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arXiv 1504.01154math.DSmath.CO
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2-球面的自分支覆盖的实现

Realizations of self branched coverings of the 2-sphere

J. Tomasini

摘要:

对于2-球面的d度自分支覆盖,一个值得注意的组合不变量是2d - 2的整数划分,由临界点的重数组成。一个更好的不变式是所谓的赫尔维茨护照。赫维茨护照的实现问题直到今天仍然很大程度上是开放的。在这篇文章中,我们介绍了两种不同类型的精细不变量:二分映射和关联矩阵。然后,我们完全解决他们的实现问题,证明了一个映射,或矩阵,实现了一个分支覆盖,当且仅当它满足一定的平衡条件。二分图方法的一个变体是由W.瑟斯顿我们的研究结果为赫尔维茨护照问题提供了一些新的线索。

英文摘要:

For a degree d self branched covering of the 2-sphere, a notable combinatorial invariant is an integer partition of 2d -- 2, consisting of the multiplicities of the critical points. A finer invariant is the so called Hurwitz passport. The realization problem of Hurwitz passports remain largely open till today. In this article, we introduce two different types of finer invariants: a bipartite map and an incident matrix. We then settle completely their realization problem by showing that a map, or a matrix, is realized by a branched covering if and only if it satisfies a certain balanced condition. A variant of the bipartite map approach was initiated by W. Thurston. Our results shed some new lights to the Hurwitz passport problem.

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