Discretely approximable locally compact groups and Jessen's theorem for nilmanifolds

Hamrouni, Hatem; Kadri, Bilel

doi:10.1016/j.bulsci.2015.11.002

Cited by 3 publications

(1 citation statement)

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“…Moreover, note that a connected ADS locally compact group G is necessarily nilpotent (see [75,Theorem 2.18]) and that a connected simply connected Lie group is ADS if and only if G is nilpotent and if it admits a discrete cocompact subgroup ([73, Theorem 1.6, 1.7 and 1.9]. We refer to [73] [74] [75] [101] [150] and [152] for more information on this notion. Now, we introduce different notions of approximation by discrete groups.…”

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confidence: 99%

Projections, multipliers and decomposable maps on noncommutative $\mathrm{L}^p$-spaces

Arhancet¹,

Kriegler²

2017

Preprint

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We describe a noncommutative analogue of the absolute value of a regular operator acting on a noncommutative L p -space. We equally prove that two classical operator norms, the regular norm and the decomposable norm are identical. We also describe precisely the regular norm of several classes of regular multipliers. This includes Schur multipliers and Fourier multipliers on some unimodular locally compact groups which can be approximated by discrete groups in various senses. A main ingredient is to show the existence of a bounded projection from the space of completely bounded L p operators onto the subspace of Schur or Fourier multipliers, preserving complete positivity. On the other hand, we show the existence of bounded Fourier multipliers which cannot be approximated by regular operators, on large classes of locally compact groups, including all infinite abelian locally compact groups. We finish by introducing a general procedure for proving positive results on selfadjoint contractively decomposable Fourier multipliers, beyond the amenable case.

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confidence: 99%