curve fitting with equations

Hi, I have 5 data points that i want to curve fit. I used the matlab curve fitter to fit it to a fourier series because the model equation i have is periodic (i think). It works, it seems to plot a nice curve through the points.

However I need certain parameters from my equation to use else where in other equations and therefore wanted to curve fit to a "custom equation" however matlab cant converge and I wondered if anyone could help me. my data is % k = [1 2 3 4 5]; % yk = [1.994 1.806 1.632 1.493 1.2];

the equation and parameters I am looking for are func(i) = (1/(a*b))*(c0-asinh(sinh(c0)*exp(a*t*2*pi)))

parameters a, b and c0.

Can anyone shed any light on why matlab possibly cant converge?

[Alex]

t in the equation should be the k values. sorry