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we may transform S so that
with e1, em+1 =0,1,2or3 and ai = 1 or 2 for i = 1,..., m and m > 0. Multiplying by a suitable power of X we obtain
with m > 0 and a =0,1,2 or 3. Assume that m > 1 and let
We show by induction that
This claim ...