University of Marne-la-Vallée

About: University of Marne-la-Vallée is a based out in . It is known for research contribution in the topics: Estimator & Context (language use). The organization has 831 authors who have published 1855 publications receiving 55316 citations.

Asymptotic lower bounds in estimating jumps

Abstract: We study the problem of the efficient estimation of the jumps for stochastic processes. We assume that the stochastic jump process (Xt )t∈[0,1] is observed discretely, with a sampling step of size 1/n. In the spirit of Hajek’s convolution theorem, we show some lower bounds for the estimation error of the sequence of the jumps (�X Tk )k. As an intermediate result, we prove a LAMN property, with rate √ n, when the marks of the underlying jump component are deterministic. We deduce then a convolution theorem, with an explicit asymptotic minimal variance, in the case where the marks of the jump component are random. To prove that this lower bound is optimal, we show that a threshold estimator of the sequence of jumps (�X Tk )k based on the discrete observations, reaches the minimal variance of the previous convolution theorem.

Vibrations in the B4 rhombic structure.

Abstract: A double minimum six-dimensional potential energy surface (PES) is determined in symmetry coordinates for the most stable rhombic (D2h) B4 isomer in its 1Ag electronic ground state by fitting to energies calculated ab initio. The PES exhibits a barrier to the D4h square structure of 255 cm(-1). The vibrational levels (J=0) are calculated variationally using an approach which involves the Watson kinetic energy operator expressed in normal coordinates. The pattern of about 65 vibrational levels up to 1600 cm(-1) for all stable isotopomers is analyzed. Analogous to the inversion in ammonia-like molecules, the rhombus rearrangements lead to splittings of the vibrational levels. In B4 it is the B1g (D4h) mode which distorts the square molecule to its planar rhombic form. The anharmonic fundamental vibrational transitions of 11B4 are calculated to be (splittings in parentheses): G(0)=2352(22) cm(-1), nu1(A1g)=1136(24) cm(-1), nu2(B1g)=209(144) cm(-1), nu3(B2g)=1198(19) cm(-1), nu4(B2u)=271(24) cm(-1), and nu5(Eu)=1030(166) cm(-1) (D4h notation). Their variations in all stable isotopomers were investigated. Due to the presence of strong anharmonic resonances between the B1g in-plane distortion and the B2u out-of-plane bending modes, the higher overtones and combination levels are difficult to assign unequivocally.

Archeological applications of XAFS: prehistorical paintings and medieval glasses

Abstract: High-resolution manganese and iron K-edges XANES spectra were collected on several samples of archeological interest: prehistorical paintings and medieval glasses. XANES spectra were collected at the ID21 facility (ESRF, Grenoble, France) using a micro-beam device and at the 11-2 beamline (SSRL, Stanford, USA) using a submillimetric beam. The medieval glasses studied are from gothic glass windows from Normandy (XIVth century). The aim of this study is to help understand the chemical durability of these materials, exposed to weathering since the XIVth century. They are used as analogues of weathered glasses used to dump metallic wastes. These glasses show surficial enrichment in manganese, due to its oxidation from II (glass) to III/IV (surface), which precipitates as amorphous oxy-hydroxides. Similarly, iron is oxidized on the surface and forms ferrihydrite-type aggregates. The prehistorical paintings are from Lascaux and Ekain (Basque country). We choose in that study the black ones, rich in manganese to search for potential evidences of some 'savoir-faire' that the Paleolithic men could have used to realize their paint in rock art, as shown earlier for Fe-bearing pigments. A large number of highly valuable samples, micrometric scaled, were extracted from these frescoes and show large variation in the mineralogical nature ofmore » the black pigments used, from an amorphous psilomelane-type to a well-crystallized pyrolusite. Correlation with the crystals morphology helps understanding the know-how of these early artists.« less

Optimal replacement policy for a series system with obsolescence

Abstract: Most maintenance policies assume that failed or used components are replaced with identical units. Actually, such a hypothesis neglects the possible obsolescence of the components. When a new, more reliable and less consuming technology becomes available, a decision has to be made as for the replacement strategy to be used: old-type components can all be immediately replaced, or new-type units can be introduced progressively, each time a corrective action is undertaken. Partly corrective, partly preventive policies can also be envisioned. This work tackles this issue in the case of a series system made of n identical and independent components with a constant failure rate. It provides, under given modelling assumptions, the fully analytical expression of the mean total cost induced by each possible strategy, as well as the optimal replacement policy, as a function of the problem parameters. This is performed by accounting for different costs for preventive or corrective replacements, with some economical dependence between replacements, different energy consumption rates for old-type and new-type components as well as a discount rate. Copyright © 2004 John Wiley & Sons, Ltd.