Abstract
Excluding a precise list of groups like alternating, symmetric, cyclic and dihedral, from 1st year algebra (§7.2.3), we expect there are only finitely many monodromy groups of primitive genus 0 covers. Denote this nearly proven genus 0 problem as Problem g=00 2. We call the exceptional groups 0-sporadic. Example: Finitely many Chevalley groups are 0-sporadic. A proven result: Among polynomial 0-sporadic groups, precisely three produce covers falling in nontrivial reduced families. Each (miraculously) defines one natural genus 0 ℚ cover of the j-line. The latest Nielsen class techniques apply to these dessins d’enfant to see their subtle arithmetic and interesting cusps.
John Thompson earlier considered another genus 0 problem: To find θ-functions uniformizing certain genus 0 (near) modular curves. We call this Problem g=00 1. We pose uniformization problems for j-line covers in two cases. First: From the three 0-sporadic examples of Problem g=00 2. Second: From finite collections of genus 0 curves with aspects of Problem g=00 1.
Thanks to NSF Grant #DMS-0202259, support for the Thompson Semester at the University of Florida
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Fried, M.D. (2005). Relating Two Genus 0 Problems of John Thompson. In: Voelklein, H., Shaska, T. (eds) Progress in Galois Theory. Developments in Mathematics, vol 12. Springer, Boston, MA. https://doi.org/10.1007/0-387-23534-5_4
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