Hi,all:
I met a problem of nls.
 
My data:
x    y
60 0.8
80 6.5
100 20.5
120 45.9
 
I want to fit exp curve of data.
 
My code:
> nls(y ~ exp(a + b*x)+d,start=list(a=0,b=0,d=1))
Error in nlsModel(formula, mf, start, wts) :
  singular gradient matrix at initial parameter estimates

I can't find out the reason for the error.
Any suggesions are welcome.
 
Many thanks.
 
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