Analogues of hyperlogarithm functions on affine complex curves

Abstract

For $C$ a smooth affine complex curve, there is a unique minimal subalgebra $A_C$ of the algebra $\mathcal O_{hol}(\tilde C)$ of holomorphic functions on its universal cover $\tilde C$, which is stable under all the operations $f\mapsto \int f\omega$, for $\omega$ in the space $\Omega(C)$ of regular differentials on $C$. We identify $A_C$ with the image of the iterated integration map $I_{x_0} : \mathrm{Sh}(\Omega(C))\to\mathcal O_{hol}(\tilde C)$ based at any point $x_0$ of $\tilde C$ (here $\mathrm{Sh}(-)$ denotes the shuffle algebra of a vector space), as well as with the unipotent part, with respect to the action of $\mathrm{Aut}(\tilde C/C)$, of a subalgebra of $\mathcal O_{hol}(\tilde C)$ of moderate growth functions. We show that any regular Maurer-Cartan (MC) element $J$ on $C$ with values in the topologically free Lie algebra over $\mathrm H^1_{\mathrm{dR}}(C)^*$ gives rise to an isomorphism of $A_C$ with $\mathcal O(C) \otimes\mathrm{Sh}(\mathrm H^1_{\mathrm{dR}}(C))$, where $\mathcal O(C)$ is the algebra of regular functions on $C$, leading to the assignment of a subalgebra $\mathcal H_C(J)$ of $A_C$ (isomorphic to $\mathrm{Sh}(\mathrm H^1_{\mathrm{dR}}(C))$) to any MC element. We also associate a MC element $J_\sigma$ to each section $\sigma$ of the projection $\Omega(C)\to \mathrm H^1_{\mathrm{dR}}(C)$; when $C$ has genus $0$, we exhibit a particular section $\sigma_0$ for which $\mathcal H_C(J_{\sigma_0})$ is the algebra of hyperlogarithm functions (Poincar\'e, Lappo-Danilevsky).