The pointwise operator-norm divergence conjecture for stable LTI systems

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Let Λ={λn}n∈Z\Lambda = \{\lambda_n\}_{n\in\mathbb{Z}} be a complete interpolating sequence for PWπ2\mathcal{PW}^{2}_{\pi}. Let H:PWπ1→PWπ1\mathrm{H}:\mathcal{PW}^{1}_{\pi}\to\mathcal{PW}^{1}_{\pi} be a stable LTI system, let HN\mathrm{H}_N be its sampling-based approximation, and for t∈Rt\in\mathbb{R} and β∈(0,1]\beta\in(0,1] define

∥HN∥t,β=sup⁡{∣(HNf)(t)∣:f∈PWβπ1, ∥f∥PWβπ1≤1}.\|\mathrm{H}_N\|_{t,\beta}=\sup\{ |(\mathrm{H}_Nf)(t)|: f\in\mathcal{PW}^{1}_{\beta\pi},\ \|f\|_{\mathcal{PW}^{1}_{\beta\pi}}\leq 1\}.

Pointwise operator-norm divergence conjecture. For every t∈Rt\in\mathbb{R}, there exists a stable LTI system H:PWπ1→PWπ1\mathrm{H}:\mathcal{PW}^{1}_{\pi}\to\mathcal{PW}^{1}_{\pi} such that, for every β∈(0,1]\beta\in(0,1],

lim⁡N→∞∥HN∥t,β=+∞.\lim_{N\to\infty}\|\mathrm{H}_N\|_{t,\beta}=+\infty.

This would give a negative answer to the question of whether a subsequence of the approximation operators can converge globally uniformly to Hf\mathrm{H}f for every f∈PWπ1f\in\mathcal{PW}^{1}_{\pi}. The source notes that only a limsup divergence result is currently known.

References

Primary source

Holger Boche and Volker Pohl, “System Approximations and Generalized Measurements in Modern Sampling Theory”, arXiv:1410.5872 (2014).

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