Besides the oscillator group, there is another four-dimensional non-abelian solvable Lie group that admits a bi-invariant pseudo-Riemannian metric.It is called hyperbolic oscillator group (sometimes also split oscillator group or Boidol's group).We parametrise the set of lattices in this group and develop a method to classify these lattices up automorphisms of the ambient group.We show that their commensurability classes are in bijection with the set of real quadratic fields.
Galiay et al. (Sat,) studied this question.
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