Fitting a Complicated Function

*To*: mathgroup at smc.vnet.net*Subject*: [mg127226] Fitting a Complicated Function*From*: mpdgeorgin at gmail.com*Date*: Tue, 10 Jul 2012 00:40:30 -0400 (EDT)*Delivered-to*: l-mathgroup@mail-archive0.wolfram.com*Delivered-to*: mathgroup-newout@smc.vnet.net*Delivered-to*: mathgroup-newsend@smc.vnet.net

I'm trying to fit a very complicated function and I'm having some problems. Right now I'm trying to do a Constrained fit where my parameters are R,m,a. FindFit[data, { Exp[-x/R] + Exp[-x/R]* Sum[((-a*x^(1 - m))^n)* Hypergeometric1F1[-m*n, 1 + n - m*n, x/R]/ Gamma[1 + n - m*n], {n, 1, 15}], 1 < R < 30, 0 < a < .5, .4 < m < 1}, {R, m, a}, x] The error that appears is : Power:infy : infinite expression FindFit:nrmnum : the function value Indeterminate is not a real number at{R,m,a} = {2, 0.94, 0.45} This is strange because I can evaluate the function at these values and it gives me a real, finite result. I have a couple questions: 1) Is there a way to check what the Find fit function is actually doing? As in can I look at the code to see exactly where its going wrong. 2) What should my next step be? according to my PI, simplification of the function doesn't lead anywhere. Thanks Marcel

**Follow-Ups**:**Re: Fitting a Complicated Function***From:*Peter Klamser <klamser@googlemail.com>