Problem Fitting Custom Equation (Beta Amplitude) using CFTOOL

Hy everyone!

I need to fit some experimental XY data on the function reported in the attached image

I'm tryng to use cftool with Custom Equation option but when I insert (a3-1) or (a4-1) as index exponentials the fit returns the error:

Complex value computed by model function, fitting cannot continue. Try using or tightening upper and lower bounds on coefficients.

I dont' understand if I need to best define some range for the coefficents a1, a2,,,, for example starting with positive values

Thank you!

[Torsten]

Constrain a3 and a4 to be greater than 1.

Best wishes

Torsten.

[Ricky80]

Thanks a lot Torsten