Hi again ! In your reply, you told me :
this means f is a sum of exponetial
However. For the case wher f is of the form f=A•t•exp(b•t), i don't know how to convert that as a sum of exponential... I'm asking because i feel like i could just take x to be an indefinite sum of exponentials which might end up giving me some quite nice solutions... If only i could get a sum of exponentials on the other side of the equation ;)