Reidemeister-Schreier's Algorithm For 2-Coverings of Seifert Manilfolds
  It is classical that given any Seifert structure on N, Reidemeister-Schreier's algorithm produces a presentation of all index 2 subgroups of the fundamental group of N, described as the fundamental group of some Seifert manifolds. The new result of this article is concise formulas that gather all possible cases. 
A. Bauval, C. Hayat
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