912 research outputs found
Blurring-Sharpening Process Models for Collaborative Filtering
Collaborative filtering is one of the most fundamental topics for recommender
systems. Various methods have been proposed for collaborative filtering,
ranging from matrix factorization to graph convolutional methods. Being
inspired by recent successes of graph filtering-based methods and score-based
generative models (SGMs), we present a novel concept of blurring-sharpening
process model (BSPM). SGMs and BSPMs share the same processing philosophy that
new information can be discovered (e.g., new images are generated in the case
of SGMs) while original information is first perturbed and then recovered to
its original form. However, SGMs and our BSPMs deal with different types of
information, and their optimal perturbation and recovery processes have
fundamental discrepancies. Therefore, our BSPMs have different forms from SGMs.
In addition, our concept not only theoretically subsumes many existing
collaborative filtering models but also outperforms them in terms of Recall and
NDCG in the three benchmark datasets, Gowalla, Yelp2018, and Amazon-book. In
addition, the processing time of our method is comparable to other fast
baselines. Our proposed concept has much potential in the future to be enhanced
by designing better blurring (i.e., perturbation) and sharpening (i.e.,
recovery) processes than what we use in this paper.Comment: Accepted by SIGIR 202
Quantum-Gravity Analysis of Gamma-Ray Bursts using Wavelets
In some models of quantum gravity, space-time is thought to have a foamy
structure with non-trivial optical properties. We probe the possibility that
photons propagating in vacuum may exhibit a non-trivial refractive index, by
analyzing the times of flight of radiation from gamma-ray bursters (GRBs) with
known redshifts. We use a wavelet shrinkage procedure for noise removal and a
wavelet `zoom' technique to define with high accuracy the timings of sharp
transitions in GRB light curves, thereby optimizing the sensitivity of
experimental probes of any energy dependence of the velocity of light. We apply
these wavelet techniques to 64 ms and TTE data from BATSE, and also to OSSE
data. A search for time lags between sharp transients in GRB light curves in
different energy bands yields the lower limit GeV on
the quantum-gravity scale in any model with a linear dependence of the velocity
of light . We also present a limit on any quadratic dependence.Comment: This version is accepted for publication in Astronomy & Astrophysics.
The discussion and introduction are extended making clear why the wavelet
analysis should be superior to straight cross-correlation analysis. More
details on compiled data are elaborated. 18 pages, 9 figures, A&A forma
Front Propagation in Random Media
This PhD thesis deals with the problem of the propagation of fronts under random circumstances. A
statistical model to represent the motion of fronts when are evolving in a media characterized by
microscopical randomness is discussed and expanded, in order to cope with three distinct
applications: wild-land fire simulation, turbulent premixed combustion, biofilm modeling. In the
studied formalism, the position of the average front is computed by making use of a sharp-front
evolution method, such as the level set method. The microscopical spread of particles which takes
place around the average front is given by the probability density function linked to the underlying
diffusive process, that is supposedly known in advance. The adopted statistical front propagation
framework allowed a deeper understanding of any studied field of application. The application of
this model introduced eventually parameters whose impact on the physical observables of the front
spread have been studied with Uncertainty Quantification and Sensitivity Analysis tools. In
particular, metamodels for the front propagation system have been constructed in a non intrusive
way, by making use of generalized Polynomial Chaos expansions and Gaussian Processes.The Thesis received funding from Basque Government through the BERC 2014-2017 program.
It was also funded by the Spanish Ministry of Economy and Competitiveness MINECO via the BCAM Severo Ochoa SEV-2013-0323 accreditation.
The PhD is fundend by La Caixa Foundation through the PhD grant “La Caixa 2014”.
Funding from “Programma Operativo Nazionale Ricerca e Innovazione” (PONRI 2014-2020) , “Innotavive PhDs with Industrial Characterization” is kindly acknowledged for a research visit at the department of Mathematics and Applications “Renato Caccioppoli” of University “Federico II” of Naples
Multiscale analysis of high frequency exchange rate time series
Imperial Users onl
Front propagation in random media.
244 p.This PhD thesis deals with the problem of the propagation of fronts under random circumstances. Astatistical model to represent the motion of fronts when are evolving in a media characterized bymicroscopical randomness is discussed and expanded, in order to cope with three distinctapplications: wild-land fire simulation, turbulent premixed combustion, biofilm modeling. In thestudied formalism, the position of the average front is computed by making use of a sharp-frontevolution method, such as the level set method. The microscopical spread of particles which takesplace around the average front is given by the probability density function linked to the underlyingdiffusive process, that is supposedly known in advance. The adopted statistical front propagationframework allowed a deeper understanding of any studied field of application. The application ofthis model introduced eventually parameters whose impact on the physical observables of the frontspread have been studied with Uncertainty Quantification and Sensitivity Analysis tools. Inparticular, metamodels for the front propagation system have been constructed in a non intrusiveway, by making use of generalized Polynomial Chaos expansions and Gaussian Processes.bcam:basque center for applied mathematic
Development and validation of hybrid grid-based and grid-free computational VĎ€LES method
A novel hybrid grid-based and grid-free computational method which is called VĎ€LES is proposed and validated over several benchmark cases. VĎ€LES splits the flow structures into large scale ones, resolved on the grid (Eulerian approach), and small scale ones, represented by vortex particles (Lagrangian approach). Two transport equations for grid and particle solution are derived which are dynamically coupled through the existence of coupling terms in each of them. The method resembles LES with an effort to directly reproduce the subgrid motion at least in the statistical sense
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