
Dear 
Dr. Soetaert,
 
My 
name is Raffaele Vardavas and I work mainly on 
infectious disease modeling. 
I have 
come across your excellent online source on the R package DeSolve. I usually use 
Mathematica for solving 
my ODE 
models but since I am getting more familiar with R, I would like to transition 
to using DeSolve package in R.
 
I have 
however a small problem and I was wondering if you may kindly sparse some time 
to help me with.
 
I have 
written some code that integrates the following model for smallpox vaccination 
dynamics specified by Bauch et al. in PNAS 2003
 
 SEIDV_model <- function(t, Omega, parameters) 
{
 with(as.list(c(Omega,parameters)), {
   dV = ifelse(t<tres | S<=0 
,0, nu)
   dS = -beta*S*I-dV
   dE =  beta*S*I-sigma*E
   dI =  
sigma*E-gamma*I
   dD =  gamma*I
   return(list(c(dS, dE, dI, dD, 
dV)))
   })
   }
 
As you 
can see this set of ODEs have a condition: the vaccination rate dV/dt is nonzero 
as long as the susceptible population is positive and
vaccines have been made available (after time tres).
 
When I 
integrate this model I get many warnings:
 
Warning messages:
1: In lsoda(y, times, func, parms, ...) :
  
DLSODA-  At current T (=R1), MXSTEP (=I1) steps                                 

2: In lsoda(y, times, func, parms, ...) :
        taken on this call 
before reaching TOUT                                   
3: In lsoda(y, times, 
func, parms, ...) :
        In above message,  I1 =      
5000                                         
4: In lsoda(y, times, func, 
parms, ...) :
        In above message,  R1 =  
0.2381220791874D+02                              
5: In lsoda(y, times, func, 
parms, ...) :
  an excessive amount of work (> maxsteps ) was done, but 
integration was successful - increase maxsteps
6: In lsoda(y, times, func, 
parms, ...) : 
 
I have 
increased the option maxstep and played around with hmax and atol options - but 
I seem not to be able to get rid of these warnings. 
I do 
get results in agreement with the authors of the paper. However, 
I am going to be making this model more complicated later on with 
additional boundaries ( thus more if statements 
in the 
ODE equations )- so I would rather be
comfortable in knowing and managing these warnings before 
proceeding.
 
Could 
you kindly shed some light on this. 

 
Your 
help would be very much appreciated.
 
Thank 
you.
 
Raff.
 
  		 	   		  
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