Dear coll WG, Torsten and I have tried to write up more systematically some of the issues and possibilities that arise in the design of sparse collectives for MPI, and made a concrete proposal for a small set of sparse collectives. We hope this will generate some serious discussion; all sorts of comments, suggestions, criticisms of course most welcome. Leaving aside improvements and changes to the interface, the more interesting question to the Forum is of course whether this kind of functionality helps enough users and/or library builders enough that it should be included in a future MPI standard The discussion/proposal has been put on the Wiki, Collectives WG under III. Sparse/Topological collectives best regards Jesper