4,028 research outputs found
Efficient computation of hashes
The sequential computation of hashes at the core of many distributed storage systems and found, for example, in grid services can hinder efficiency in service quality and even pose security challenges that can only be addressed by the use of parallel hash tree modes. The main contributions of this paper are, first, the identification of several efficiency and security challenges posed by the use of sequential hash computation based on the Merkle-Damgard engine. In addition, alternatives for the parallel computation of hash trees are discussed, and a prototype for a new parallel implementation of the Keccak function, the SHA-3 winner, is introduced
Mapping customer needs to engineering characteristics: an aerospace perspective for conceptual design
Designing complex engineering systems, such as an aircraft or an aero-engine, is immensely challenging. Formal Systems Engineering (SE) practices are widely used in the aerospace industry throughout the overall design process to minimise the overall design effort, corrective re-work, and ultimately overall development and manufacturing costs. Incorporating the needs and requirements from customers and other stakeholders into the conceptual and early design process is vital for the success and viability of any development programme. This paper presents a formal methodology, the Value-Driven Design (VDD) methodology that has been developed for collaborative and iterative use in the Extended Enterprise (EE) within the aerospace industry, and that has been applied using the Concept Design Analysis (CODA) method to map captured Customer Needs (CNs) into Engineering Characteristics (ECs) and to model an overall ‘design merit’ metric to be used in design assessments, sensitivity analyses, and engineering design optimisation studies. Two different case studies with increasing complexity are presented to elucidate the application areas of the CODA method in the context of the VDD methodology for the EE within the aerospace secto
A laser diode based system for calibration of fast time-of-flight detectors
A system based on commercially available items, such as a laser diode,
emitting in the visible range nm,and multimode fiber patches, fused
fiber splitters and optical switches may be assembled,for time calibration of
multi-channels time-of-flight (TOF) detectors with photomultipliers' (PMTs')
readout. As available laser diode sources have unfortunately limited peak
power, the main experimental problem is the tight light power budget of such a
system. In addition, while the technology for fused fiber splitters is common
in the Telecom wavelength range ( nm), it is not
easily available in the visible one. Therefore, extensive laboratory tests had
to be done on purpose, to qualify the used optical components, and a full scale
timing calibration prototype was built. Obtained results show that with such a
system, a calibration resolution () in the range 20-30 ps may be within
reach. Therefore, fast multi-channels TOF detectors, with timing resolutions in
the range 50-100 ps, may be easily calibrated in time. Results on tested
optical components may be of interest also for time calibration of different
light detection systems based on PMTs, as the ones used for detection of the
vacuum ultraviolet scintillation light emitted by ionizing particles in large
LAr TPCs.Comment: submitted to JINS
Effect of the Coulomb interaction on the electron relaxation of weakly-confined quantum dot systems
We study acoustic-phonon-induced relaxation of charge excitations in single
and tunnel-coupled quantum dots containing few confined interacting electrons.
The Full Configuration Interaction approach is used to account for the
electron-electron repulsion. Electron-phonon interaction is accounted for
through both deformation potential and piezoelectric field mechanisms. We show
that electronic correlations generally reduce intradot and interdot transition
rates with respect to corresponding single-electron transitions, but this
effect is lessened by external magnetic fields. On the other hand,
piezoelectric field scattering is found to become the dominant relaxation
mechanism as the number of confined electrons increases. Previous proposals to
strongly suppress electron-phonon coupling in properly designed single-electron
quantum dots are shown to hold also in multi-electron devices. Our results
indicate that few-electron orbital degrees of freedom are more stable than
single-electron ones.Comment: 20 pages (preprint format), 7 figures, submitted to Phys. Rev.
