1,717 research outputs found
Approximation for minimum triangulation of convex polyhedra
The minimum triangulation of a convex polyhedron is a triangulation that contains the minimum number of tetrahedra over all its possible triangulations. Since finding the minimum triangulation of convex polyhedra was recently shown to be NP-hard, it becomes significant to find algorithms that give good approximation. In this paper, we give a new triangulation algorithm with an improved approximation ratio 2 - &OHgr;(l/√). We also show that this is best possible for algorithms that only consider the combinatorial structure of the polyhedra. Copyright © 2009 ACM, Inc.published_or_final_versio
Protecting privacy of users in brain-computer interface applications
Machine learning (ML) is revolutionizing research and industry. Many ML applications rely on the use of large amounts of personal data for training and inference. Among the most intimate exploited data sources is electroencephalogram (EEG) data, a kind of data that is so rich with information that application developers can easily gain knowledge beyond the professed scope from unprotected EEG signals, including passwords, ATM PINs, and other intimate data. The challenge we address is how to engage in meaningful ML with EEG data while protecting the privacy of users. Hence, we propose cryptographic protocols based on secure multiparty computation (SMC) to perform linear regression over EEG signals from many users in a fully privacy-preserving(PP) fashion, i.e., such that each individual's EEG signals are not revealed to anyone else. To illustrate the potential of our secure framework, we show how it allows estimating the drowsiness of drivers from their EEG signals as would be possible in the unencrypted case, and at a very reasonable computational cost. Our solution is the first application of commodity-based SMC to EEG data, as well as the largest documented experiment of secret sharing-based SMC in general, namely, with 15 players involved in all the computations
Determination of the Fermion Pair Size in a Resonantly Interacting Superfluid
Fermionic superfluidity requires the formation of pairs. The actual size of
these fermion pairs varies by orders of magnitude from the femtometer scale in
neutron stars and nuclei to the micrometer range in conventional
superconductors. Many properties of the superfluid depend on the pair size
relative to the interparticle spacing. This is expressed in BCS-BEC crossover
theories, describing the crossover from a Bardeen-Cooper-Schrieffer (BCS) type
superfluid of loosely bound and large Cooper pairs to Bose-Einstein
condensation (BEC) of tightly bound molecules. Such a crossover superfluid has
been realized in ultracold atomic gases where high temperature superfluidity
has been observed. The microscopic properties of the fermion pairs can be
probed with radio-frequency (rf) spectroscopy. Previous work was difficult to
interpret due to strong and not well understood final state interactions. Here
we realize a new superfluid spin mixture where such interactions have
negligible influence and present fermion-pair dissociation spectra that reveal
the underlying pairing correlations. This allows us to determine the
spectroscopic pair size in the resonantly interacting gas to be 2.6(2)/kF (kF
is the Fermi wave number). The pairs are therefore smaller than the
interparticle spacing and the smallest pairs observed in fermionic superfluids.
This finding highlights the importance of small fermion pairs for superfluidity
at high critical temperatures. We have also identified transitions from fermion
pairs into bound molecular states and into many-body bound states in the case
of strong final state interactions.Comment: 8 pages, 7 figures; Figures updated; New Figures added; Updated
discussion of fit function
Subcortical nuclei volumes are associated with cognition in children post-convulsive status epilepticus: Results at nine years follow-up
PURPOSE: The purpose of the present study was to investigate the relationship between subcortical nuclei volume and cognition in children with post-convulsive status epilepticus (CSE). METHODS: Structural T1-weighted magnetic resonance imaging (MRI) scans (Siemens Avanto, 1.5 T) and neuropsychological assessments (full-scale intelligence quotient (FSIQ) and Global Memory Scores (GMS)) were collected from subjects at a mean 8.5 years post-CSE (prolonged febrile seizures (PFS), n = 30; symptomatic/known, n = 28; and other, n = 12) and from age- and sex-matched healthy controls (HC). Subjects with CSE were stratified into those with lower cognitive ability (LCA) (CSE+, n = 22) and those without (CSE-, n = 48). Quantitative volumetric analysis using Functional MRI of the Brain Software Library (FSL) (Analysis Group, FMRIB, Oxford) provided segmented MRI brain volumes. Univariate analysis of covariance (ANCOVA) was performed to compare subcortical nuclei volumes across subgroups. Multivariable linear regression was performed for each subcortical structure and for total subcortical volume (SCV) to identify significant predictors of LCA (FSIQ <85) while adjusting for etiology, age, socioeconomic status, sex, CSE duration, and intracranial volume (ICV); Bonferroni correction was applied for the analysis of individual subcortical nuclei. RESULTS: Seventy subjects (11.8 ± 3.4 standard deviation (SD) years; 34 males) and 72 controls (12.1 ± 3.0SD years; 29 males) underwent analysis. Significantly smaller volumes of the left thalamus, left caudate, right caudate, and SCV were found in subjects with CSE+ compared with HC, after adjustment for intracranial, gray matter (GM), or cortical/cerebellar volume. When compared with subjects with CSE-, subjects with CSE+ also had smaller volumes of the left thalamus, left pallidum, right pallidum, and SCV. Individual subcortical nuclei were not associated, but SCV was associated with FSIQ (p = 0.005) and GMS (p = 0.014). Intracranial volume and etiology were similarly predictive. CONCLUSIONS: Nine years post-CSE, SCV is significantly lower in children who have LCA compared with those that do not. However, in this cohort, we are unable to determine whether the relationship is independent of ICV or etiology. Future, larger scale studies may help tease this out
Evidence for Superfluidity of Ultracold Fermions in an Optical Lattice
The study of superfluid fermion pairs in a periodic potential has important
ramifications for understanding superconductivity in crystalline materials.
