limitations of fit?

[<pl...@pi...>]

Hi,

I have about 2300 data points that I need to fit a simple cosine to. 
However, fit says it is converging at the first iteration and 
effectively does nothing.

The data has a good signal to noise and I have primed the parameters 
with values which are pretty close. It just needs to shift the constant 
term closer to zero.

I had this working fine with less data points (about 200)  on 
essentially the same dataset.

Is the large number of points causing fit to fail to converge properly?

TIA, Peter.



  fcos1(x)= a1*cos(2*pi*(x-yz1)/p1)+c1
a1=c1=1; yz1=1990; p1=4.5
  fit [:1991.5] fcos1(x) datafile1  u 1:2 via  a1,c1,p1,yz1

After 1 iterations the fit converged.
final sum of squares of residuals : 3036.32
rel. change during last iteration : 0