algebraic condition

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Hi,

I have a very simple problem: I have a parameter a which is either 1 or -1. I have some very complicated expressions in which this parameter occurs raised to integer powers a^2, a^3, a^4 etc. I would like to implify those equations substituting the condition a^2=1, but with expr.subs a^3, a^4 etc are not replaced by a, 1, etc.

I would appreciate some help in this issue.