Subgroup Structure of Fundamental Groups in Positive Characteristic

Author: Bary-Soroker Lior   Kumar Manish  

Publisher: Taylor & Francis Ltd

ISSN: 0092-7872

Source: Communications in Algebra, Vol.41, Iss.10, 2013-10, pp. : 3705-3719

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Abstract

Let Π be the étale fundamental group of a smooth affine curve over an algebraically closed field of characteristic p > 0. We establish a criterion for profinite freeness of closed subgroups of Π. Roughly speaking, if a closed subgroup of Π is “captured” between two normal subgroups, then it is free, provided it contains most of the open subgroups of index p. In the proof we establish a strong version of “almost ω-freeness” of Π and then apply the Haran-Shapiro induction.