Robust multivariate association and dimension reduction using density divergences

In this article, we introduce two new families of multivariate association measures based on power divergence and alpha divergence that recover both linear and nonlinear dependence relationships between multiple sets of random vectors. Importantly, this novel approach not only characterizes independence, but also provides a smooth bridge between well-known distances that are inherently robust against outliers. Algorithmic approaches are developed for dimension reduction and the selection of the optimal robust association index. Extensive simulation studies are performed to assess the robustness of these association measures under different types and proportions of contamination. We illustrate the usefulness of our methods in application by analyzing two socioeconomic datasets that are known to contain outliers or extreme observations. Some theoretical properties, including the consistency of the estimated coefficient vectors, are investigated and computationally efficient algorithms for our nonparametric methods are provided. (C) 2013 Elsevier Inc. All rights reserved

Mechanistic and pathological study of the genesis, growth, and rupture of abdominal aortic aneurysms

Postprint (published version

Finite temperature properties of the triangular lattice t-J model, applications to Nax_xCoO2_2

We present a finite temperature ($T$) study of the t-J model on the two-dimensional triangular lattice for the negative hopping $t$, as relevant for the electron-doped Na$_x$CoO$_2$ (NCO). To understand several aspects of this system, we study the $T$-dependent chemical potential, specific heat, magnetic susceptibility, and the dynamic Hall-coefficient across the entire doping range. We show systematically, how this simplest model for strongly correlated electrons describes a crossover as function of doping ($x$) from a Pauli-like weakly spin-correlated metal close to the band-limit (density $n=2$) to the Curie-Weiss metallic phase ($1.5<n<1.75$) with pronounced anti-ferromagnetic (AFM) correlations at low temperatures and Curie-Weiss type behavior in the high-temperature regime. Upon further reduction of the doping, a new energy scale, dominated by spin-interactions ($J$) emerges (apparent both in specific heat and susceptibility) and we identify an effective interaction $J_{eff}(x)$, valid across the entire doping range. This is distinct from Anderson's formula, as we choose here $t<0$, hence the opposite sign of the usual Nagaoka-ferromagnetic situation. This expression includes the subtle effect of weak kinetic AFM - as encountered in the infinitely correlated situation ($U=\infty$). By explicit computation of the Kubo-formulae, we address the question of practical relevance of the high-frequency expression for the Hall coefficient $R_H^*$. We hope to clarify some open questions concerning the applicability of the t-J model to real experimental situations through this study

Theory of Suspension Segregation in Partially Filled Horizontal Rotating Cylinders

It is shown that a suspension of particles in a partially-filled, horizontal, rotating cylinder is linearly unstable towards axial segregation and an undulation of the free surface at large enough particle concentrations. Relying on the shear-induced diffusion of particles, concentration-dependent viscosity, and the existence of a free surface, our theory provides an explanation of the experiments of Tirumkudulu et al., Phys. Fluids 11, 507-509 (1999); ibid. 12, 1615 (2000).Comment: Accepted for publication in Phys Fluids (Lett) 10 pages, two eps figure

