> y
[1]   1  11  42  64 108 173 214
   
  > t
[1] 1 2 3 4 5 6 7
   
  > nls(1/y ~ c*exp(-a*b*t)+1/b, start=list(a=0.001,b=250,c=5), trace=TRUE)
29.93322 :    0.001 250.000   5.000 
Error in numericDeriv(form[[3]], names(ind), env) : 
        Missing value or an infinity produced when evaluating the model
   
  # the start value for b is almost close to final estimates, 
  # a is usually small positive close to 0
  # c is a positive number
  # Do I need different estimation method?
   
  Regards
  xj
   
   

 
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