non linear fit of multiple data sets with recursive function

Hello every one

I am in trouble with a fit because I am working with a recursive function and I am not able to write a code to correctly fit my data. Morover, I have to fit several data set at the same time.

The fitting function is: , where . In this expression, . In essence, the functions τ and are interconnected and I have no idea in which way write these two in my fitting code.

The perameters I want to know are: while the quantities I know are .

Do you have any idea on the way to write this code?

[Daniele Sonaglioni]

@Star Strider do you have any suggestions? I think you are the most suited person who can help me.