Let α be an automorphism of the totally disconnected group G. The compact open subgroup, V, of G is tidy for α if [α(V') : α(V')∩ V'] is minimised at V, where V' ranges over all compact open subgroups of G. Identifying a subgroup tidy for α is analogous to identifying a basis which puts a linear transformation into Jordan canonical form. This analogy is developed here by showing that commuting automorphisms have a common tidy subgroup of G and, conversely, that a group \siH of automorphisms having a common tidy subgroup V is abelian modulo the automorphisms which leave V invariant. Certain subgroups of G are the analogues of eigenspaces and corresponding real characters of \siH the analogues of eigenvalues.

Research supported by A.R.C. Grant A69700321