Houjun Liu

SIR Model

The SIR Model is a model to show how diseases spread.

  • Susceptible – # of susceptible people
  • Infectious — # of infectious people
  • Removed — # of removed people

Compartmental SIR model

S => I => R [ => S]

So then, the question is: what is the transfer rate between populations between these compartments?


  • \(R_0\) “reproductive rate”: the number of people that one infectious person will infect over the duration of their entire infectious period, if the rest of the population is entirely susceptible (only appropriate for a short duration)
  • \(D\) “duration”: duration of the infectious period
  • \(N\) “number”: population size (fixed)

Transition I to R:

\begin{equation} \frac{I}{D} \end{equation}

\(I\) is the number of infectious people, and \(\frac{1}{D}\) is the number of people that recover/remove per day (i.e. because the duration is \(D\).)

Transition from S to I:

\begin{equation} I \frac{R_0}{D} \frac{S}{N} \end{equation}

So for \(\frac{R_0}{D}\) is the number of people able to infect per day, \(\frac{S}{N}\) is the percentage of population that’s able to infect, and \(I\) are the number of people doing the infecting.

And so therefore—

  • \(\dv{S}{T} = -\frac{SIR_{0}}{DN}\)
  • \(\dv{I}{T} = \frac{SIR_{0}}{DN}\)
  • \(\dv{I}{T} = \frac{I}{D}\)

Evolutionary Game Theory

Suppose that we have two strategies, \(A\) and \(B\), and they have some payoff matrix:


and we have some values:

\begin{equation} \mqty(x_{a} \\x_{b}) \end{equation}

are the relative abundances (i.e. that \(xa+xb\)).

The finesses (“how much are you going to reproduce”) of the strategies are determined by—

  • \(f_{A}(x_{A}, x_{B}) = ax_{A} + bx_{B}\)
  • \(f_{B}(x_{A}, x_{B}) = cx_{A} + dx_{B}\)

Except for payoff constants \((a,b,c,d)\), everything else is a function of time.

The mean fitness, then:

\begin{equation} q = x_{A}f_{A} + x_{B}f_{B} \end{equation}

Let’s have the actual, absolute number of individuals:

\begin{equation} \mqty(N_{A}\\ N_{B}) \end{equation}

So, we can talk about the change is individuals using strategy \(A\):

\begin{equation} \dv t x_{A} = \dv t \frac{N_{A}}{N} = X_{A}(f_{a}) \end{equation}