Tree representations of Galois groups

Nigel Boston

Published 2000-09-18Version 1

Much work has gone into matrix representations of Galois groups, but there is a whole new class of naturally occurring representations that have as yet gone almost unnoticed. In fact, it is well-known in various areas of mathematics that the main sources of totally disconnected groups are matrix groups over local fields AND automorphism groups of locally finite trees. It is perhaps surprising then that representations of Galois groups into the latter have been almost ignored, while at the same time Galois representations into the former have been enormously effective in resolving long-standing problems in number theory. These ``tree'' representations are important as regards topics such as the unramified Fontaine-Mazur conjecture. This conjecture states that any p-adic representation of the Galois group of an extension unramified at p (and ramified at only finitely many primes) should have finite image. In other words, p-adic representations say little about such Galois groups. This paper proposes the conjecture that these Galois groups should, on the other hand, have representations with large image (measured by Hausdorff dimension) in the automorphism group of a rooted tree. Thus, they might allow us to investigate the structure of the Galois group of infinite pro-p extensions (such as Hilbert p-class towers), something unapproachable by standard p-adic representation methods.