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Let $v$ be a vector $v \in \mathbb{R}^p$, with non-negative entries and $p$ prime. The Hausdorff-Young inequality gives bounds of the form: $$\|\mathcal{F}v\|_a \le C_{a,b} \|v\|_b$$ where the constants $C_{a,b}$ are explicitly known. There is a nice paper that gives an explicit characterization of all the $v$ for which this inequality is achieved. The proof is to break up the Riesz square into three different "regimes"; understand the extreme points; and then use Riesz-Thorin to interpolate to the rest of the square.

My question: I would like a quantitative understanding of what $v$ looks like when this inequality is far from sharp. Is there a quantitative version of the Hausdorff-Young inequality or Reisz interpolation theorem?

It appears Michael Christ showed exactly this, for functions over $\mathbb{R}^d$ instead of over $\mathbb{Z}_p$, https://arxiv.org/pdf/1406.1210.pdf. I am looking for an analogue for finite cyclic groups. (Not sure if this is easier or harder).

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    $\begingroup$ Thank you for the catches! Edited. Regarding your first question; yes my group structure is just the finite cyclic group. $\endgroup$ – DJA Jan 27 at 19:43

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