On the enumerative geometry of branched covers of curves
By: Carl Lian
Publisher: [publisher not identified]
Published: 2020
Place of publication: [New York, N.Y.?]
Language: eng
Description: In this thesis, we undertake two computations in enumerative geometry involving branched covers of algebraic curves. Firstly, we consider the general problem of enumerating branched covers of the projective line from a fixed general curve subject to ramification conditions at possibly moving points. Our main computations are in genus 1; the theory of limit linear series allows one to reduce to this case. We first obtain a simple formula for a weighted count of pencils on a fixed elliptic curve E, where base-points are allowed. We then deduce, using an inclusion-exclusion procedure, formulas for the numbers of maps E → P1 with moving ramification conditions. A striking consequence is the invariance of these counts under a certain involution. Our results generalize work of Harris, Logan, Osserman, and Farkas-Moschetti-Naranjo-Pirola. Secondly, we consider the loci of curves of genus 2 and 3 admitting a d-to-1 map to a genus 1 curve. After compactifying these loci via admissible covers, we obtain formulas for their Chow classes, recovering results of Faber-Pagani and van Zelm when d = 2.

The answers exhibit quasimodularity properties similar to those in the Gromov- Witten theory of a fixed genus 1 curve; we conjecture that the quasimodularity persists in higher genus, and indicate a number of possible variants.
Notes: Department: Mathematics.

Thesis advisor: Aise Johan de Jong.

Thesis (Ph.D.)--Columbia University, 2020.