Quadratic-residue distribution conjecture for breaking the square-root barrier

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Let pp be prime, let χ(x)=(xp)\chi(x)=\left(\frac{x}{p}\right) denote the Legendre symbol, and let {r1,…,rk}⊆Zp\{r_1,\ldots,r_k\}\subseteq\mathbb{Z}_p. For distinct indices i,ji,j, set a=rj−ria=r_j-r_i and

Iri,rj:={ri−rℓ:1≤ℓ≤k, ℓ≠i, ℓ≠j},I_{r_i,r_j}:=\{r_i-r_\ell:1\leq \ell\leq k,\ \ell\ne i,\ \ell\ne j\},

calling Iri,rjI_{r_i,r_j} a one-sided difference set. Quadratic-residue distribution conjecture. There exist constants 0<α<10<\alpha<1, ν>1/2\nu>1/2, and a positive integer mαm_\alpha such that, for any set {r1,…,rk}\{r_1,\ldots,r_k\} in Zp\mathbb{Z}_p with mα≤k≤pνm_\alpha\leq k\leq p^\nu, there exist indices 1≤i<j≤k1\leq i<j\leq k satisfying

∣∑ℓ∈Iri,rjχ(ℓ)χ(ℓ+a)∣∣Iri,rj∣<α.\frac{\left|\sum_{\ell\in I_{r_i,r_j}}\chi(\ell)\chi(\ell+a)\right|}{|I_{r_i,r_j}|}<\alpha.

This conjecture would provide a route to breaking the square-root barrier for the specified compressed-sensing construction through improved eigenvalue bounds; its resolution is not supplied here.

References

Primary source

Arman Arian and Ozgur Yilmaz, “RIP constants for deterministic compressed sensing matrices-beyond Gershgorin”, arXiv:1911.07428 (2019).

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