Proposed norm inequalities for extending the radius estimate to 1<p≤21<p\leq2

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Let 1<p<21<p<2, set r=2p/(2−p)r=2p/(2-p), and let ip2 ⁣:ℓp→ℓ2i_{p2}\colon\ell_p\to\ell_2 be the formal identity map. Let πr,2(⋅)\pi_{r,2}(\cdot) denote the (r,2)(r,2)-absolutely summing norm. For a linear map B ⁣:ℓ2→ℓpB\colon\ell_2\to\ell_p, let ∥B∥Sr\|B\|_{S_r} denote its rr-Schatten norm,

∥B∥Sr=(Tr⁡(B∗B)r/2)1/r.\|B\|_{S_r}=\bigl(\operatorname{Tr}(B^*B)^{r/2}\bigr)^{1/r}.

For a positive semi-definite operator A ⁣:ℓp′→ℓpA\colon\ell_{p'}\to\ell_p, where p′=p/(p−1)p'=p/(p-1), set r=p/(2−p)r=p/(2-p) when referring to the third inequality. Norm-inequality conjectures. The following statements are proposed as problems/conjectures:

(a) πr,2(ip2)≤1\pi_{r,2}(i_{p2})\leq1 for p∈(1,2)p\in(1,2), with r=2p/(2−p)r=2p/(2-p);

(b) ∥B ⁣:ℓ2→ℓp∥≥∥B∥Sr\|B\colon\ell_2\to\ell_p\|\geq\|B\|_{S_r} with r=2p/(2−p)r=2p/(2-p);

(c) if AA is positive semi-definite, then ∥A ⁣:ℓp′→ℓp∥≥∥A∥Sr\|A\colon\ell_{p'}\to\ell_p\|\geq\|A\|_{S_r} with r=p/(2−p)r=p/(2-p) and p′=p/(p−1)p'=p/(p-1).

These are presented as possible counterparts to the lemma used to obtain a sharp estimate on λ′\lambda' and are not resolved in the supplied text. They correspond to different proofs of the preceding norm lemma.

References

Primary source

Stanislaw Szarek and Pawel Wolff, “Radii of Euclidean sections of _p-balls”, arXiv:2410.15118 (2025).

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