Behaviour in Magnetic Fields of Fast Conventional and Fine-Mesh Photomultipliers
The performance of both conventional and fine-mesh Hamamatsu photomultipliers
has been measured inside moderate magnetic fields. This has allowed the test of
effective shielding solutions for photomultipliers, to be used in
time-of-flight detectors based on scintillation counters. Both signal amplitude
reduction or deterioration of the timing properties inside magnetic fields have
been investigated
Phonon-induced electron relaxation in weakly-confined single and coupled quantum dots
We investigate charge relaxation rates due to acoustic phonons in
weakly-confined quantum dot systems, including both deformation potential and
piezoelectric field interactions. Single-electron excited states lifetimes are
calculated for single and coupled quantum dot structures, both in homonuclear
and heteronuclear devices. Piezoelectric field scattering is shown to be the
dominant relaxation mechanism in many experimentally relevant situations. On
the other hand, we show that appropriate structure design allows to minimize
separately deformation potential and piezolectric field interactions, and may
bring electron lifetimes in the range of microseconds.Comment: 20 pages (preprint format), 7 figures, submitted to Physical Review
How alternative food networks work in a metropolitan area? An analysis of Solidarity Purchase Groups in Northern Italy
Our paper focuses on Solidarity Purchase Group (SPG) participants located in a highly urbanized area, with the aim to investigate the main motivations underlining their participation in a SPG and provide a characterization of them. To this end, we carried out a survey of 795 participants involved in 125 SPGs in the metropolitan area of Milan (Italy). Taking advantage of a questionnaire with 39 questions, we run a factor analysis and a two-step cluster analysis to identify different profiles of SPG participants. Our results show that the system of values animating metropolitan SPG practitioners does not fully conform to that traditionally attributed to an alternative food network (AFN). In fact, considerations linked to food safety and healthiness prevail on altruistic motives such as environmental sustainability and solidarity toward small producers. Furthermore, metropolitan SPGs do not consider particularly desirable periurban and local food products. Observing the SPGs from this perspective, it emerges as such initiatives can flourish also in those places where the lack of connection with the surrounding territory is counterbalanced by the high motivation to buy products from trusted suppliers who are able to guarantee genuine and safe products, not necessarily located nearby
Topological properties of the bond-modulated honeycomb lattice
We study the combined effects of lattice deformation, e-e interaction and
spin-orbit coupling in a two-dimensional (2D) honeycomb lattice. We adopt
different kinds of hopping modulation--generalized dimerization and a Kekule
distortion--and calculate topological invariants for the non-interacting system
and for the interacting system. We identify the parameter range (Hubbard U,
hopping modulation, spin-orbit coupling) where the 2D system behaves as a
trivial insulator or Quantum Spin Hall Insulator.Comment: 8 pages, 4 figures: discussion improved, typos corrected, references
updated. Matches version published in PR
Bayesian DNA copy number analysis
BACKGROUND: Some diseases, like tumors, can be related to chromosomal aberrations, leading to
changes of DNA copy number. The copy number of an aberrant genome can be represented as a
piecewise constant function, since it can exhibit regions of deletions or gains. Instead, in a healthy
cell the copy number is two because we inherit one copy of each chromosome from each our
parents.
Bayesian Piecewise Constant Regression (BPCR) is a Bayesian regression method for data that are
noisy observations of a piecewise constant function. The method estimates the unknown segment
number, the endpoints of the segments and the value of the segment levels of the underlying
piecewise constant function. The Bayesian Regression Curve (BRC) estimates the same data with
a smoothing curve. However, in the original formulation, some estimators failed to properly
determine the corresponding parameters. For example, the boundary estimator did not take into
account the dependency among the boundaries and succeeded in estimating more than one
breakpoint at the same position, losing segments.
RESULTS: We derived an improved version of the BPCR (called mBPCR) and BRC, changing the
segment number estimator and the boundary estimator to enhance the fitting procedure. We also
proposed an alternative estimator of the variance of the segment levels, which is useful in case of
data with high noise. Using artificial data, we compared the original and the modified version of
BPCR and BRC with other regression methods, showing that our improved version of BPCR
generally outperformed all the others. Similar results were also observed on real data.
CONCLUSION: We propose an improved method for DNA copy number estimation, mBPCR, which
performed very well compared to previously published algorithms. In particular, mBPCR was more
powerful in the detection of the true position of the breakpoints and of small aberrations in very
noisy data. Hence, from a biological point of view, our method can be very useful, for example, to
find targets of genomic aberrations in clinical cancer samples
An integrated Bayesian analysis of LOH and copy number data
BACKGROUND Cancer and other disorders are due to genomic lesions. SNP-microarrays are able to measure simultaneously both genotype and copy number (CN) at several Single Nucleotide Polymorphisms (SNPs) along the genome. CN is defined as the number of DNA copies, and the normal is two, since we have two copies of each chromosome. The genotype of a SNP is the status given by the nucleotides (alleles) which are present on the two copies of DNA. It is defined homozygous or heterozygous if the two alleles are the same or if they differ, respectively. Loss of heterozygosity (LOH) is the loss of the heterozygous status due to genomic events. Combining CN and LOH data, it is possible to better identify different types of genomic aberrations. For example, a long sequence of homozygous SNPs might be caused by either the physical loss of one copy or a uniparental disomy event (UPD), i.e. each SNP has two identical nucleotides both derived from only one parent. In this situation, the knowledge of the CN can help in distinguishing between these two events. RESULTS To better identify genomic aberrations, we propose a method (called gBPCR) which infers the type of aberration occurred, taking into account all the possible influence in the microarray detection of the homozygosity status of the SNPs, resulting from an altered CN level. Namely, we model the distributions of the detected genotype, given a specific genomic alteration and we estimate the parameters involved on public reference datasets. The estimation is performed similarly to the modified Bayesian Piecewise Constant Regression, but with improved estimators for the detection of the breakpoints.Using artificial and real data, we evaluate the quality of the estimation of gBPCR and we also show that it outperforms other well-known methods for LOH estimation. CONCLUSIONS We propose a method (gBPCR) for the estimation of both LOH and CN aberrations, improving their estimation by integrating both types of data and accounting for their relationships. Moreover, gBPCR performed very well in comparison with other methods for LOH estimation and the estimated CN lesions on real data have been validated with another technique.This work was supported by Swiss National Science Foundation (grants
205321-112430, 205320-121886/1); Oncosuisse grants OCS-1939-8-2006 and
OCS - 02296-08-2008; Cantone Ticino ("Computational life science/Ticino in
rete” program); Fondazione per la Ricerca e la Cura sui Linfomi (Lugano,
Switzerland)
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