ULTRAZVUČNA PROSUDBA MIGRACIJE PLACENTE PREVIJE U ODNOSU NA MAJČINE DEMOGRAFSKE ČIMBENIKE

Objective. To assess the association between maternal age, parity, history of prior cesarean delivery and placental location in evaluating the persistence and rate of placental migration in low-lying or complete placenta previas followed by serial ultrasound examination. Study design. This is a retrospective study of 92 cases of low-lying/placenta previa diagnosed at 28 weeks of gestation followed serially by transvaginal ultrasound. The patients were stratified into three groups depending on the placenta to internal cervical os distance: (1) an overlap of 0.0 cm and over the cervical os (complete previa), (2) 0.1 to 2.9 cm (marginal placenta previa), (3) 3.0 cm or above (normal placental location). The ¬prevalence of complete and marginal placenta previas, and the mean rate of placental »migration« (mm/week) were ¬obtained at 28 and 36 weeks of gestation, and compared with maternal age, parity, history of prior cesarean delivery and placental location. Results. At the time of delivery, 51 patients had placenta previa: 22 complete and 29 marginal placenta previas. In contrast, 41 patients had sufficient placental ’migration’ to be categorized into the normal placental location group. The prevalence of complete placenta of 3.3% and 6.5% at 28 weeks, and 3.3% and 5.4% at 36 weeks’ gestation, for patients who had parity more than 1, or history or prior cesarean delivery (CD), respectively, was not statistically significant. The rate of placental migration was significantly associated with maternal age (p=0.002), while did not differ when stratified by parity (p=0.672) or prior history of CD (p=0.805), or placental location (p=0.147). Conclusion. Maternal age significantly modifies the rate of placenta previa migration. A history of prior CD, maternal parity and placental location did not affect the rate of placental migration in our sample of patients with complete or marginal placenta previa diagnosed by ¬ultrasound at 28 weeks’ gestation.Cilj rada. Prosuditi povezanost dobi majke, pariteta, prethodnog carskog reza i smještaja posteljice, s perzistiranjem ili migracijom posteljice kod nisko nasjele ili predležeće placente previje, praćene serijskim ultrazvučnim pregledima. Način istraživanja. Retrospektivna studija 92 nisko nasjele posteljice ili placente previje, dijagnosticirane s 28 tjedana i serijski ultrazvučno praćene. Bolesnice su bile podijeljene u tri skupine, ovisno o udaljenosti posteljice od unutrašnjeg ušća cerviksa: 1) preraštanje više od 0,0 mm preko ušća cerviksa (kompletna previja); 2) 0,1 do 2,9 cm (marginalna previja); 3) 3,0 ili više cm od ušća cerviksa (normalni smještaj posteljice). Zastupljenost kompletnih i marginalnih placenta previja i srednja vrijednost »migracije« posteljice (mm/tjedan) su utvrđeni s 28 i 36 tjedana trudnoće te uspoređeni s dobi majke, paritetom, ranijim carskim rezom i smještajem posteljice. Rezultati. Od 92 trudnice s 28 tjedana, u vrijeme poroda 51 trudnica je imala placentu previju: 22 kompletnu i 29 marginalnu, dok je u 41 trudnice posteljica dovoljno »migrirala« da bi bila razvrstana u skupinu s normalnim smještajem. Zastupljenost kompletne previje za trudnice s više od jednog poroda od 3,3% s 28 i 3,3% s 36 tjedana, odnosno s prethodnim carskim rezom od 6,5% s 28 i 5,4% s 36 tjedana, nije statistički signifikantno različita. Stopa »migracije« posteljice je znakovito povezana s dobi trudnice (p=0,002), a nije s paritetom (p=0,672), ranijim carskim rezom (p=0,805) ili ležištem posteljice (p=0,147). Zaključak. Dob trudnice znakovito modificira stupanj migracije placente previje. U našem uzorku kompletnih i marginalnih posteljica otkrivenih ultrazvukom s 28 tjedana, raniji carski rez, paritet majke i ležište posteljice (sprijeda/straga) ne utječu na stupanj migracije posteljice

Fluctuation limits of strongly degenerate branching systems

Functional limit theorems for scaled fluctuations of occupation time processes of a sequence of critical branching particle systems in $\R^d$ with anisotropic space motions and strongly degenerated splitting abilities are proved in the cases of critical and intermediate dimensions. The results show that the limit processes are constant measure-valued Wienner processes with degenerated temporal and simple spatial structures.Comment: 15 page

Quantum-Mechanical Position Operator and Localization in Extended Systems

We introduce a fundamental complex quantity, $z_{L}$, which allows us to discriminate between a conducting and non-conducting thermodynamic phase in extended quantum systems. Its phase can be related to the expectation value of the position operator, while its modulus provides an appropriate definition of a localization length. The expressions are valid for {\it any} fractional particle filling. As an illustration we use $z_{L}$ to characterize insulator to ``superconducting'' and Mott transitions in one-dimensional lattice models with infinite on-site Coulomb repulsion at quarter filling.Comment: 4 pages, REVTEX, 1 ps figure

A new class of exactly solvable interacting fermion models in one dimension

We investigate a model containing two species of one-dimensional fermions interacting via a gauge field determined by the positions of all particles of the opposite species. The model can be solved exactly via a simple unitary transformation. Nevertheless, correlation functions exhibit nontrivial interaction-dependent exponents. A similar model defined on a lattice is introduced and solved. Various generalizations, e.g. to the case of internal symmetries of the fermions, are discussed. The present treatment also clarifies certain aspects of Luttinger's original solution of the ``Luttinger model''.Comment: 11 pages, revtex 3.0, no figures, some typos correcte

X-ray Characteristics of NGC 3516: A View through the Complex Absorber

We consider new Suzaku data for NGC 3516 taken during 2009, along with other recent X-ray observations of the source. The cumulative characteristics of NGC 3516 cannot be explained without invoking changes in the line-of-sight absorption. Contrary to many other well-studied Seyfert galaxies, NGC 3516 does not show a positive lag of hard X-ray photons relative to soft photons over the timescales sampled. In the context of reverberation models for the X-ray lags, the lack of such a signal in NGC 3516 is consistent with flux variations being dominated by absorption changes. The lack of any reverberation signal in such a highly variable source disfavors intrinsic continuum variability in this case. Instead, the colorless flux variations observed at high flux states for NGC 3516 are suggested to be a consequence of Compton-thick clumps of gas crossing the line-of-sight.Comment: 12 pages, 6 figures, accepted for publication in The Astrophysical Journa

Does the attractive Hubbard model support larger persistent currents than the repulsive one ?

We consider a one-dimensional Hubbard model in the presence of disorder. We compute the charge stiffness for a mesoscopic ring, as a function of the size $L$, which is a measure of the permanent currents. We find that for finite disorder the permanent currents of the system with repulsive interactions are larger than those of the system with attractive interactions. This counter intuitive result is due to the fact that local density fluctuations are reduced in the presence of repulsive interactions.Comment: 14 pages; Revtex 3.0; 3 postscript figures uuencoded with uufile