#  simulated data used in the fit
x = 1:20
y = c(1.857049, 4.175308, 5.200348, 11.487756,  5.194808, 12.745572, 17.638813,
      15.070816, 21.180778, 21.511097, 24.631538, 26.603505, 24.568025, 29.077487,
      29.959046, 32.288661, 36.562983, 38.402021, 41.681277, 40.411469)