Surface Algebras I: Dessins D'enfants, Surface Algebras, and Dessin Orders

Abstract

In this paper, a construction of an infinite dimensional associative algebra, which will be called a \emph{Surface Algebra}, is associated in a "canonical" way to a dessin d'enfant, or more generally, a cellularly embedded graph in a Riemann surface. Once the surface algebras are constructed we will see a construction of what we call here the associated \emph{Dessin Order} or more generally the \emph{Surface Order}. This provides a way of associating to every algebraic curve $X$, with function field $k(X)$ (defined over an arbitrary field $k$) the representation theory of its Surface Algebra and the lattices over Surface Orders, which are defined as pullbacks of certain matrix algebras over commutative $k$-algebras. We will then be able to prove that the center and the (noncommutative) normalization of the surface orders are invariant under the action of the absolute Galois group $\mathcal{G}(\overline{\mathbb{Q}}/\mathbb{Q})$. We will see that the surface algebras and surface orders are closely related to the fundamental group(oid) of the Riemann surfaces and the associated monodromy group. A description of the projective resolutions of the simple modules over the surface order is given and it will be shown that one can completely recover the dessin with the projective resolutions of the simple modules alone. In particular, the projective resolutions of the simple modules encode all combinatorial and topological data of the monodromy group (or cartographic group) of a dessin. Finally, as a corollary we are able to say that classifying dessins in an orbit of $\mathcal{G}(\overline{\mathbb{Q}}/\mathbb{Q})$ is equivalent to classifying dessin orders with a given normalization. We end with some further examples of surface algebras and surface orders related to the classical and geometric version of the Langlands Program.