[R] deSolve ODE Output Question

[walke554]

Hello,

I am working on a simple ODE problem with the deSolve package, and I was
hoping that someone could answer a question about how the deSolve package
does integration.  

Here is my program:

#The function
STDMod<-function(t,y,p){
	IH = y[1];
	IL = y[2];
	with(as.list(p), {
		dIH.dt = Beta[1,1]*(nH-IH)*IH + Beta[1,2]*(nH-IH)*IL - gamma*IH;
		dIL.dt = Beta[2,1]*(nL-IL)*IH + Beta[2,2]*(nL-IL)*IL - gamma*IL;
		return(list(c(dIH.dt,dIL.dt)));
	})
}

#giving the parameters

Beta = matrix(data=c(10, 0.1, 0.1, 1.0), ncol=2, nrow=2)
nH = 0.2
nL = 0.8
IH0 = 1e-5
IL0 = 0

gamma = 1

p = list(Beta=Beta, gamma=gamma, nH=nH, nL=nL)

y0 = c(IH0, IL0)

#Running the ode integrator

steps= 10;
t = seq(from=0, to=30, by=.01);
out = ode(y=y0, times=t, func=STDMod, parms=p);

My understanding is that the 'out' matrix would be the values of the STDmod
function at each time step, given the initial values of the state parameters
IH and IL.  However, for the very first time step, I am getting a value
different than what the derivative and the initial values add up to.

My output:
<http://r.789695.n4.nabble.com/file/n4707448/ROutput.png> 

but 'IH0' + 'the value of dIH.dt = Beta[1,1]*(nH-IH)*IH +
Beta[1,2]*(nH-IH)*IL - gamma*IH for timestep one' = 1.999e-5.  Is there
something that I am missing?  I am hoping to use this for more complicated
derivatives, but I want to make sure I am using the package correctly first.

Thanks!



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