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Alexander polynomials of Hurwitz curves. (English. Russian original) Zbl 1112.14019
Izv. Math. 70, No. 1, 69-86 (2006); translation from Izv. Ross. Akad. Nauk, Ser. Mat. 70, No. 1, 75-94 (2006).
The work under review is a sequel of the paper [G.-M.Greuel and V. S. Kulikov, Izv. Math. 69, No. 4, 667–701 (2005; Zbl 1093.14023)], where the Alexander polynomials of Hurwitz $$C$$-groups and the corresponding Hurwitz curves in $${\mathbb C}{\mathbb P}^2$$ are studied.
In particular, the author proves that the necessary condition from [loc. cit.] for a polynomial $$P(t) \in \mathbb Z [t]$$ to be the Alexander polynomial of an irreducible $$C$$-group is, in fact, also a sufficient one. This result leads to a complete description of the set of the Alexander polynomials of irreducible Hurwitz $$C$$-groups; it consists of all the polynomials $$P(t)$$ such that $$P(1)=1$$ and the roots of $$P(t)$$ are roots of unity. The second theorem describes sufficient conditions on $$P(t)$$ to be the Alexander polynomial of a reducible Hurwitz $$C$$-groups. The author also conjectures the necessity of these conditions.
As an application he proves that $$P(t)=(-1)^{n+k}(t-1)^{n}(t+ 1)^{k}$$ is the Alexander polynomial of a Hurwitz $$C$$-group if and only if $$n\geq k.$$ In conclusion two examples with explicitly given $$C$$-presentations are considered; they don’t correspond to a Hurwitz $$C$$-group since their Alexander polynomials $$(t-1)^2$$ and ($$(1-t)(t+1)^2$$ do not satisfy the necessary conditions.
##### MSC:
 14F35 Homotopy theory and fundamental groups in algebraic geometry 14H30 Coverings of curves, fundamental group 14H50 Plane and space curves
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