[R-sig-dyn-mod] deSolve::ode workaround for solutions with NaN

[Setzer.Woodrow at epamail.epa.gov]

What would happen if you re-wrote the system in terms of d log(Q) / dt =
(1/Q) dQ / dt?

R. Woodrow Setzer, Ph. D.
National Center for Computational Toxicology
http://www.epa.gov/comptox
US Environmental Protection Agency
Mail Drop B205-01/US EPA/RTP, NC 27711
Ph: (919) 541-0128    Fax: (919) 541-1194


                                                                                           
  From:       Maik Renner <maikrenner at googlemail.com>                                      
                                                                                           
  To:         r-sig-dynamic-models at r-project.org                                           
                                                                                           
  Date:       06/28/2010 02:37 PM                                                          
                                                                                           
  Subject:    [R-sig-dyn-mod] deSolve::ode workaround for solutions with NaN               
                                                                                           
  Sent by:    r-sig-dynamic-models-bounces at r-project.org                                   
                                                                                           





Dear List,

I have a question regarding unstable solutions using deSolve. Due to
my model equation I get NaN, when my state drops below zero.
Now, I wish to set the state to some default value, when this happens,
such that the simulation is continuing without NaNs.
However I could not figure out how to do that technically.

An if clause in the Model equation did not help. And I also tried the
event / root example in the vignette, but it did not run, having some
problems with my forcing inputs.

I have a model with one simple differential equation and one state and
a forcing vector. Here is the model implementation:

KirchSimeq19 <- function(t, state, parameters, input1, input2) {
 with(as.list(c(state, parameters)),{
  P <- input1(t)
  E <- input2(t)
  dQ <- exp(c1 + c2 * log(Q) + c3 * (log(Q))^2) * ( (P - k_E * E) / Q  -
1)
 list(c(dQ))
 }) # end with(as.list...
}



Any help  is appreciated,

Maik



--
Maik Renner, Dipl. Hydrologist
TU Dresden Germany
Tel.: +49 351 463-31341
http://tu-dresden.de/meteorologie

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