130,574 research outputs found
Two-dimensional convection with a self-lubricating, simple-damage rheology
No full textWe present 2-D simulations of convection in the Earth's mantle with temperature and damage-dependent viscosity in a basally heated system. The equation governing the temporal evolution of damage includes a source term for damage, a healing term and an advection term. A systematic study of the influence of the different damage terms on the convection pattern shows that: (1) at least four different convective regimes can be distinguished depending on the size of the damage source term and (2) self-lubricating behaviour is possible only if the advection term in the damage equation is small enough compared with the other terms. We also demonstrate, that good plate-like behaviour in terms of (a) focused low-viscosity bands (LVB), (b) homogeneous surface velocities within the plates and abrupt velocity jumps across the plate boundaries and (c) asymmetric subduction, can be obtained with this kind of rheology, although an increase in subduction asymmetry tends to make convection highly time-dependent
Free completely random measures
No full textFree probability is a noncommutative probability theory introduced by Voiculescu where the concept of independence of classical probability is replaced by the concept of freeness. An important connection between free and classical infinite divisibility was established by Bercovici and Pata (1999) in form of a bijection, mapping the class of classical infinitely divisible laws into the class of free infinitely divisible laws. A particular class of infinitely divisible laws are the completely random measures introduced by Kingman (1967). In this paper, a free analogous of completely random measures is introduced and, a free Poisson process characterization is provided as well as a representation through a free cumulant transform. Furthermore, some examples are displayed.Bayesian non parametrics, Bercovici-Pata bijection, Free completely random measures, Free infinite divisibility, Free probability
Linear Algebraic Properties of C_{0}-Operators
No full textThesis (PhD) - Indiana University, Mathematics, 2006The theory of Jordan models for contractions is due to B. Sz.-Nagy - C. Foias, B.
Moore - E.A. Nordgren, and H. Bercovici - D. Voiculescu. J.A. Ball introduced the
class of C0-operators relative to a multiply connected domain. A. Zucchi provided
a classification of C0-operators relative to a multiply connected domain. Since no analogue of the characteristic
function of a contraction is available in that context, that study does not yield some
of the results available for the unit disk. In this thesis we use a substitute for the
characteristic function, suggested by an analogue of Beurling's theorem provided by
M.A. Abrahamse and R.G. Douglas. This allows us to prove a relationship between
the Jordan models of a C0-operator relative to a multiply connected domain, of its restriction to an invariant
subspace, and of its compression to the orthocomplement of that subspace.
This thesis is organized as follows. In Chapter 2, by defining a quasi-inner function, we provide a generalized Beurling's Theorem. In Chapter 3, we primarily deal
with C0-operators relative to a multiply connected domain. Finally, in Chapter 4, we study the modular lattice
for C0-Operators relative to the open unit disc
MeSH term explosion and author rank improve expert recommendations
No full textInformation overload is an often-cited phenomenon that reduces the productivity, efficiency and efficacy of scientists. One challenge for scientists is to find appropriate collaborators in their research. The literature describes various solutions to the problem of expertise location, but most current approaches do not appear to be very suitable for expert recommendations in biomedical research. In this study, we present the development and initial evaluation of a vector space model-based algorithm to calculate researcher similarity using four inputs: 1) MeSH terms of publications; 2) MeSH terms and author rank; 3) exploded MeSH terms; and 4) exploded MeSH terms and author rank. We developed and evaluated the algorithm using a data set of 17,525 authors and their 22,542 papers. On average, our algorithms correctly predicted 2.5 of the top 5/10 coauthors of individual scientists. Exploded MeSH and author rank outperformed all other algorithms in accuracy, followed closely by MeSH and author rank. Our results show that the accuracy of MeSH term-based matching can be enhanced with other metadata such as author rank
Energetics of a two-phase model of lithospheric damage, shear localization and plate-boundary formation
No full textcited By 102International audienceThe two-phase theory for compaction and damage proposed by Bercovici et al. (2001a, J. Geophys. Res., 106, 8887-8906) employs a nonequilibrium relation between interfacial surface energy, pressure and viscous deformation, thereby providing a model for damage (void generation and microcraking) and a continuum description of weakening, failure and shear localization. Here we examine further variations of the model which consider (1) how interfacial surface energy, when averaged over the mixture, appears to be partitioned between phases; (2) how variability in deformational-work partitioning greatly facilitates localization; and (3) how damage and localization are manifested in heat output and bulk energy exchange. Microphysical considerations of molecular bonding and activation energy suggest that the apparent partitioning of surface energy between phases goes as the viscosity of the phases. When such partitioning is used in the two-phase theory, it captures the melt-compaction theory of McKenzie (1984, J. Petrol., 25, 713-765) exactly, as well as the void-damage theory proposed in a companion paper (Ricard & Bercovici, submitted). Calculations of 1-D shear localization with this variation of the theory still show at least three possible regimes of damage and localization: at low stress is weak localization with diffuse slowly evolving shear bands; at higher stress strong localization with narrow rapidly growing bands exists; and at yet higher shear stress it is possible for the system to undergo broadly distributed damage and no localization. However, the intensity of localization is strongly controlled by the variability of the deformational-work partitioning with dilation rate, represented by the parameter γ. For γ ≫ 1, extreme localization is allowed, with sharp profiles in porosity (weak zones), nearly discontinuous separation velocities and effectively singular dilation rates. Finally, the bulk heat output is examined for the 1-D system to discern how much deformational work is effectively stored as surface energy. In the high-stress, distributed-damage cases, heat output is reduced as more interfacial surface energy is created. Yet, in either the weak or strong localizing cases, the system always releases surface energy, regardless of the presence of damage or not, and thus slightly more heat is in fact released than energy is input through external work. Moreover, increased levels of damage (represented by the maximum work-partitioning f*) make the localizing system release surface energy faster as damage enhances phase separation and focusing of the porosity field, thus yielding more rapid loss of net interfacial surface area. However, when cases with different levels of damage are compared at similar stages of development (say, the peak porosity of the localization) it is apparent that increased damage causes smaller relative heat release and retards loss of net interfacial surface energy. The energetics and energy partitioning of this damage and shear-localization model are applied to estimating the energy costs of forming plate boundaries and generating plates from mantle convection
Going Beyond Counting First Authors in Author Co-citation Analysis
No full textThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Toeplitz operators and skew carleson measures for weighted bergman spaces on strongly pseudoconvex domains
No full textIn this paper we study properties of Toeplitz operators on weighted Bergman spaces of bounded strongly pseudoconvex domains. We prove that a Toeplitz operator built using a weighted Bergman kernel of weight β and integrating against a measure μ maps continuously a weighted Bergman space Ap1α1 (D) into Ap2α2 (D) if and only if μ is a (λ, γ)-skew Carleson measure, where λ = 1 + 1/p1 - 1/p2 and γ = 1 l (β + α1/p1 - α2/p2). This generalizes results obtained by Pau and Zhao on the unit ball, and by Abate, Raissy and Saracco on a smaller class of Toeplitz operators on strongly pseudoconvex domains
"Closing the R&D Gap, Evaluating the Sources of R&D Spending"
No full textBoth spending and tax policies have been implemented in the United States with the goal of stimulating private sector research and development (R&D). Karier questions whether current R&D policy, especially the research and experimentation tax credit, can contribute to closing the gap between nondefense expenditures on R&D in the United States and such expenditures in other countries, such as Japan and Germany. He also explores possible changes to our current R&D policy to make it more effective.
Dispelling the Myths Behind First-author Citation Counts
No full textWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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