously investigated julolidine-based fluorescent molecular rotors dispersed in PS films. This phenomenon was attributed to the better control of the JCAEM moiety distribution in the polymer matrix conferred by the covalent approach, combined with a minimal film thickness of 4 microns. These factors, in concert, strongly accelerate the deactivation pathways of the JCAEM units in the presence of VOCs which interact well. Overall, the present results support the use of julolidine-enriched styrene copolymers as effective chromogenic materials suitable for the fast detection of VOCs
âN-alkyl diketopyrrolopyrrole-based fluorophores for luminescent solar concentrators: effect of the alkyl chain on dye efficiencyâ
We report on the preparation of luminescent solar concentrators (LSCs) made of poly(methyl methacrylate) (PMMA) thin films doped with six new diketopyrrolopyrrole (DPP) fluorophores obtained in good yields by using simple N-alkylation and direct C-H arylation synthetic strategies. Spectroscopic investigations in solution and in PMMA thin films combined with photocurrent measurements revealed that the branched alkyl chains were efficient in preventing DPP segregation from the PMMA matrix thanks to their higher steric hindrance. The aromatic substituent was found to expand DPP conjugation but favoured DPP adverse aggregation, thus affecting fluorescence emission and photocurrents of PMMA films. The worthwhile combination of the appropriate alkyl chain and aromatic moieties assured to achieve optical efficiency of 6.8% that was comparable to that of PMMA LSCs of the state-of-the art (7.2%)
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