1,010 research outputs found
A note on approximate accelerated forward-backward methods with absolute and relative errors, and possibly strongly convex objectives
In this short note, we provide a simple version of an accelerated
forward-backward method (a.k.a. Nesterov's accelerated proximal gradient
method) possibly relying on approximate proximal operators and allowing to
exploit strong convexity of the objective function. The method supports both
relative and absolute errors, and its behavior is illustrated on a set of
standard numerical experiments. Using the same developments, we further provide
a version of the accelerated proximal hybrid extragradient method of Monteiro
and Svaiter (2013) possibly exploiting strong convexity of the objective
function.Comment: Minor modifications in notations and acknowledgments. These methods
were originally presented in arXiv:2006.06041v2. Code available at
https://github.com/mathbarre/StronglyConvexForwardBackwar
Principled Analyses and Design of First-Order Methods with Inexact Proximal Operators
Proximal operations are among the most common primitives appearing in both
practical and theoretical (or high-level) optimization methods. This basic
operation typically consists in solving an intermediary (hopefully simpler)
optimization problem. In this work, we survey notions of inaccuracies that can
be used when solving those intermediary optimization problems. Then, we show
that worst-case guarantees for algorithms relying on such inexact proximal
operations can be systematically obtained through a generic procedure based on
semidefinite programming. This methodology is primarily based on the approach
introduced by Drori and Teboulle (Mathematical Programming, 2014) and on convex
interpolation results, and allows producing non-improvable worst-case analyzes.
In other words, for a given algorithm, the methodology generates both
worst-case certificates (i.e., proofs) and problem instances on which those
bounds are achieved.
Relying on this methodology, we provide three new methods with conceptually
simple proofs: (i) an optimized relatively inexact proximal point method, (ii)
an extension of the hybrid proximal extragradient method of Monteiro and
Svaiter (SIAM Journal on Optimization, 2013), and (iii) an inexact accelerated
forward-backward splitting supporting backtracking line-search, and both (ii)
and (iii) supporting possibly strongly convex objectives. Finally, we use the
methodology for studying a recent inexact variant of the Douglas-Rachford
splitting due to Eckstein and Yao (Mathematical Programming, 2018).
We showcase and compare the different variants of the accelerated inexact
forward-backward method on a factorization and a total variation problem.Comment: Minor modifications including acknowledgments and references. Code
available at https://github.com/mathbarre/InexactProximalOperator
Telematics Combined Actuarial Neural Networks for Cross-Sectional and Longitudinal Claim Count Data
We present novel cross-sectional and longitudinal claim count models for
vehicle insurance built upon the Combined Actuarial Neural Network (CANN)
framework proposed by Mario W\"uthrich and Michael Merz. The CANN approach
combines a classical actuarial model, such as a generalized linear model, with
a neural network. This blending of models results in a two-component model
comprising a classical regression model and a neural network part. The CANN
model leverages the strengths of both components, providing a solid foundation
and interpretability from the classical model while harnessing the flexibility
and capacity to capture intricate relationships and interactions offered by the
neural network. In our proposed models, we use well-known log-linear claim
count regression models for the classical regression part and a multilayer
perceptron (MLP) for the neural network part. The MLP part is used to process
telematics car driving data given as a vector characterizing the driving
behavior of each insured driver. In addition to the Poisson and negative
binomial distributions for cross-sectional data, we propose a procedure for
training our CANN model with a multivariate negative binomial (MVNB)
specification. By doing so, we introduce a longitudinal model that accounts for
the dependence between contracts from the same insured. Our results reveal that
the CANN models exhibit superior performance compared to log-linear models that
rely on manually engineered telematics features.Comment: 30 pages, 10 tables, 6 figure
Topographically induced internal solitary waves in a pycnocline: Ultrasonic probes and stereo-correlation measurements
Internal solitary waves (ISWs) are large amplitude stable waves propagating in regions of high density gradients such as the ocean pycnocline. Their dynamics has often been investigated in two-dimensional approaches, however, their three-dimensional evolution is still poorly known. Experiments have been conducted in the large stratified water tank of CNRM-GAME to study the generation of ISWs in two academic configurations inspired by oceanic regimes. First, ultrasonic probes are used to measure the interfacial displacement in the two configurations. In the primary generation case for which the two layers are of constant density, the generation of ISWs is investigated in two series of experiments with varying amplitude and forcing frequency. In the secondary generation case for which the lower layer is stratified, the generation of ISWs from the impact of an internal wave beam on the pycnocline and their subsequent dynamics is studied. The dynamics of ISWs in these two regimes accords well with analytical approaches and numerical simulations performed in analogous configurations. Then, recent developments of a stereo correlation technique are used to describe the three-dimensional structure of propagating ISWs. In the primary generation configuration, small transverse effects are observed in the course of the ISW propagation. In the secondary generation configuration, larger transverse structures are observed in the interfacial waves dynamics. The interaction between interfacial troughs and internal waves propagating in the lower stratified layer are a possible cause for the generation of these structures. The magnitude of these transverse structures is quantified with a nondimensional parameter in the two configurations. They are twice as large in the secondary generation case as in the primary generation case
Particle Effects on the ISGRI Instrument On-Board the INTEGRAL Satellite
The INTEGRAL satellite was launched on October 17, 2002. All on-board
instruments are operating successfully. In this paper, we focus on radiation
effects on the Cadmium Telluride camera ISGRI. The spectral response of the
camera is affected by cosmic particles depositing huge amount of energy,
greater than the high threshold of the electronics. Our study raises the
contribution of cosmic ray protons. Solutions are proposed to limit the
degradation of spectral response of large pixel gamma cameras operating in
space
Petrography and Geochemistry of the Intrusive Rocks at the Diorite-Hosted Regnault Au Mineralization
Archean greenstone belts are renowned for their Au endowment. Gold can be associated with orogenic Au style of mineralization and with diorite-hosted Au-Cu disseminated sulfides interpreted as a porphyry style of mineralization. The Regnault Au project, located in the Frotet–Evans greenstone belt (Superior craton), is a structurally-controlled and diorite-hosted mineralization with an unclear metallogenic model. The aim of this study is to evaluate the fertility of the Regnault granodiorite-diorite-gabbro intrusive. Using whole-rock chemistry and petrological observations, it is concluded that the intrusive suite derives from a differentiated and water-bearing magma extracted from the metasomatized mantle. Amphibole chemistry indicates that the magma was moderately oxidized and that it emplaced at a shallow depth. It is concluded that the Regnault intrusive suite displays several characteristics favorable to the exsolution of magmatic fluids in the upper crust and that the Regnault Au mineralization potentially corresponds to, at least in part, a magmatic-hydrothermal system
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