Using cold atomic gases, various condensed matter models can be studied in a
highly controllable environment. Weakly repulsive fermions in an optical
lattice could undergo d-wave pairing at low temperatures, a possible mechanism
for high temperature superconductivity in the cuprates. The lattice potential
could also strongly increase the critical temperature for s-wave superfluidity.
Recent experimental advances in the bulk include the observation of fermion
pair condensates and high-temperature superfluidity. Experiments with fermions
and bosonic bound pairs in optical lattices have been reported, but have not
yet addressed superfluid behavior. Here we show that when a condensate of
fermionic atom pairs was released from an optical lattice, distinct
interference peaks appear, implying long range order, a property of a
superfluid. Conceptually, this implies that strong s-wave pairing and
superfluidity have now been established in a lattice potential, where the
transport of atoms occurs by quantum mechanical tunneling and not by simple
propagation. These observations were made for unitarity limited interactions on
both sides of a Feshbach resonance. For larger lattice depths, the coherence
was lost in a reversible manner, possibly due to a superfluid to insulator
transition. Such strongly interacting fermions in an optical lattice can be
used to study a new class of Hamiltonians with interband and atom-molecule
couplings.Comment: accepted for publication in Natur
Hippocampal volume loss following childhood convulsive status epilepticus is not limited to prolonged febrile seizures.
Childhood convulsive status epilepticus (CSE), in particular prolonged febrile seizures (PFS), has been linked with mesial temporal sclerosis (MTS). Previous studies have shown that hippocampal injury occurs in the acute phase immediately following CSE, but little is known about the longer term evolution of such injury. This study aimed to investigate the longer term outcome of childhood CSE with sequential magnetic resonance imaging (MRI) looking for progressive hippocampal injury during the first year post-CSE
Observation of pseudogap behavior in a strongly interacting Fermi gas
Ultracold atomic Fermi gases present an opportunity to study strongly
interacting Fermi systems in a controlled and uncomplicated setting. The
ability to tune attractive interactions has led to the discovery of
superfluidity in these systems with an extremely high transition temperature,
near T/T_F = 0.2. This superfluidity is the electrically neutral analog of
superconductivity; however, superfluidity in atomic Fermi gases occurs in the
limit of strong interactions and defies a conventional BCS description. For
these strong interactions, it is predicted that the onset of pairing and
superfluidity can occur at different temperatures. This gives rise to a
pseudogap region where, for a range of temperatures, the system retains some of
the characteristics of the superfluid phase, such as a BCS-like dispersion and
a partially gapped density of states, but does not exhibit superfluidity. By
making two independent measurements: the direct observation of pair
condensation in momentum space and a measurement of the single-particle
spectral function using an analog to photoemission spectroscopy, we directly
probe the pseudogap phase. Our measurements reveal a BCS-like dispersion with
back-bending near the Fermi wave vector k_F that persists well above the
transition temperature for pair condensation
Patient Access to U.S. Physicians Who Conduct Internet or E-mail Consults
BACKGROUND: E-mail communication has the potential to improve communication between patients and doctors. OBJECTIVE: The objective of the study is to describe the access of patients to physicians who conduct e-mail consults. METHODS: We analyzed data from the National Ambulatory Medical Care Survey (NAMCS), a nationally representative cross-sectional survey of office-based physician visits, in 2001, 2002, and 2003. The main outcome measure was the percentage of visits to a provider who reported doing internet or e-mail consults. RESULTS: There was fewer than 1 in 10 outpatient visits in 2001 (9.2%) to physicians who reported doing internet or e-mail consults, and this did not increase in 2002 (5.8%) or 2003 (5.5%). Access to these physicians was greater among patients who were male, nonminority, lived in the Western United States, seen for pre-/postoperative care, seen by a primary care provider, and not seen by a nurse during their visit. Access to physicians who conducted internet or e-mail consults was independent of other patient (e.g., chronic conditions), provider (e.g., office setting), and visit (e.g., medications prescribed) characteristics. CONCLUSIONS: Access to physicians who do internet or e-mail consults is generally low and did not increase between 2001 and 2003, despite growth in internet access and in other internet-related activities
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