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It is classical that given any Seifert structure on N,Reidemeister-Schreier's algorithm produces a presentation of all index 2subgroups of the fundamental group of N, described as the fundamental group ofsome Seifert manifolds. The new result of this article is concise formulas thatgather all possible cases.

Reidemeister-Schreier's Algorithm For 2-Coverings of Seifert Manilfolds