Totally disconnected group
Interest centres on locally compact totally disconnected groups (variously referred to as groups of td-type, locally profinite groups, t.d. groups). The compact case has been heavily studied – these are the profinite groups – but for a long time not much was known about the general case. A theorem of van Dantzig from the 1930s, stating that every such group contains a compact open subgroup, was all that was known. Then groundbreaking work on this subject was done in 1994, when George Willis showed that every locally compact totally disconnected group contains a so-called tidy subgroup and a special function on its automorphisms, the scale function, thereby advancing the knowledge of the local structure. Advances on the global structure of totally disconnected groups have been obtained in 2011 by Caprace and Monod, with notably a clasification of characteristically simple groups and of Noetherian groups.
Locally compact case
In a locally compact, totally disconnected group, every neighbourhood of the identity contains a compact open subgroup. Conversely, if a group is such that the identity has a neighbourhood basis consisting of compact open subgroups, then it is locally compact and totally disconnected.
The scale function
The index of in is shown to be finite and independent of the U which is tidy for . Define the scale function as this index. Restriction to inner automorphisms gives a function on G with interesting properties. These are in particular:
Define the function on G by , where is the inner automorphism of on G.
Calculations and applications
- G.A. Willis - The structure of totally disconnected, locally compact groups, Mathematische Annalen 300, 341-363 (1994)