question on trying to fit an exponential decay curve to data

[American Citizen <website.reader3@gmail.com>]

Hello:

While working with the L-series code that I have, in trying to find the 
regulator of an elliptic curve, I've hit a problem with curve fitting.

I have a set of 15,026 points which shows a nice decay curve to 
(purportedly) the final value = regulator, but trying to fit the 
equation (1) eludes me.

(1)  y = a*exp(x*k1) + b*exp(x*k2) + c

My attempts to optimize (1)  to the data failed in both a Python3 
module, and gnuplot curve fitting, so I am stuck at the moment. Python3 
says maxfev > 800, so optimization attempts failed and gnuplot says bad 
data at data point 710, which makes absolutely no sense at all.

How would you use GP Pari to do a least squares fit to the equation (1) 
?? I need to know [a, b, c, k1, k2] of course.

Randall