Paper detail

Multiplicities of the Betti map associated to a section of an elliptic surface from a differential-geometric perspective

For the study of the Mordell-Weil group of an elliptic curve ${\bf E}$ over a complex function field of a projective curve $B$, the first author introduced the use of differential-geometric methods arising from Kähler metrics on $\mathcal H \times \mathbb C$ invariant under the action of the semi-direct product ${\rm SL}(2,\mathbb R) \ltimes \mathbb R^2$. To a properly chosen geometric model $π: \mathcal E \to B$ of ${\bf E}$ as an elliptic surface and a non-torsion holomorphic section $σ: B \to \mathcal E$ there is an associated ``verticality'' $η_σ$ of $σ$ related to the locally defined Betti map. The first-order linear differential equation satisfied by $η_σ$, expressed in terms of invariant metrics, is made use of to count the zeros of $η_σ$, in the case when the regular locus $B^0\subset B$ of $π: \mathcal E \to B$ admits a classifying map $f_0$ into a modular curve for elliptic curves with level-$k$ structure, $k \ge 3$, explicitly and linearly in terms of the degree of the ramification divisor $R_{f_0}$ of the classifying map, and the degree of the log-canonical line bundle of $B^0$ in $B$. Our method highlights ${\rm deg}(R_{f_0})$ in the estimates, and recovers the effective estimate obtained by a different method of Ulmer-Urzúa on the multiplicities of the Betti map associated to a non-torsion section, noting that the finiteness of zeros of $η_σ$ was due to Corvaja-Demeio-Masser-Zannier. The role of $R_{f_0}$ is natural in the subject given that in the case of an elliptic modular surface there is no non-torsion section by a theorem of Shioda, for which a differential-geometric proof had been given by the first author. Our approach sheds light on the study of non-torsion sections of certain abelian schemes.