Hurwitz existence problem and fiber products
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arXiv
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| Format: | Preprint |
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2024
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| _version_ | 1866909603212558336 |
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| author | Pakovich, Fedor |
| author_facet | Pakovich, Fedor |
| contents | With each holomorphic map $f: R \rightarrow \mathbb C\mathbb P^1$, where $R$ is a compact Riemann surface, one can associate a combinatorial datum consisting of the genus $g$ of $R$, the degree $n$ of $f$, the number $q$ of branching points of $f$, and the $q$ partitions of $n$ given by the local degrees of $f$ at the preimages of the branching points. These quantities are related by the Riemann-Hurwitz formula, and the Hurwitz existence problem asks whether a combinatorial datum that fits this formula actually corresponds to some map $f$. In this paper, using results and techniques related to fiber products of holomorphic maps between compact Riemann surfaces, we prove a number of results that enable us to uniformly explain the non-realizability of many previously known non-realizable branch data, and to construct a large amount of new such data. We also deduce from our results the theorem of Halphen, proven in 1880, concerning polynomial solutions of the equation $A(z)^a+B(z)^b=C(z)^c$, where $a,b,c$ are integers greater than one. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2408_10874 |
| institution | arXiv |
| publishDate | 2024 |
| record_format | arxiv |
| spellingShingle | Hurwitz existence problem and fiber products Pakovich, Fedor Geometric Topology Complex Variables With each holomorphic map $f: R \rightarrow \mathbb C\mathbb P^1$, where $R$ is a compact Riemann surface, one can associate a combinatorial datum consisting of the genus $g$ of $R$, the degree $n$ of $f$, the number $q$ of branching points of $f$, and the $q$ partitions of $n$ given by the local degrees of $f$ at the preimages of the branching points. These quantities are related by the Riemann-Hurwitz formula, and the Hurwitz existence problem asks whether a combinatorial datum that fits this formula actually corresponds to some map $f$. In this paper, using results and techniques related to fiber products of holomorphic maps between compact Riemann surfaces, we prove a number of results that enable us to uniformly explain the non-realizability of many previously known non-realizable branch data, and to construct a large amount of new such data. We also deduce from our results the theorem of Halphen, proven in 1880, concerning polynomial solutions of the equation $A(z)^a+B(z)^b=C(z)^c$, where $a,b,c$ are integers greater than one. |
| title | Hurwitz existence problem and fiber products |
| topic | Geometric Topology Complex Variables |
| url | https://arxiv.org/abs/2408.10874 |