Hi Colleen,
this error was not uncommon and is usually a sign of a numerically
problematic or wrongly implemented model. Please use package deSolve,
the successor of odesolve, that is more robust and has also a bunch
alternative solvers for difficult cases.
I tested your code with deSolve (on R 2.8.0, Windows) and it runs
without problems.
Thomas Petzoldt
BTW: your system worked also with odesolve, so my question: which
versions (R, odesolve) and operating system are you using?
Colleen Carlson wrote:
Dear list -
Does anyone have any ideas / comments about why I am receiving the following
warning when I run lsoda:
1: lsoda-- at t (=r1), too much accuracy requested in: lsoda(start, times,
model, parms)
2: for precision of machine.. see tolsf (=r2) in: lsoda(start, times,
model, parms)
I have tried changing both rtol and atol but without success. I saw the
thread in the R-archive of 11 June 2004 but this has not helped me.
I have built the model in stages and the problem only occurs when the
exponent beta in the third DE is anything other than 0 or 1. If beta = 0 or
1 then the solver gives me perfectly justifiable results. Just changing
beta to 0.9 or similar causes the problem.
I am still new to R so I am unsure if it is my programming or my
understanding of the way lsoda works.
Any comments or input would be welcome.
Many thanks
Colleen
___________
My code is:
library(odesolve)
SI <- 80
model <- function(t, x, parms) {
H <- x[1]
BA <- x[2]
N <- x[3]
with(as.list(parms), {
dHdt <- (b/c)*(((a**c)*((H)**(1-c))-H))
dBAdt <- -(BA*b)*(c0+(c1*SI)-log(BA))/(log(1-((H/a)**c)))
dNdt <- N*alpha*(((log(1-((H/a)**c)))/b)**beta) - (gamma*BA)
list(c(dHdt, dBAdt, dNdt))
})
}
times <- seq(0, 40, 1)
parms <- c(a=(SI*1.258621)-1.32759, b=0.1, c=0.4, c0=4.6012, c1=0.013597,
alpha=0.0005, beta=0.5, gamma=0.01)
start <- c(H=0.1, BA=0.1, N=600)
out <- as.data.frame(lsoda(start, times, model, parms))
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--
Thomas Petzoldt
Technische Universitaet Dresden
Institut fuer Hydrobiologie [EMAIL PROTECTED]
01062 Dresden http://tu-dresden.de/hydrobiologie/
GERMANY
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