
Hi,    I am using package deSolve to run some ordinary differential equations (ODE) as part of a mathematical modeling project.  I have solved for the following equilibrium states: Seq1<-a*(1-Neq1)/(f*Veq1+m+d)
Ceq1<-(f*Seq1*Veq1+g*Ieq1+r*(1-Neq1)-b1*Veq1*Ieq1)/(b2+m+d+g)
Ieq1<-(-b2*Ceq1)-r*(1-Neq1)/(b1*Veq1-g-u)
Veq1<-o*(Ceq1+Ieq1)/e  I want to take the differential of both sides of the equation and then solve for the inverse of the first as follows (the parameter values are made up): library(deSolve)
rkMethod("rk45dp7")
CSIeq1<-function(t,yeq1,pars) {
with (as.list(c(yeq1,pars)),{
Neq1<-Seq1+Ceq1+Ieq1
dSeq1<-d[a*(1-Neq1)/(f*Veq1+m+d)]
dCeq1<-d[(f*Seq1*Veq1+g*Ieq1+r*(1-Neq1)-b1*Veq1*Ieq1)/(b2+m+d+g)]
dIeq1<-d[(-b2*Ceq1)-r*(1-Neq1)/(b1*Veq1-g-u)]
dVeq1<-d[o*(Ceq1+Ieq1)/e]
return(list(c(Seq1,Ceq1,Ieq1,Veq1),Neq1))
})
}pars <- c(a=0.1, 
m=0.0005, 
u=0.00005, 
b1=0.7, 
b2=0.2, 
f=0.01, 
g=0.4,      
r=0.3, 
o=20, 
e=90, 
d=0.01)
# initial conditions
yeq1 <- c(Seq1=0.99,Ceq1=0.01,Ieq1=0,Veq1=1)
t <-seq(0,365, by=50)
o1 <- ode(yeq1, t, CSIeq1, pars, method = rkMethod("rk45dp7"))1/o1$Seq1
*************************************************** The output gives the following error message so I am wondering about the syntax of the differentials such as dSeq1<-d[a*(1-Neq1)/(f*Veq1+m+d)]:
 > library(deSolve)
> rkMethod("rk45dp7")
$ID
[1] "rk45dp7"$varstep
[1] TRUE$FSAL
[1] TRUE$A
           [,1]       [,2]      [,3]       [,4]       [,5]      [,6]
[1,] 0.00000000   0.000000 0.0000000  0.0000000  0.0000000 0.0000000
[2,] 0.20000000   0.000000 0.0000000  0.0000000  0.0000000 0.0000000
[3,] 0.07500000   0.225000 0.0000000  0.0000000  0.0000000 0.0000000
[4,] 0.97777778  -3.733333 3.5555556  0.0000000  0.0000000 0.0000000
[5,] 2.95259869 -11.595793 9.8228929 -0.2908093  0.0000000 0.0000000
[6,] 2.84627525 -10.757576 8.9064227  0.2784091 -0.2735313 0.0000000
[7,] 0.09114583   0.000000 0.4492363  0.6510417 -0.3223762 0.1309524$b1
[1]  0.08991319  0.00000000  0.45348907  0.61406250 -0.27151238  0.08904762
[7]  0.02500000$b2
[1]  0.09114583  0.00000000  0.44923630  0.65104167 -0.32237618  0.13095238
[7]  0.00000000$c
[1] 0.0000000 0.2000000 0.3000000 0.8000000 0.8888889 1.0000000 1.0000000$d
[1] -1.127018  0.000000  2.675424 -5.685527  3.521932 -1.767281  2.382469$densetype
[1] 1$stage
[1] 7$Qerr
[1] 4attr(,"class")
[1] "list"     "rkMethod"
> CSIeq1<-function(t,yeq1,pars) {
+ with (as.list(c(yeq1,pars)),{
+ Neq1<-Seq1+Ceq1+Ieq1
+ dSeq1<-d[a*(1-Neq1)/(f*Veq1+m+d)]
+ dCeq1<-d[(f*Seq1*Veq1+g*Ieq1+r*(1-Neq1)-b1*Veq1*Ieq1)/(b2+m+d+g)]
+ dIeq1<-d[(-b2*Ceq1)-r*(1-Neq1)/(b1*Veq1-g-u)]
+ dVeq1<-d[o*(Ceq1+Ieq1)/e]
+ return(list(c(Seq1,Ceq1,Ieq1,Veq1),Neq1))
+ })
+ }
> 
> pars <- c(a=0.1, 
m=0.0005, 
u=0.00005, 
b1=0.7, 
b2=0.2, 
f=0.01, 
g=0.4,      
r=0.3, 
o=20, 
e=90, 
d=0.01)
> # initial conditions
> yeq1 <- c(Seq1=0.99,Ceq1=0.01,Ieq1=0,Veq1=1)
> t <-seq(0,365, by=50)
> o1 <- ode(yeq1, t, CSIeq1, pars, method = rkMethod("rk45dp7"))
There were 50 or more warnings (use warnings() to see the first 50)
> 
> 1/o1$Seq1
Error in o1$Seq1 : $ operator is invalid for atomic vectors
> warnings()
Warning messages:
1: In eval(expr, envir, enclos) : NAs introduced by coercion
 ******************************************** Any thoughts or suggestions would be appreciated.  If you run the program with only the differential on the left, it runs just fine.  Do I need to use a different 'R' package?
-- AAP
 
Anil A. Panackal, MD, ScM, FACP

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