We propose another model of early propagation of an epidemic, by counting the number of indi- viduals spreading the disease as follows: At day 0, there is one person infected. From a day to the next, every infected individual independently: 1) remains a spreader with probability (w.p.) p, and has either infected exactly one new individual (who becomes a spreader) with conditional probability q, or has not spread the disease with con- ditional probability 1-q. 2) gets definitively removed from the pool of spreaders without infecting anyone else (w.p. 1-p). a. Justify why this model can be described as a branching process (what is the number of spreaders at time n + 1 given the number of spreaders at time n, what is the reproduction law). Find the generating function of the reproduction law. b. We define the contact number o as the average number of individuals directly contaminated

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Chapter1: Combinatorial Analysis
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Problem 3
We propose another model of early propagation of an epidemic, by counting the number of indi-
viduals spreading the disease as follows: At day 0, there is one person infected. From a day to the
next, every infected individual independently:
1) remains a spreader with probability (w.p.) p, and has either infected exactly one new individual
(who becomes a spreader) with conditional probability q, or has not spread the disease with con-
ditional probability 1-q.
2) gets definitively removed from the pool of spreaders without infecting anyone else (w.p. 1-p).
a. Justify why this model can be described as a branching process (what is the number of spreaders
at time n + 1 given the number of spreaders at time n, what is the reproduction law). Find the
generating function of the reproduction law.
b. We define the contact number o as the average number of individuals directly contaminated
by an infected individual before this individual is removed from the pool of spreaders. Show that
o= (hint: you can first consider how long on average can an infected person spread the
disease)
C. Show that the epidemic has a probability> 0 to propagate forever if and only if > 1, and
that in this case, this probability is 1-
Transcribed Image Text:Problem 3 We propose another model of early propagation of an epidemic, by counting the number of indi- viduals spreading the disease as follows: At day 0, there is one person infected. From a day to the next, every infected individual independently: 1) remains a spreader with probability (w.p.) p, and has either infected exactly one new individual (who becomes a spreader) with conditional probability q, or has not spread the disease with con- ditional probability 1-q. 2) gets definitively removed from the pool of spreaders without infecting anyone else (w.p. 1-p). a. Justify why this model can be described as a branching process (what is the number of spreaders at time n + 1 given the number of spreaders at time n, what is the reproduction law). Find the generating function of the reproduction law. b. We define the contact number o as the average number of individuals directly contaminated by an infected individual before this individual is removed from the pool of spreaders. Show that o= (hint: you can first consider how long on average can an infected person spread the disease) C. Show that the epidemic has a probability> 0 to propagate forever if and only if > 1, and that in this case, this probability is 1-
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