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My equations contain different cases which differ only by some signs. So in order to work with all cases simultaneously, I introduce a placeholder for a sign:

s[x]

where x labels which sign that is. Now when I perform manipulations of my terms, I encounter higher powers of the same sign, like s[1]^2. Of course the square of a sign should be just s[1]^2=1, so I try to define a rule:

s[x_]^2:=1

SetDelayed::write: Tag Power in s[x_]^2 is Protected.

As you can see, there is an error message and the rule does not get set. How should I be going about defining a rule that sets the square of any s[x] to 1?

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1 Answer 1

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You can use TagSetDelayed i.e.:

s /: s[x_]^2 := 1

This will create a substitution rule (UpValue) for symbol s, which is only applied, when s is inside Power[s[_], 2].

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    $\begingroup$ Thank you! I went with s /: s[x_]^n_Integer /; n > 1 := s[x]^Mod[n, 2]. $\endgroup$
    – Kagaratsch
    May 19, 2017 at 18:43

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