Theta characteristics of hyperelliptic graphs

Abstract

We study theta characteristics of hyperelliptic metric graphs of genus $g$ with no bridge edges. These graphs have a harmonic morphism of degree two to a metric tree that can be lifted to morphism of degree two of a hyperelliptic curve $X$ over $K$ to the projective line, with $K$ an algebraically closed field of char$(K) \not =2$, complete with respect to a non-Archimedean valuation, with residue field $k$ of char$(k)\not=2$. The hyperelliptic curve has $2^{2g}$ theta characteristics. We show that for each effective theta characteristics on the graph, $2^{g-1}$ even and $2^{g-1}$ odd theta characteristics on the curve specialize to it; and $2^g$ even theta characteristics on the curve specialize to the unique not effective theta characteristics on the graph.