Hardy spaces on Riemann surfaces under ramified coverings

Abstract

We extend the theory of indefinite Hardy spaces on finite bordered Riemann surfaces to the setting of ramified analytic coverings. Given a finite $n$-sheeted ramified covering $F\colon S_1\to S_2$ of finite bordered Riemann surfaces satisfying a spin-compatibility hypothesis, we construct (i) the direct image of a unitary flat vector bundle $\VxX{1}\otimes \Del{1}$ on the double $X_1$ under $F$, taking full account of the ramification divisor $R_F$ and establishing the extension across the branch locus via a local analysis; (ii) a canonical matrix function $G_2$ encoding the parahermitian structure on $X_2$, together with the induced representation $\chi_2$ of $\piX{X_2}{p_0}$; (iii) an explicit isometric isomorphism $\phi_F\colon H^{2,J_1(p)}(S_1,\VxS{1}\otimes\Del{1}) \xrightarrow{\;\sim\;} H^{2,J_2(p)}(S_2,\VxS{2}\otimes\Del{2})$ between the associated Hardy-Kre\u{\i}n spaces, provided that $h^0(X_1,\VxX{1}\otimes\Del{1})=0$ and that the branch locus is disjoint from $\partial S_2$. We then develop the resulting operator theory in terms of vessels and Bezoutian operators. To each object in the category $\mathcal{RH}$ of finite bordered surfaces with unitary flat bundles we attach a triangular vessel whose input and output spaces are the Hardy-Kre\u{\i}n spaces on the two surfaces. The Bezoutian of the vessel is expressed as a finite-rank operator on $\mathcal{H}_2$ whose kernel is built from bounded holomorphic point-evaluation functionals in $\mathcal{H}_2$ evaluated at the interior ramification images $F(r_\nu)\in S_2$, consistently with the boundary-transversality hypothesis $\partial S_2\cap B_F=\left\{\varnothing\right\}$. We prove that the assignment $(S,\Vx{},J)\mapsto H^{2,J(p)}(S,\Vx{}\otimes\Delta)$ extends to a covariant functor from $\mathcal{RH}$ (with ramified morphisms) to the category of Kre\u{\i}n spaces.