Quoting Chris Landauer <cal@rush.aero.org>:

I remember a paper from one of the SIAM journals (probly numerical methods or something) from many years ago whose title was something like how not to fit an exponential.

the author(s?) describe I think 20 ways that are plausible, but do not work, and explain what the ill-conditioning means and even why (or at least that) it happens a lot.

I've run into this fitting some laser data to the energy transfer equations between different levels of different rare earths (not my problem, my favorite physicist friend's problem; she does the experiments and gets LOTSA data)

C. Moler and C. Van Loan. Nineteen dubious ways to compute the exponential of a matrix. SIAM Rev., 20(4):801-836, 1978. Their topic was calculating the exponential of a matrix (as distinguished from the exponential of a floating point number). Most everything that can go wrong does; they analyze the why's and wherefore's in considerable detail. This is not a curve fitting problem for a set of exponentials, although the problem relates to the different scales arising from the various eigenvalues of the matrix. Of course, there may be a better reference for your problem; this article is the one that sprang to mind when I read your message. - hvm ------------------------------------------------- www.correo.unam.mx UNAMonos Comunicándonos