[R] invalid "mode" of argument optimize

[voodooochild@gmx.de]

Hello everybody,

i have the following function, which i want to solve for b

i=1,..,n,  and n is the length of t and t is the last element of t, if 
you do cumsum() before.

(t_n*exp(-b*t_n)*sum_{i=1}^{n} f_i) / (1-exp(-b*t_n)) - (sum_{i=1}^{n} 
(f_i*(t_i*exp(-b*t_i)-t_{i-1}*exp(-b*t_{i-1}))) / 
(exp(-b*t_{i-1})-exp(-b*t_i)) )  = 0

i implemented this function in the following way

ll2<-function(b,f,t) {
  t<-cumsum(t)
  tn<-t[length(t)]
  i<-seq(along=f)
  s1<-tn*exp(-b*tn)*sum(f[i])
  s2<-(1-exp(-b*tn))
  i[length(i)+1]<-0
  i<-sort(i)
  f[length(f)+1]<-1
  t[length(t)+1]<-0
  t<-sort(t)
  s3<-f[2]*(t[2]*exp(-b*t[2])-t[1]*exp(-b*t[1]))
  s4<-exp(-b*t[1])-exp(-b*t[2])
  i<-i[3:length(i)]
  s5<-sum(f[i]*(t[i]*exp(-b*t[i])-t[i-1]*exp(-b*t[i-1])))
  s6<-sum(exp(-b*t[i-1])-exp(-b*t[i]))
  (s1/s2)-((s3/s4)+(s5/s6))
}

# i have the given data
f=c(1,1,1)
t=c(320,14390,9000)

# now i wanted to use optimize() to get the minimum of ll2() with the 
given data

xmin<-optimize(ll2,c(0,10000),tol=0.001,f=f,t=t)

and i got always the error "invalid "mode" of argument  optimize", but i 
don't now what is really wrong? Or is there any mistake in my 
implementation of the function?

best regards
Andreas