http://en.wikipedia.org/wiki/Profinite_group. http://eom.springer.de/p/p075050.htm. http://planetmath.org/encyclopedia/ProfiniteGroup.html. http://www.msri.org/publications/ln/msri/1999/vonneumann/haran/1/. http://www.msri.org/publications/ln/msri/1999/vonneumann/mestre/1/.