Hey ffezz.

I recall when I did a first year applied math course that they derived the annuity equations for an nth time period using difference equations like this:

http://www.ziegenbalg.ph-karlsruhe.d...ClosedForm.pdf

Can you outline your model in terms of a difference equation for the value of the annuity at x_(t+1) given the annuities value at x_t along with the other information? This will make it a lot easier to give a more specific answer.