5,988 research outputs found
Quartically hyponormal weighted shifts need not be 3-hyponormal
We give the first example of a quartically hyponormal unilateral weighted
shift which is not 3-hyponormal
A new approach to the 2-variable subnormal completion problem
We study the Subnormal Completion Problem (SCP) for 2-variable weighted
shifts. We use tools and techniques from the theory of truncated moment
problems to give a general strategy to solve SCP. We then show that when all
quadratic moments are known (equivalently, when the initial segment of weights
consists of five independent data points), the natural necessary conditions for
the existence of a subnormal completion are also sufficient. To calculate
explicitly the associated Berger measure, we compute the algebraic variety of
the associated truncated moment problem; it turns out that this algebraic
variety is precisely the support of the Berger measure of the subnormal
completion
Why Optimal States Recruit Fewer Reactions in Metabolic Networks
The metabolic network of a living cell involves several hundreds or thousands
of interconnected biochemical reactions. Previous research has shown that under
realistic conditions only a fraction of these reactions is concurrently active
in any given cell. This is partially determined by nutrient availability, but
is also strongly dependent on the metabolic function and network structure.
Here, we establish rigorous bounds showing that the fraction of active
reactions is smaller (rather than larger) in metabolic networks evolved or
engineered to optimize a specific metabolic task, and we show that this is
largely determined by the presence of thermodynamically irreversible reactions
in the network. We also show that the inactivation of a certain number of
reactions determined by irreversibility can generate a cascade of secondary
reaction inactivations that propagates through the network. The mathematical
results are complemented with numerical simulations of the metabolic networks
of the bacterium Escherichia coli and of human cells, which show,
counterintuitively, that even the maximization of the total reaction flux in
the network leads to a reduced number of active reactions.Comment: Contribution to the special issue in honor of John Guckenheimer on
the occasion of his 65th birthda
Hyponormality and subnormality for powers of commuting pairs of subnormal operators
Let H_0 (resp. H_\infty denote the class of commuting pairs of subnormal
operators on Hilbert space (resp. subnormal pairs), and for an integer k>=1 let
H_k denote the class of k-hyponormal pairs in H_0. We study the hyponormality
and subnormality of powers of pairs in H_k. We first show that if (T_1,T_2) is
in H_1, then the pair (T_1^2,T_2) may fail to be in H_1. Conversely, we find a
pair (T_1,T_2) in H_0 such that (T_1^2,T_2) is in H_1 but (T_1,T_2) is not.
Next, we show that there exists a pair (T_1,T_2) in H_1 such that T_1^mT_2^n is
subnormal (all m,n >= 1), but (T_1,T_2) is not in H_\infty; this further
stretches the gap between the classes H_1 and H_\infty. Finally, we prove that
there exists a large class of 2-variable weighted shifts (T_1,T_2) (namely
those pairs in H_0 whose cores are of tensor form) for which the subnormality
of (T_1^2,T_2) and (T_1,T_2^2) does imply the subnormality of (T_1,T_2)
k-hyponormality of multivariable weighted shifts
We characterize joint k-hyponormality for 2-variable weighted shifts. Using
this characterization we construct a family of examples which establishes and
illustrates the gap between k-hyponormality and (k+1)-hyponormality for each
k>=1. As a consequence, we obtain an abstract solution to the Lifting Problem
for Commuting Subnormals.Comment: 13 pages; to appear in J. Funct. Ana
Development or Despair? The Intentions and Realities of South-South Migration
The literature on the motivations and the impacts of remittances sent by international migrants to their families and communities strongly focus on their economic impacts and the flows of money from the global North to the South. The driving factors for remittance sending, closely embedded in motivations for migration, often overlook the conditions in which remittances are not sent, where households do not prosper, and community development does not happen. This ethnographic study uses the case of Nicaraguan migration to Costa Rica to examine the lives
of the families of migrants in the context of rural to rural and South to South migration. The paper examines the intentions and realities of South-South migration through the lens of migrants and their families, and illuminates the
other side of migration often invisible in the migrant’s home communities and the migration and development literature. The paper also shows the disparity in physical, economic, and emotional distance as it illustrates the unique structural challenges of South-South and especially rural to rural migrants. These structural barriers that create an unbridgeable distance between migrants and their families embody the chasm between the intentions and the realities of migration for individuals on both sides of the border
Development or Despair? The Intentions and Realities of South-South Migration
The literature on the motivations and the impacts of remittances sent by international migrants to their families and communities strongly focus on their economic impacts and the flows of money from the global North to the South. The driving factors for remittance sending, closely embedded in motivations for migration, often overlook the conditions in which remittances are not sent, where households do not prosper, and community development does not happen. This ethnographic study uses the case of Nicaraguan migration to Costa Rica to examine the lives of the families of migrants in the context of rural to rural and South to South migration. The paper examines the intentions and realities of South-South migration through the lens of migrants and their families, and illuminates the other side of migration often invisible in the migrant’s home communities and the migration and development literature. The paper also shows the disparity in physical, economic, and emotional distance as it illustrates the unique structural challenges of South-South and especially rural to rural migrants. These structural barriers that create an unbridgeable distance between migrants and their families embody the chasm between the intentions and the realities of migration for individuals on both sides of the border
Particle-in-cell and weak turbulence simulations of plasma emission
The plasma emission process, which is the mechanism for solar type II and
type III radio bursts phenomena, is studied by means of particle-in-cell and
weak turbulence simulation methods. By plasma emission, it is meant as a loose
description of a series of processes, starting from the solar flare associated
electron beam exciting Langmuir and ion-acoustic turbulence, and subsequent
partial conversion of beam energy into the radiation energy by nonlinear
processes. Particle-in-cell (PIC) simulation is rigorous but the method is
computationally intense, and it is difficult to diagnose the results. Numerical
solution of equations of weak turbulence (WT) theory, termed WT simulation, on
the other hand, is efficient and naturally lends itself to diagnostics since
various terms in the equation can be turned on or off. Nevertheless, WT theory
is based upon a number of assumptions. It is, therefore, desirable to compare
the two methods, which is carried out for the first time in the present paper
with numerical solutions of the complete set of equations of the WT theory and
with two-dimensional electromagnetic PIC simulation. Upon making quantitative
comparisons it is found that WT theory is largely valid, although some
discrepancies are also found. The present study also indicates that it requires
large computational resources in order to accurately simulate the radiation
emission processes, especially for low electron beam speeds. Findings from the
present paper thus imply that both methods may be useful for the study of solar
radio emissions as they are complementary.Comment: 21 pages, 9 figure
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Pascalammetry with operando microbattery probes: Sensing high stress in solid-state batteries.
Energy storage science calls for techniques to elucidate ion transport over a range of conditions and scales. We introduce a new technique, pascalammetry, in which stress is applied to a solid-state electrochemical device and induced faradaic current transients are measured and analyzed. Stress-step pascalammetry measurements are performed on operando microbattery probes (Li2O/Li/W) and Si cathodes, revealing stress-assisted Li+ diffusion. We show how non-Cottrellian lithium diffusional kinetics indicates stress, a prelude to battery degradation. An analytical solution to a diffusion/activation equation describes this stress signature, with spatiotemporal characteristics distinct from Cottrell's classic solution for unstressed systems. These findings create an unprecedented opportunity for quantitative detection of stress in solid-state batteries through the current signature. Generally, pascalammetry offers a powerful new approach to study stress-related phenomena in any solid-state electrochemical system
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