# Avoid Recursion in lag and difference operations

I have a panel data situation and I'm interested in taking lags and differences and using Mathematica to simplify the algebra of the result. Currently I'm working with a model like this:

y[t_] := y[t] = b + b x[t] + b x[t - 1] + \[Eta][t]


which serves my needs if I want to take a d difference (y[t]-y[t-d]) or a multiple m of a d difference (y[t]-m*y[t-d]).

However, if I change the model slightly where y[t] now includes a lag of itself,

y[t_] := y[t] = b + b x[t] + b x[t - 1] + b y[t-1] + \[Eta][t]


then this no longer works. It tries to recursively plug in all the y lags and I get the recursion limit error. I don't want it to recursively substitute at all. I would like the output it gives me of y[t]-y[t-1] to contain y[t-1] and y[t-1-d].

I'm planning on taking multiple differences, so whatever the solution I would still need to do things like ExpandAll and Collect.

I'm quite new to Mathematica, so I apologize in advance for obvious mistakes. If there are any comments on any part of my code/approach, please share them!

• hey mike_s welcome! If I understand correctly you're trying to do algebraic manipulations on the structural form of y[t]; unless there are other considerations, using something like y[t_]:=y[t]=... (ie memoization) is probably not necessary; one way to evaluate something like eg y[t]-m*y[t-d] is to use ReplaceAll with Rule; – user42582 Oct 25 '18 at 13:15
• Thank you for the response! Can you give an example of how I would use ReplaceAll with Rule and then collect terms? – mike_s Oct 25 '18 at 17:09
• in order to calc y[t] - m y[t - d] define yrl[t_] := b + b x[t] + b x[t - 1] + b y[t - 1] + η[t] (instead of using y[t_]:=...) and then evaluate y[t] - m y[t - d] /. y[t_] :> yrl[t]; you should be able to collect terms in the later expression – user42582 Oct 25 '18 at 18:33