a Given these assumptions about the fifictitia epidemic, write down the two laws of motion for St and It in terms of St, It, C, p, α, β, γ , and the λs. b What conditions must be true if the populations of infected and uninfected people are in steady state (that is, they do not change over time)? c Assume γ = 0. Interpret this assumption.

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a Given these assumptions about the fifictitia epidemic, write down the two laws of motion for St and It in terms of St, It, C, p, α, β, γ , and the λs.

b What conditions must be true if the populations of infected and uninfected people are in steady state (that is, they do not change over time)?

c Assume γ = 0. Interpret this assumption.

d Now assume γ >β. Interpret this assumption.

e Assuming γ >β, solve for I∗ as a function of C, p, α, β, γ , and the λs. [Hint:As a fifirst step, you should add your two equations from part (b) to derive a relationship between I∗ and S∗.]

f How does a change in the price of p affffect the steady-state level of I∗?

14 The SIR-style diagram in Figure 21.8 demonstrates the dynamics of a new disease
called fictitia that is sweeping the nation. Fictitia is different from the disease we
diagrammed in Section 21.2 in a number of ways:
• There is no vaccine for fictitia and no way to recover from it, so there is no recovered
population.
● Fictitia renders patients sterile, so the birth rate is proportional to the size of the
susceptible population, not the whole population.
• Fortunately, there are some costly self-protective activities h that people can take to
avoid contracting fictitia. Their rate of use depends on both the cost of the activities
p and the prevalence of the disease I,. Specifically, the rate of use h(p,1)=C-ap+
yl, for constant C.
Birth
Susceptible
X St
AbSt
[31,-h(p.)] St
Infected
Xa, It
Death
Figure 21.8. The SIR model space for fictitia.
S₂
4
Abst
[3,-h(p.)]S,
size of the susceptible group
size of the infected group
number of births
Р
h(p. l)
Xds
X,
number of infections
price of costly self-protective activities
rate of costly self-protective activities
death rate of susceptible group
death rate of infected group
Transcribed Image Text:14 The SIR-style diagram in Figure 21.8 demonstrates the dynamics of a new disease called fictitia that is sweeping the nation. Fictitia is different from the disease we diagrammed in Section 21.2 in a number of ways: • There is no vaccine for fictitia and no way to recover from it, so there is no recovered population. ● Fictitia renders patients sterile, so the birth rate is proportional to the size of the susceptible population, not the whole population. • Fortunately, there are some costly self-protective activities h that people can take to avoid contracting fictitia. Their rate of use depends on both the cost of the activities p and the prevalence of the disease I,. Specifically, the rate of use h(p,1)=C-ap+ yl, for constant C. Birth Susceptible X St AbSt [31,-h(p.)] St Infected Xa, It Death Figure 21.8. The SIR model space for fictitia. S₂ 4 Abst [3,-h(p.)]S, size of the susceptible group size of the infected group number of births Р h(p. l) Xds X, number of infections price of costly self-protective activities rate of costly self-protective activities death rate of susceptible group death rate of infected group
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