I have estimated and ARMA process in Mathematica using data which I have had to take the 2nd difference of to remove the trend. I have managed to fit a model to the detrended data but cannot work out how to undo the differences when forecasting. I can undo the difference if the set is a list of numbers but because the forecast uses temporal data I am stuck. I am fairly new to Mathematica and do not have very much coding experience so an example would appreciated.
Here is a link to my dropbox with code I have done so far.
Thanks for any help in advance.