Topology & Geometric Group Theory Seminar

Fall 2007

1:30 - 2:30, Malott 253

Tuesday, September 4

Roland Roeder, University of Toronto

Computing arithmetic invariants for hyperbolic reflection groups

The following is collaborative work with Omar Antolin-Camarena and Gregory Maloney from the University of Toronto.

I will demonstrate a collection of computer scripts written in PARI/GP to compute, for reflection groups determined by finite-volume polyhedra in H3, the commensurability invariants known as the invariant trace field and invariant quaternion algebra. These scripts also allow one to determine arithmeticity of such groups and the isomorphism class of the invariant quaternion algebra by analyzing its ramification.

I present many computed examples of these invariants. This is enough to show that most of the groups that we consider are pairwise incommensurable. For pairs of groups with identical invariants, not all is lost: when both groups are arithmetic, having identical invariants guarantees commensurability. We discover many "unexpected" commensurable pairs this way. We also present a non-arithmetic pair with identical invariants for which we cannot determine commensurability.

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