p-Spread and Restricted Isometry Properties of Sparse Random Matrices

Venkatesan Guruswami, Peter Manohar, Jonathan Mosheiff

Research output: Chapter in Book/Report/Conference proceedingConference contributionpeer-review


Random subspaces X of ℝn of dimension proportional to n are, with high probability, well-spread with respect to the ℓ2-norm. Namely, every nonzero x ∈ X is “robustly non-sparse” in the following sense: x is ε ∥x∥2-far in ℓ2-distance from all δn-sparse vectors, for positive constants ε, δ bounded away from 0. This “ℓ2-spread” property is the natural counterpart, for subspaces over the reals, of the minimum distance of linear codes over finite fields, and corresponds to X being a Euclidean section of the ℓ1 unit ball. Explicit ℓ2-spread subspaces of dimension Ω(n), however, are unknown, and the best known explicit constructions (which achieve weaker spread properties), are analogs of low density parity check (LDPC) codes over the reals, i.e., they are kernels of certain sparse matrices. Motivated by this, we study the spread properties of the kernels of sparse random matrices. We prove that with high probability such subspaces contain vectors x that are o(1) · ∥x∥2-close to o(n)-sparse with respect to the ℓ2-norm, and in particular are not ℓ2-spread. This is strikingly different from the case of random LDPC codes, whose distance is asymptotically almost as good as that of (dense) random linear codes. On the other hand, for p < 2 we prove that such subspaces are ℓp-spread with high probability. The spread property of sparse random matrices thus exhibits a threshold behavior at p = 2. Our proof for p < 2 moreover shows that a random sparse matrix has the stronger restricted isometry property (RIP) with respect to the ℓp norm, and in fact this follows solely from the unique expansion of a random biregular graph, yielding a somewhat unexpected generalization of a similar result for the ℓ1 norm [6]. Instantiating this with suitable explicit expanders, we obtain the first explicit constructions of ℓp-RIP matrices for 1 ≤ p < p0, where 1 < p0 < 2 is an absolute constant.

Original languageEnglish
Title of host publication37th Computational Complexity Conference, CCC 2022
EditorsShachar Lovett
PublisherSchloss Dagstuhl- Leibniz-Zentrum fur Informatik GmbH, Dagstuhl Publishing
ISBN (Electronic)9783959772419
StatePublished - 1 Jul 2022
Externally publishedYes
Event37th Computational Complexity Conference, CCC 2022 - Philadelphia, United States
Duration: 20 Jul 202223 Jul 2022

Publication series

NameLeibniz International Proceedings in Informatics, LIPIcs
ISSN (Print)1868-8969


Conference37th Computational Complexity Conference, CCC 2022
Country/TerritoryUnited States


  • Euclidean Sections
  • Restricted Isometry Property
  • Sparse Matrices
  • Spread Subspaces

ASJC Scopus subject areas

  • Software

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