Spectral theory of approximate lattices in nilpotent Lie groups

We show that an approximate lattice in a nilpotent Lie group admits a relatively dense subset of central $(1-epsilon)$-Bragg peaks for every $epsilon > 0$. For the Heisenberg group we deduce that the union of horizontal and vertical $(1-epsilon)$-Bragg peaks is relatively dense in the unitary dual. More generally we study uniform approximate lattices in extensions of lcsc groups. We obtain necesary and sufficient conditions for the existence of a continuous horizontal factor of the associated hull-dynamical system, and study the spectral theory of the hull-dynamical system relative to this horizontal factor.