12,451 research outputs found
On multivariate quantiles under partial orders
This paper focuses on generalizing quantiles from the ordering point of view.
We propose the concept of partial quantiles, which are based on a given partial
order. We establish that partial quantiles are equivariant under
order-preserving transformations of the data, robust to outliers, characterize
the probability distribution if the partial order is sufficiently rich,
generalize the concept of efficient frontier, and can measure dispersion from
the partial order perspective. We also study several statistical aspects of
partial quantiles. We provide estimators, associated rates of convergence, and
asymptotic distributions that hold uniformly over a continuum of quantile
indices. Furthermore, we provide procedures that can restore monotonicity
properties that might have been disturbed by estimation error, establish
computational complexity bounds, and point out a concentration of measure
phenomenon (the latter under independence and the componentwise natural order).
Finally, we illustrate the concepts by discussing several theoretical examples
and simulations. Empirical applications to compare intake nutrients within
diets, to evaluate the performance of investment funds, and to study the impact
of policies on tobacco awareness are also presented to illustrate the concepts
and their use.Comment: Published in at http://dx.doi.org/10.1214/10-AOS863 the Annals of
Statistics (http://www.imstat.org/aos/) by the Institute of Mathematical
Statistics (http://www.imstat.org
Weak momentum scattering and the conductivity of graphene
Electrical transport in graphene offers a fascinating parallel to spin
transport in semiconductors including the spin-Hall effect. In the weak
momentum scattering regime the steady-state density matrix contains two
contributions, one linear in the carrier number density and characteristic
scattering time , the other independent of either. In this paper we take
the Liouville equation as our starting point and demonstrate that these two
contributions can be identified with pseudospin conservation and
non-conservation respectively, and are connected in a non-trivial manner by
scattering processes. The scattering term has a distinct form, which is
peculiar to graphene and has important consequences in transport. The
contribution linear in is analogous to the part of the spin density
matrix which yields a steady state spin density, while the contribution
independent of , is analogous to the part of the spin density matrix
which yields a steady state spin current. Unlike in systems with spin-orbit
interactions, the and -independent part of the conductivity is
reinforced in the weak momentum scattering regime by scattering between the
conserved and non-conserved pseudospin distributions.Comment: 10 pages. Accepted for publication in Phys. Rev.
Statistical characterization of phenolic-novolak structures
Three statistical methods of general validity are valuable for characterizing any polymer which results from chain polymerization of multifunctional branching monomers linked through bifunctional monomers
Moving to Diversity
In this brief, authors Richelle Winkler and Kenneth Johnson, using new data and techniques, find that net migration between U.S. counties increased racial diversity in each of the last two decades. However, migration’s influence on diversity was far from uniform: it varied by race, age group, and location, sometimes starkly. Overall, net migration of the population under age 40 increased diversity, while net migration of people over age 60 diminished diversity. Blacks and Hispanics are migrating to predominantly white counties, while white young adults are moving to urban core counties with relatively high proportions of blacks and Hispanics. The movement of older whites is not contributing to the growing diversity, because older whites tend to move to predominantly white counties. Winkler and Johnson conclude that, while migration contributed to the growing diversity of the nation, the process was complex and varied from place to place with significant social, economic, and political implications for both the more diverse and less diverse places
External gates and transport in biased bilayer graphene
We formulate a theory of transport in graphene bilayers in the weak momentum
scattering regime in such a way as to take into account contributions to the
electrical conductivity to leading and next-to-leading order in the scattering
potential. The response of bilayers to an electric field cannot be regarded as
a sum of terms due to individual layers. Rather, interlayer tunneling and
coherence between positive- and negative-energy states give the main
contributions to the conductivity. At low energies, the dominant effect of
scattering on transport comes from scattering within each energy band, yet a
simple picture encapsulating the role of collisions in a set of scattering
times is not applicable. Coherence between positive- and negative-energy states
gives, as in monolayers, a term in the conductivity which depends on the order
of limits. The application of an external gate, which introduces a gap between
positive- and negative-energy states, does not affect transport. Nevertheless
the solution to the kinetic equation in the presence of such a gate is very
revealing for transport in both bilayers and monolayers.Comment: 6 pages, accepted for publication in Physical Review
Sound predictability as a higher-order cue in auditory scene analysis
A major challenge for the auditory system is to disentangle signals emitted by two or more sound sources that are active in a temporally interleaved manner (sequential stream segregation). Besides distinct characteristics of the individual signals (e.g., their timbre, location, and pitch), one important cue for distinguishing the sound sources is how their emitted signals unfold over time. It seems intuitively plausible that signals that unfold predictably with respect to their acoustic features and time-points of occurrence, such as the repetitive signature of a train moving on the rails, can be more readily identified as originating from one sound source. Based on this rationale, predictive elements have successfully been incorporated into computational models of auditory scene analysis for many years
Reentrant nu = 1 quantum Hall state in a two-dimensional hole system
We report the observation of a reentrant quantum Hall state at the Landau
level filling factor nu = 1 in a two-dimensional hole system confined to a
35-nm-wide (001) GaAs quantum well. The reentrant behavior is characterized by
a weakening and eventual collapse of the nu = 1 quantum Hall state in the
presence of a parallel magnetic field component B||, followed by a
strengthening and reemergence as B|| is further increased. The robustness of
the nu = 1 quantum Hall state during the transition depends strongly on the
charge distribution symmetry of the quantum well, while the magnitude of B||
needed to invoke the transition increases with the total density of the system
Proton-Coupled Electron Flow in Protein Redox Machines
Electron transfer (ET) reactions are fundamental steps in biological redox processes. Respiration is a case in point: at least 15 ET reactions are required to take reducing equivalents from NADH, deposit them in O_2, and generate the electrochemical proton gradient that drives ATP synthesis. Most of these reactions involve quantum tunneling between weakly coupled redox cofactors (ET distances > 10 Ă…) embedded in the interiors of folded proteins. Here we review experimental findings that have shed light on the factors controlling these distant ET events. We also review work on a sensitizer-modified copper protein photosystem in which multistep electron tunneling (hopping) through an intervening tryptophan is orders of magnitude faster than the corresponding single-step ET reaction.If proton transfers are coupled to ET events, we refer to the processes as proton coupled ET, or PCET, a term introduced by Huynh and Meyer in 1981. Here we focus on two protein redox machines, photosystem II and ribonucleotide reductase, where PCET processes involving tyrosines are believed to be critical for function. Relevant tyrosine model systems also will be discussed
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