Proposed norm inequalities for extending the radius estimate to 1<p21<p\leq2

Let 1<p<21<p<2, set r=2p/(2p)r=2p/(2-p), and let ip2 ⁣:p2i_{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 ⁣:2pB\colon\ell_2\to\ell_p, let BSr\|B\|_{S_r} denote its rr-Schatten norm,

BSr=(Tr(BB)r/2)1/r.\|B\|_{S_r}=\bigl(\operatorname{Tr}(B^*B)^{r/2}\bigr)^{1/r}.

For a positive semi-definite operator A ⁣:ppA\colon\ell_{p'}\to\ell_p, where p=p/(p1)p'=p/(p-1), set r=p/(2p)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/(2p)r=2p/(2-p);

(b) B ⁣:2pBSr\|B\colon\ell_2\to\ell_p\|\geq\|B\|_{S_r} with r=2p/(2p)r=2p/(2-p);

(c) if AA is positive semi-definite, then A ⁣:ppASr\|A\colon\ell_{p'}\to\ell_p\|\geq\|A\|_{S_r} with r=p/(2p)r=p/(2-p) and p=p/(p1)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.

Sources & referencesView supporting material

Primary source

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

Progress summary

Never refreshed

Nothing recorded yet. Refresh searches the literature and the public web for attempts on this problem, and writes the first summary here.

Solutions 0

No solutions have been posted yet.