Computably totally disconnected locally compact groups

Abstract

We study totally disconnected, locally compact (t.d.l.c.) groups from an algorithmic perspective. We give various approaches to defining computable presentations of t.d.l.c. groups, and show their equivalence. In the process, we obtain an algorithmic Stone-type duality between t.d.l.c. groups and certain countable ordered groupoids given by the compact open cosets. We exploit the flexibility given by these different approaches to show that several natural groups, such as Aut(T d ) and SLn(Qp), have computable presentations. We show that many construction leading from t.d.l.c. groups to new t.d.l.c. groups have algorithmic versions that stay within the class of computably presented t.d.l.c. groups. This leads to further examples, such as PGLn(Qp). We study whether objects associated with computably t.d.l.c. groups are computable: the modular function, the scale function, and Cayley-Abels graphs in the compactly generated case. We give a criterion when computable presentations of t.d.l.c. groups are unique up to computable isomorphism, and apply it to Qp as an additive group, and the semidirect product Z ⋉ Qp.