A combinatorial Li-Yau inequality and rational points on curves

We present a method to control gonality of nonarchimedean curves based
on graph theory. Let k denote a complete nonarchimedean valued field.We first prove
a lower bound for the gonality of a curve over the algebraic closure of k in terms of
the minimal degree of a class of graph maps, namely: one should minimize over all
so-called finite harmonic graph morphisms to trees, that originate from any refinement
of the dual graph of the stable model of the curve. Next comes our main result: we
prove a lower bound for the degree of such a graph morphism in terms of the first
eigenvalue of the Laplacian and some “volume” of the original graph; this can be seen
as a substitute for graphs of the Li–Yau inequality from differential geometry, although
we also prove that the strict analogue of the original inequality fails for general graphs.
Finally,we apply the results to give a lower bound for the gonality of arbitraryDrinfeld
modular curves over finite fields and for general congruence subgroups Γ of Γ (1) that
is linear in the index [Γ (1) : Γ ], with a constant that only depends on the residue field
degree and the degree of the chosen “infinite” place. This is a function field analogue
of a theorem of Abramovich for classical modular curves. We present applications
to uniform boundedness of torsion of rank two Drinfeld modules that improve upon
existing results, and to lower bounds on the modular degree of certain elliptic curves
over function fields that solve a problem of Papikian.