On the exceptional fibres of Kleinian singularities

We give a new proof, avoiding case-by-case analysis, of a theorem of Y. Ito and I. Nakamura which provides a module-theoretic interpretation of the bijection between the irreducible components of the exceptional fibre for a Kleinian singularity, and the nontrivial simple modules for the corresponding finite subgroup of SL (2, [inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="01i" /]). Our proof uses a classification of certain cyclic modules for preprojective algebras.