Ultrametrics and surface singularities

The present lecture notes give an introduction to works of García Barroso, González Pérez, Ruggiero and the author. The starting point of those works is a theorem of Płoski, stating that one defines an ultrametric on the set of branches drawn on a smooth surface singularity by associating to any pair of distinct branches the quotient of the product of their multiplicities by their intersection number. We show how to construct ultrametrics on certain sets of branches drawn on any normal surface singularity from their mutual intersection numbers and how to interpret the associated rooted trees in terms of the dual graphs of adapted embedded resolutions. The text begins by recalling basic properties of intersection numbers and multiplicities on smooth surface singularities and the relation between ultrametrics on finite sets and rooted trees. On arbitrary normal surface singularities one has to use Mumford's definition of intersection numbers of curve singularities drawn on them, which is also recalled.

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