Representative functions on discrete groupoids and duality with Hopf algebroids

The aim of this paper is to establish a duality between the category of discrete groupoids and the category of geometrically transitive commutative Hopf algebroids in the sense of P. Deligne and A. Brugui`eres. In one direction we have the usual contravariant functor which assigns to each Hopf algebroid its characters groupoid (the fiber groupoid at the ground field). In the other direction we construct the contravariant functor which associated to each discrete groupoid its Hopf algebroid of representative functions. This duality extends the well known duality between discrete groups and commutative Hopf algebras, and also sheds light on a new approach to Tannaka-Krein duality for compact topological groupoids. Our results are supported by several illustrative examples including topological ones.