Sharp distortion conjecture for LqL_q integer grids in LpL_p

For 2<q<p2<q<p and m,nNm,n\in\mathbb N, let [m]qn[m]_q^n denote the nn-dimensional integer grid with its q\ell_q metric, and let cp([m]qn)c_p([m]_q^n) be its least bi-Lipschitz distortion in LpL_p. Let J(qp;n)RJ^R_{(q\to p;n)} and J(qp;n)SJ^S_{(q\to p;n)} be the Rosenthal and Schoenberg embeddings introduced in the source. Sharp grid distortion conjecture. The better of these two embeddings is optimal up to constants depending only on pp and qq; equivalently,

cp([m]qn)p,qmin{n(pq)(q2)q2(p2),m12q}.c_p([m]_q^n)\asymp_{p,q}\min\left\{n^{\frac{(p-q)(q-2)}{q^2(p-2)}},m^{1-\frac{2}{q}}\right\}.

In particular, there is η(p,q)(0,1)\eta(p,q)\in(0,1) such that mnpqq(p2)m\geqslant n^{\frac{p-q}{q(p-2)}} implies cp([m]qn)η(p,q)cp(qn)c_p([m]_q^n)\geqslant\eta(p,q)c_p(\ell_q^n), whereas m=o(npqq(p2))m=o(n^{\frac{p-q}{q(p-2)}}) implies cp([m]qn)=o(cp(qn))c_p([m]_q^n)=o(c_p(\ell_q^n)). The conjecture compares the two known constructions and is explicitly left open.

Sources & referencesView supporting material

Primary source

Assaf Naor and Gideon Schechtman, “Metric X_p inequalities”, arXiv:1408.5819 (2015).

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