[R] Problems with the deSolve package

[Abdel Halloway]

I think your parameters are off. If you look at the simul data frame, it
gives you a bunch of NaNs after the first initialization. If you put lower
the timesteps s.t.

> times<-seq(0,200, by=0.01)

it begins to run but soon your values diverge, i1 & i2 going negative while
i12 goes way high. Not sure what you are modeling but I assume those values
aren't to be like that. Try again with different parameters and see.

On Fri, Feb 19, 2016 at 2:42 AM, Alexandre Suire <alexandresuire at hotmail.fr>
wrote:

> Hello R-users,
>
> I'm trying to build a SIR-like model under R,
> using the "deSolve" package. I'm trying to do simulations of its dynamic
>  over time, with three differential equations. I'm also looking to
> calculate the equilibrium state.
>
> So far, my code looks like this
>
> library(deSolve)
> #This is my system, with three differential equations
> system<-function(times, init, parameters){
> with(as.list(c(init, parameters)),{
>
> di1_dt=(alpha1*(N-i1-i2-i12)*(i1+i12))+(beta2*i12+gamma1*(N-i1-i2-i12))-(beta1*i1)-(delta*alpha2*i1*(i2+i12))
>
> di2_dt=(alpha2*(N-i1-i2-i12)*(i2+i12))+(beta1*i12+gamma2*(N-i1-i2-i12))-(beta2*i2)-(delta*alpha1*i2*(i1+i12))
>
> di12_dt=(delta*alpha2*i1*(i12+i2))+(delta*alpha1*i2*(i12*i1))+(delta*gamma1*i1)+(delta*gamma2*i2)-((beta1+beta2)*i12)
> return(list(c(di1_dt,di2_dt,di12_dt)))
> })
> }
>
> # Initials values and parameters
> init<-c(i1=10, i2=10, i12=0)
> parameters<-c(alpha1=0.7, alpha2=0.5, beta1=0.5, beta2=0.3, gamma1=0.5,
> gamma2=0.5, delta=0.5, N=100)
> times<-seq(0,200, by=1)
> simul <- as.data.frame(ode(y = init, times = times, func = system, parms =
> parameters, method="ode45"))
> simul$time <- NULL
> head(simul,200)
>
> #Plotting the results
> matplot(times,
>  simul, type = "l", xlab = "Time", ylab = "i1 i2 i12", main =
> "Simulation", lwd = 2, lty = 2, bty = "l", col=c("darkblue",
> "darkred","mediumseagreen"))
> legend("bottomright", c("i1", "i2","i12"), lty=2,lwd=2, col =
> c("darkblue", "darkred", "mediumseagreen"))
>
> At
>  first, I just tried studying with only the first two equations, and it
> seems to work completely fine, but when I wanted to add the 3rd
> equation, I sometimes get this message, even when I juggle the
> parameters, when i launch the line:
> #simul <- as.data.frame(ode(y = init, times = times, func = system, parms
> = parameters))
> Warning messages:
> 1: In lsode(y, times, func, parms, mf = 10, ...) :
>   an excessive amount of work (> maxsteps ) was done, but integration was
> not successful - increase maxsteps
> 2: In lsode(y, times, func, parms, mf = 10, ...) :
>   Returning early. Results are accurate, as far as they go
>
> Have I overlooked something ? I tried to use methods="ode45" and
> methods="adams", without any sucess.
> Thank you for your time
> Alex
>
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>
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