16 citations · 60 across the 17 of their papers we have counts for
18 papers
A Balian-Low theorem for nilpotent Lie groups
Ulrik Enstad, Jordy Timo van Velthoven
We prove strict necessary density conditions for smooth coherent frames and Riesz sequences in the orbit of a connected, simply connected, nilpotent Lie group. This extends previou…
On the Calderón sum formula for wavelet systems
Ulrik Enstad, Jordy Timo van Velthoven
We show that the Calderón sum formula for orthonormal wavelet bases holds for arbitrary dilation and translation matrices under a mild condition on the wavelet function. This parti…
Exponential frames and Riesz sequences at the critical density
Ulrik Enstad, Jordy Timo van Velthoven
We characterize exponential systems on sets of finite measure that form a frame or a Riesz sequence at the critical density. Namely, they are precisely those systems for which the…
Bessel duality of Gabor systems: A von Neumann algebraic perspective
Ulrik Enstad, Franz Luef
Bessel duality of regular Gabor systems states that a Gabor system over a lattice is a Bessel sequence if and only if the corresponding Gabor system over the adjoint lattice is a B…
A Duflo-Moore theorem for ergodic group actions on semifinite von Neumann algebras
Ulrik Enstad, Hannes Wendt
We prove a generalization of the orthogonality relations of Duflo and Moore for ergodic, trace-preserving group actions on von Neumann algebras that are integrable in a suitable se…
Z-stability of twisted group C*-algebras of nilpotent groups
Ulrik Enstad, Eduard Vilalta
We prove that the twisted group C*-algebra of a finitely generated nilpotent group is -stable if and only if it is nowhere scattered, a condition that we characterize…
Criteria for the existence of Schwartz Gabor frames over rational lattices
Ulrik Enstad, Hannes Thiel, Eduard Vilalta
We give an explicit criterion for a rational lattice in the time-frequency plane to admit a Gabor frame with window in the Schwartz class. The criterion is an inequality formulated…
Free actions of polynomial growth Lie groups and classifiable C*-algebras
Ulrik Enstad, Gabriel Favre, Sven Raum
We show that any free action of a connected Lie group of polynomial growth on a finite dimensional locally compact space has finite tube dimension. This is shown to imply that the…
Linear independence of coherent systems associated to discrete subgroups
Ulrik Enstad, Jordy Timo van Velthoven
This note considers the finite linear independence of coherent systems associated to discrete subgroups. We show by simple arguments that such coherent systems of amenable groups a…
Coherent systems over approximate lattices in amenable groups
Ulrik Enstad, Jordy Timo van Velthoven
Let be a second-countable amenable group with a uniform -approximate lattice . For a projective discrete series representation of of formal degre…
A dynamical approach to sampling and interpolation in unimodular groups
Ulrik Enstad, Sven Raum
We introduce a notion of covolume for point sets in locally compact groups that simultaneously generalizes the covolume of a lattice and the reciprocal of the Beurling density for…
On sufficient density conditions for lattice orbits of relative discrete series
Ulrik Enstad, Jordy Timo van Velthoven
This note provides new criteria on a unimodular group and a discrete series representation of formal degree under which any lattice wit…
Smooth lattice orbits of nilpotent groups and strict comparison of projections
Erik Bédos, Ulrik Enstad, Jordy Timo van Velthoven
This paper provides sufficient density conditions for the existence of smooth vectors generating a frame or Riesz sequence in the lattice orbit of a square-integrable projective re…
The density theorem for projective representations via twisted group von Neumann algebras
Ulrik Enstad
We consider converses to the density theorem for irreducible, projective, unitary group representations restricted to lattices using the dimension theory of Hilbert modules over tw…
Deformations and Balian-Low theorems for Gabor frames on the adeles
Ulrik Enstad, Mads S. Jakobsen, Franz Luef +1
We generalize Feichtinger and Kaiblinger's theorem on linear deformations of uniform Gabor frames to the setting of a locally compact abelian group . More precisely, we show tha…
The Balian-Low theorem for locally compact abelian groups and vector bundles
Ulrik Enstad
Let be a lattice in a second countable, locally compact abelian group with annihilator . We investigate the validity of the following state…
Heisenberg modules as function spaces
Are Austad, Ulrik Enstad
Let be a closed, cocompact subgroup of , where is a second countable, locally compact abelian group. Using localization of Hilbert -modules, we s…
Time-frequency analysis on the adeles over the rationals
Ulrik B. R. Enstad, Mads S. Jakobsen, Franz Luef
We show that the construction of Gabor frames in with generators in and with respect to time-frequency shifts from a rectangular la…