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Realization of primitive branched coverings over closed surfaces following the hurwitz approach

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Bogatyi S., Gonçalves D., Kudryavtseva E., Zieschang H.

Open Mathematics

2003

tom 1

nr 2

184-197

Let V be a closed surface, H⊑π1(V) a subgroup of finite index l and D=[A 1,...,A m] a collection of partitions of a given number d≥2 with positive defect v(D). When does there exist a connected branched covering f:W→V of order d with branch data D and f∶W→V It has been shown by geometric arguments [4] that, for l=1 and a surface V different from the sphere and the projective plane, the corresponding branched covering exists (the data D is realizable) if and only if the data D fulfills the Hurwitz congruence v(D)э0 mod 2. In the case l>1, the corresponding branched covering exists if and only if v(D)э0 mod 2, the number d/l is an integer, and each partition A i ∈D splits into the union of l partitions of the number d/l. Here we give a purely algebraic proof of this result following the approach of Hurwitz [11]. The realization problem for the projective plane and l=1 has been solved in [7,8]. The case of the sphere is treated in [1, 2, 12, 7].

Ograniczanie wyników

1 Open Mathematics

1 Bogatyi S.

1 Gonçalves D.

1 Kudryavtseva E.

1 Zieschang H.

1 2003

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Realization of primitive branched coverings over closed surfaces following the hurwitz approach