178 research outputs found
A charged rotating cylindrical shell
We give an example of a spacetime having an infinite thin rotating
cylindrical shell constituted by a charged perfect fluid as a source. As the
interior of the shell the Bonnor--Melvin universe is considered, while its
exterior is represented by Datta--Raychaudhuri spacetime. We discuss the energy
conditions and we show that our spacetime contains closed timelike curves.
Trajectories of charged test particles both inside and outside the cylinder are
also examined. Expression for the angular velocity of a circular motion inside
the cylinder is given.Comment: 14 pages, 4 figures, minor corrections, to appear in Gen.Rel.Gra
Nine challenges in incorporating the dynamics of behaviour in infectious diseases models.
Traditionally, the spread of infectious diseases in human populations has been modelled with static parameters. These parameters, however, can change when individuals change their behaviour. If these changes are themselves influenced by the disease dynamics, there is scope for mechanistic models of behaviour to improve our understanding of this interaction. Here, we present challenges in modelling changes in behaviour relating to disease dynamics, specifically: how to incorporate behavioural changes in models of infectious disease dynamics, how to inform measurement of relevant behaviour to parameterise such models, and how to determine the impact of behavioural changes on observed disease dynamics
Stability of Closed Timelike Curves in Goedel Universe
We study, in some detail, the linear stability of closed timelike curves in
the Goedel metric. We show that these curves are stable. We present a simple
extension (deformation) of the Goedel metric that contains a class of closed
timelike curves similar to the ones associated to the original Goedel metric.
This extension correspond to the addition of matter whose energy-momentum
tensor is analyzed. We find the conditions to have matter that satisfies the
usual energy conditions. We study the stability of closed timelike curves in
the presence of usual matter as well as in the presence of exotic matter
(matter that does satisfy the above mentioned conditions). We find that the
closed timelike curves in Goedel universe with or whithout the inclusion of
regular or exotic matter are also stable under linear perturbations. We also
find a sort of structural stability.Comment: 12 pages, 11 figures, RevTex, several typos corrected. GRG, in pres
Effectiveness of isolation, testing, contact tracing, and physical distancing on reducing transmission of SARS-CoV-2 in different settings: a mathematical modelling study
BACKGROUND: The isolation of symptomatic cases and tracing of contacts has been used as an early COVID-19 containment measure in many countries, with additional physical distancing measures also introduced as outbreaks have grown. To maintain control of infection while also reducing disruption to populations, there is a need to understand what combination of measures-including novel digital tracing approaches and less intensive physical distancing-might be required to reduce transmission. We aimed to estimate the reduction in transmission under different control measures across settings and how many contacts would be quarantined per day in different strategies for a given level of symptomatic case incidence. METHODS: For this mathematical modelling study, we used a model of individual-level transmission stratified by setting (household, work, school, or other) based on BBC Pandemic data from 40 162 UK participants. We simulated the effect of a range of different testing, isolation, tracing, and physical distancing scenarios. Under optimistic but plausible assumptions, we estimated reduction in the effective reproduction number and the number of contacts that would be newly quarantined each day under different strategies. RESULTS: We estimated that combined isolation and tracing strategies would reduce transmission more than mass testing or self-isolation alone: mean transmission reduction of 2% for mass random testing of 5% of the population each week, 29% for self-isolation alone of symptomatic cases within the household, 35% for self-isolation alone outside the household, 37% for self-isolation plus household quarantine, 64% for self-isolation and household quarantine with the addition of manual contact tracing of all contacts, 57% with the addition of manual tracing of acquaintances only, and 47% with the addition of app-based tracing only. If limits were placed on gatherings outside of home, school, or work, then manual contact tracing of acquaintances alone could have an effect on transmission reduction similar to that of detailed contact tracing. In a scenario where 1000 new symptomatic cases that met the definition to trigger contact tracing occurred per day, we estimated that, in most contact tracing strategies, 15 000-41 000 contacts would be newly quarantined each day. INTERPRETATION: Consistent with previous modelling studies and country-specific COVID-19 responses to date, our analysis estimated that a high proportion of cases would need to self-isolate and a high proportion of their contacts to be successfully traced to ensure an effective reproduction number lower than 1 in the absence of other measures. If combined with moderate physical distancing measures, self-isolation and contact tracing would be more likely to achieve control of severe acute respiratory syndrome coronavirus 2 transmission. FUNDING: Wellcome Trust, UK Engineering and Physical Sciences Research Council, European Commission, Royal Society, Medical Research Council
Extinction times in the subcritical stochastic SIS logistic epidemic
Many real epidemics of an infectious disease are not straightforwardly super-
or sub-critical, and the understanding of epidemic models that exhibit such
complexity has been identified as a priority for theoretical work. We provide
insights into the near-critical regime by considering the stochastic SIS
logistic epidemic, a well-known birth-and-death chain used to model the spread
of an epidemic within a population of a given size . We study the behaviour
of the process as the population size tends to infinity. Our results cover
the entire subcritical regime, including the "barely subcritical" regime, where
the recovery rate exceeds the infection rate by an amount that tends to 0 as but more slowly than . We derive precise asymptotics for
the distribution of the extinction time and the total number of cases
throughout the subcritical regime, give a detailed description of the course of
the epidemic, and compare to numerical results for a range of parameter values.
We hypothesise that features of the course of the epidemic will be seen in a
wide class of other epidemic models, and we use real data to provide some
tentative and preliminary support for this theory.Comment: Revised; 34 pages; 6 figure
Dispersal delays, predator–prey stability, and the paradox of enrichment
Author Posting. © Elsevier B.V., 2007. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Theoretical Population Biology 71 (2007): 436-444, doi:10.1016/j.tpb.2007.02.002.It takes time for individuals to move from place to place. This travel time can be
incorporated into metapopulation models via a delay in the interpatch migration
term. Such a term has been shown to stabilize the positive equilibrium of the classical
Lotka-Volterra predator{prey system with one species (either the predator or the
prey) dispersing.
We study a more realistic, Rosenzweig-MacArthur, model that includes a carrying
capacity for the prey, and saturating functional response for the predator. We show
that dispersal delays can stabilize the predator{prey equilibrium point despite the
presence of a Type II functional response that is known to be destabilizing. We also
show that dispersal delays reduce the amplitude of oscillations when the equilibrium
is unstable, and therefore may help resolve the paradox of enrichment.MGN and PK were supported by grants from the National Science Foundation (OCE-0083976, DEB-0235692, DEB-9973212) and Environmental
Protection Agency (R-82908901-0). The research of PvdD is partially supported by NSERC and MITACS
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