title: Using R to fit: y ~ exp(-(x-x0)^a)
body: <p>I trying to fit y ~  exp(-(x-x0)^a) to this data but get the error:
   "Error in nlsModel(formula, mf, start, wts) : 
    singular gradient matrix at initial parameter estimates"</p>
  
  <p>Do anybody have a tip?</p>
  
  <pre><code>x=c(0.612245,   0.47619,    0.857143,   0.535714,   0.535714)
  y=c(0.865,  1.1374, 0.439,  0.8698, 0.8468)
  m &lt;- nls(y ~  exp(-(x-x0)^a), start=list(x0=-100,a=-100))
  </code></pre>