Conjecture on weighted frame representations of invertible unconditionally convergent multipliers

From papers

Let H{\cal H} be a Hilbert space, let Φ=(φn)\Phi=(\varphi_n) and Ψ=(ψn)\Psi=(\psi_n) be sequences in H{\cal H}, and let m=(mn)m=(m_n) be a complex sequence. The multiplier Mm,Φ,ΨM_{m,\Phi,\Psi} is given by

Mm,Φ,Ψf=nmnf,ψnφn.M_{m,\Phi,\Psi}f=\sum_n m_n\langle f,\psi_n\rangle\varphi_n.

It is unconditionally convergent if this series converges unconditionally for every fHf\in{\cal H}. A sequence in H{\cal H} is a frame for H{\cal H} if it satisfies the frame inequalities, and a multiplier is invertible when it is an invertible bounded operator on H{\cal H}. Weighted frame representation conjecture. Let Mm,Φ,ΨM_{m,\Phi,\Psi} be an unconditionally convergent multiplier which is invertible on H{\cal H}. Then there exist (αn)C(\alpha_n)\subset\mathbb C and (βn)C(\beta_n)\subset\mathbb C such that (αnφn)(\alpha_n\varphi_n) and (βnψn)(\beta_n\psi_n) are frames for H{\cal H} and

αnβn=mn\alpha_n\overline{\beta_n}=m_n

for all nn. This asks whether every invertible unconditionally convergent multiplier admits a representation using weighted versions of its two defining sequences that are both frames; the context states that this question was posed as a conjecture, while its general status is not otherwise resolved here.

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Sources & referencesView supporting material

Primary source

Peter Balazs, Rosario Corso and Diana Stoeva, “Weighted frames, weighted lower semi frames and unconditionally convergent multipliers”, arXiv:2310.18957 (2023).

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