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arXiv 1408.0641math.PR
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生灭过程的期望持续时间及其在分支过程和SIS流行病中的应用

On expected durations of birth-death processes, with applications to branching processes and SIS epidemics

Frank Ball, Tom Britton, Peter Neal

摘要:

本文研究连续时间生灭型过程,其中个体具有独立同分布的寿命,根据一个随机变量Q,E[Q]=1,并且如果种群当前处于状态(有大小)n,则出生率为α(n).我们着重讨论了两个重要的例子,即α(n)=λn是一个分支过程,以及α(n)=λn(N-n)/N对应于一个具有固定规模N的均匀混合社区中的SIS传染病模型.假设过程从单个个体开始,即在状态1中。设T,A_n,C和S分别表示灭绝前的(随机)时间,处于状态$n$的总时间,曾经存活的个体总数和出生-纯灭过程中所有个体的寿命之和。本文的主要结果给出了所有这些量的期望的表达式,并表明这些期望对Q的分布不敏感。我们还推导出了SIS流行病灭绝的预期时间的渐近表达式,但现在从地方病状态开始,这与Q的分布无关。结果还应用于家庭SIS流行病,表明它的门限参数R * 对Q的分布不敏感,而家庭SIR流行病的R * 依赖于Q。

英文摘要:

We study continuous-time birth-death type processes, where individuals have independent and identically distributed lifetimes, according to a random variable Q, with E[Q]=1, and where the birth rate if the population is currently in state (has size) n is α(n). We focus on two important examples, namely α(n)=λn being a branching process, and α(n)=λn(N-n)/N which corresponds to an SIS epidemic model in a homogeneously mixing community of fixed size N. The processes are assumed to start with a single individual, i.e. in state 1. Let T, A_n, C and S denote the (random) time to extinction, the total time spent in state $n$, the total number of individuals ever alive and the sum of the lifetimes of all individuals in the birth-death process, respectively. The main results of the paper give expressions for the expectation of all these quantities, and shows that these expectations are insensitive to the distribution of Q. We also derive an asymptotic expression for the expected time to extinction of the SIS epidemic, but now starting at the endemic state, which is not independent of the distribution of Q. The results are also applied to the household SIS epidemic, showing that its threshold parameter R_* is insensitive to the distribution of Q, contrary to the household SIR epidemic, for which R_* does depend on Q.

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