Solves a pairwise approximation to the SIS model on a random network of N individuals, each with n contacts.
1 
pars 

init 

time 
time sequence for which output is wanted; the first value of times must be the initial time. 
... 
further arguments passed to ode function. 
This is the R version of program 7.8 from page 285 of "Modeling Infectious Disease in humans and animals" by Keeling & Rohani.
All parameters must be positive.
list
. The first element, *$model
, is the model function. The second, third and fourth elements are the vectors *$pars
, *$init
and *$time
, containing the pars
, init
and time
arguments of the function. The fifth element *$results
is a data.frame
with up to as many rows as elements in time. First column contains the time. Second, third and fourth columns contain the number of susceptibles, infectious and susceptibleinfectious pairs.
Keeling, Matt J., and Pejman Rohani. Modeling infectious diseases in humans and animals. Princeton University Press, 2008. Modeling Infectious Diseases in Humans and Animals
ode.
1 2 3 4 5 6 7 8 9  # Parameters and initial conditions.
n < 4; N < 1e4; Y < 1; X < N  Y
parameters < c(tau = 0.1, gamma = 0.05, n = n, N = N)
initials < c(X = X, Y = Y, XY = n * Y * X / N)
# Solve and plot.
sis.pairwise.approx < SISPairwiseApprox(pars = parameters,
init = initials, time = 0:100)
PlotMods(sis.pairwise.approx)

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