Hello R-Community,
we are running aprogram to fit Non-linear differential equations to Aphid
population Data and to estimate the birth and death parameters,
here is the code:

dat<-data.frame(Time=c(0:60),Cur=c(5,6.2,59,39,38,44,20.4,19.4,34.2,35.4,38.2,48.2,55.4,113.2,
97,112,115,126,136.6,140.6,147.2,151.6,157.8,170,202,210.4,221.2,224.4,248.2,266,
277,291.4,392,461.2,470,418,410.8,395.6,365.2,189.4,43.4,33.2,32,29,26,26,25.6,24.6,
24,23.4,18.4,17.6,15.8,13.6,10.6,8.4,6.4,5.4,4.2,3.6,2.7))

times<-c(0:60)
f <- function(t,xx,pars){
 Cur<- xx[1]
 Cum<- xx[2]
 a <- pars[1]
 b <- pars[2]
 dCur <- a*Cur-b*Cur*Cum^2
 dCum <- Cur
 list(c(dCur,dCum))
}


require(nls)
fit <- nls(Cur ~ lsoda(c(Cur=1,Cum=1), times, f, c(a=a, b=b))[,2],
           data=dat,
           start=list(a=2.5, b=.01),
           trace=T
           )
print(coef(fit))

It runs the first iterations and shows the output, after the intial
eveluation it send the following error message:

92 :  2.50 0.01
Error in numericDeriv(form[[3]], names(ind), env) :
        Missing value or an infinity produced when evaluating the model
In addition: There were 50 or more warnings (use warnings() to see the first 50)
> print(coef(fit))
Error in coef(fit) : object "fit" not found

Looking forward to hear soon
Thanks
Raj

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