2,894 research outputs found
Dynamics of the time to the most recent common ancestor in a large branching population
If we follow an asexually reproducing population through time, then the
amount of time that has passed since the most recent common ancestor (MRCA) of
all current individuals lived will change as time progresses. The resulting
"MRCA age" process has been studied previously when the population has a
constant large size and evolves via the diffusion limit of standard
Wright--Fisher dynamics. For any population model, the sample paths of the MRCA
age process are made up of periods of linear upward drift with slope +1
punctuated by downward jumps. We build other Markov processes that have such
paths from Poisson point processes on
with intensity measures of the form where is
Lebesgue measure, and (the "family lifetime measure") is an arbitrary,
absolutely continuous measure satisfying and
. Special cases of this construction
describe the time evolution of the MRCA age in -stable continuous
state branching processes conditioned on nonextinction--a particular case of
which, , is Feller's continuous state branching process conditioned on
nonextinction. As well as the continuous time process, we also consider the
discrete time Markov chain that records the value of the continuous process
just before and after its successive jumps. We find transition probabilities
for both the continuous and discrete time processes, determine when these
processes are transient and recurrent and compute stationary distributions when
they exist.Comment: Published in at http://dx.doi.org/10.1214/09-AAP616 the Annals of
Applied Probability (http://www.imstat.org/aap/) by the Institute of
Mathematical Statistics (http://www.imstat.org
Impact of aging on the auditory system and related cognitive functions: A narrative review
Age-related hearing loss (ARHL), presbycusis, is a chronic health condition that affects approximately one-third of the world’s population. The peripheral and central hearing alterations associated with age-related hearing loss have a profound impact on perception of verbal and non-verbal auditory stimuli. The high prevalence of hearing loss in the older adults corresponds to the increased frequency of dementia in this population. Therefore, researchers have focused their attention on age-related central effects that occur independent of the peripheral hearing loss as well as central effects of peripheral hearing loss and its association with cognitive decline and dementia. Here we review the current evidence for the age-related changes of the peripheral and central auditory system and the relationship between hearing loss and pathological cognitive decline and dementia. Furthermore, there is a paucity of evidence on the relationship between ARHL and established biomarkers of Alzheimer’s disease, as the most common cause of dementia. Such studies are critical to be able to consider any causal relationship between dementia and ARHL. While this narrative review will examine the pathophysiological alterations in both the peripheral and central auditory system and its clinical implications, the question remains unanswered whether hearing loss causes cognitive impairment or vice versa
Quantum gate characterization in an extended Hilbert space
We describe an approach for characterizing the process of quantum gates using
quantum process tomography, by first modeling them in an extended Hilbert
space, which includes non-qubit degrees of freedom. To prevent unphysical
processes from being predicted, present quantum process tomography procedures
incorporate mathematical constraints, which make no assumptions as to the
actual physical nature of the system being described. By contrast, the
procedure presented here ensures physicality by placing physical constraints on
the nature of quantum processes. This allows quantum process tomography to be
performed using a smaller experimental data set, and produces parameters with a
direct physical interpretation. The approach is demonstrated by example of
mode-matching in an all-optical controlled-NOT gate. The techniques described
are non-specific and could be applied to other optical circuits or quantum
computing architectures.Comment: 4 pages, 2 figures, REVTeX (published version
Squeezing based on nondegenerate frequency doubling internal to a realistic laser
We investigate theoretically the quantum fluctuations of the fundamental
field in the output of a nondegenerate second harmonic generation process
occuring inside a laser cavity. Due to the nondegenerate character of the
nonlinear medium, a field orthogonal to the laser field is for some operating
conditions indepedent of the fluctuations produced by the laser medium. We show
that this fact may lead to perfect squeezing for a certain polarization mode of
the fundamental field. The experimental feasibility of the system is also
discussed.Comment: 6 pages, 5 figure
Seagrasses in an era of ocean warming: a review
Seagrasses are valuable sources of food and habitat for marine life and are one of Earth's most efficient carbon sinks. However, they are facing a global decline due to ocean warming and eutrophication. In the last decade, with the advent of new technology and molecular advances, there has been a dramatic increase in the number of studies focusing on the effects of ocean warming on seagrasses. Here, we provide a comprehensive review of the future of seagrasses in an era of ocean warming. We have gathered information from published studies to identify potential commonalities in the effects of warming and the responses of seagrasses across four distinct levels: molecular, biochemical/physiological, morphological/population, and ecosystem/planetary. To date, we know that although warming strongly affects seagrasses at all four levels, seagrass responses diverge amongst species, populations, and over depths. Furthermore, warming alters seagrass distribution causing massive die-offs in some seagrass populations, whilst also causing tropicalization and migration of temperate species. In this review, we evaluate the combined effects of ocean warming with other environmental stressors and emphasize the need for multiple-stressor studies to provide a deeper understanding of seagrass resilience. We conclude by discussing the most significant knowledge gaps and future directions for seagrass research.En prens
Phosphorus
Phosphorus (P) is one of the most important
mineral nutrients for biological systems, yet
it is also one of the most scarce nutrients in
terms of its demand in both terrestrial and
aquatic environments (Moss, 1988). In natural systems, P is tightly cycled through the
plant-soil continuum, but in agricultural
systems soil P is removed in the crop or
animal products and must be replaced if P
deficiency is to be avoided. Therefore, mineral
PO fertilizers and animal manures
are applied to agricultural land to raise soil
P levels and maintain crop yields (Sibbesen
and Sharpley, 1997)
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