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DOI: 10.4230/LIPIcs.APPROX/RANDOM.2024.61

When Can an Expander Code Correct Ω(n) Errors in O(n) Time?

Authors: Kuan Cheng, Minghui Ouyang, Chong Shangguan, and Yuanting Shen

Published in: LIPIcs, Volume 317, Approximation, Randomization, and Combinatorial Optimization. Algorithms and Techniques (APPROX/RANDOM 2024)

Abstract

Tanner codes are graph-based linear codes whose parity-check matrices can be characterized by a bipartite graph G together with a linear inner code C₀. Expander codes are Tanner codes whose defining bipartite graph G has good expansion property. This paper is motivated by the following natural and fundamental problem in decoding expander codes: What are the sufficient and necessary conditions that δ and d₀ must satisfy, so that every bipartite expander G with vertex expansion ratio δ and every linear inner code C₀ with minimum distance d₀ together define an expander code that corrects Ω(n) errors in O(n) time? For C₀ being the parity-check code, the landmark work of Sipser and Spielman (IEEE-TIT'96) showed that δ > 3/4 is sufficient; later Viderman (ACM-TOCT'13) improved this to δ > 2/3-Ω(1) and he also showed that δ > 1/2 is necessary. For general linear code C₀, the previously best-known result of Dowling and Gao (IEEE-TIT'18) showed that d₀ = Ω(cδ^{-2}) is sufficient, where c is the left-degree of G. In this paper, we give a near-optimal solution to the above question for general C₀ by showing that δ d₀ > 3 is sufficient and δ d₀ > 1 is necessary, thereby also significantly improving Dowling-Gao’s result. We present two novel algorithms for decoding expander codes, where the first algorithm is deterministic, and the second one is randomized and has a larger decoding radius.

Cite as

Kuan Cheng, Minghui Ouyang, Chong Shangguan, and Yuanting Shen. When Can an Expander Code Correct Ω(n) Errors in O(n) Time?. In Approximation, Randomization, and Combinatorial Optimization. Algorithms and Techniques (APPROX/RANDOM 2024). Leibniz International Proceedings in Informatics (LIPIcs), Volume 317, pp. 61:1-61:23, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2024)

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@InProceedings{cheng_et_al:LIPIcs.APPROX/RANDOM.2024.61,
  author =	{Cheng, Kuan and Ouyang, Minghui and Shangguan, Chong and Shen, Yuanting},
  title =	{{When Can an Expander Code Correct \Omega(n) Errors in O(n) Time?}},
  booktitle =	{Approximation, Randomization, and Combinatorial Optimization. Algorithms and Techniques (APPROX/RANDOM 2024)},
  pages =	{61:1--61:23},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-348-5},
  ISSN =	{1868-8969},
  year =	{2024},
  volume =	{317},
  editor =	{Kumar, Amit and Ron-Zewi, Noga},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.APPROX/RANDOM.2024.61},
  URN =		{urn:nbn:de:0030-drops-210543},
  doi =		{10.4230/LIPIcs.APPROX/RANDOM.2024.61},
  annote =	{Keywords: expander codes, expander graphs, linear-time decoding}
}

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DOI: 10.4230/LIPIcs.SoCG.2024.71

Beyond Chromatic Threshold via (p,q)-Theorem, and Blow-Up Phenomenon

Authors: Hong Liu, Chong Shangguan, Jozef Skokan, and Zixiang Xu

Published in: LIPIcs, Volume 293, 40th International Symposium on Computational Geometry (SoCG 2024)

Abstract

We establish a novel connection between the well-known chromatic threshold problem in extremal combinatorics and the celebrated (p,q)-theorem in discrete geometry. In particular, for a graph G with bounded clique number and a natural density condition, we prove a (p,q)-theorem for an abstract convexity space associated with G. Our result strengthens those of Thomassen and Nikiforov on the chromatic threshold of cliques. Our (p,q)-theorem can also be viewed as a χ-boundedness result for (what we call) ultra maximal K_r-free graphs. We further show that the graphs under study are blow-ups of constant size graphs, improving a result of Oberkampf and Schacht on homomorphism threshold of cliques. Our result unravels the cause underpinning such a blow-up phenomenon, differentiating the chromatic and homomorphism threshold problems for cliques. Our result implies that for the homomorphism threshold problem, rather than the minimum degree condition usually considered in the literature, the decisive factor is a clique density condition on co-neighborhoods of vertices. More precisely, we show that if an n-vertex K_r-free graph G satisfies that the common neighborhood of every pair of non-adjacent vertices induces a subgraph with K_{r-2}-density at least ε > 0, then G must be a blow-up of some K_r-free graph F on at most 2^O(r/ε log1/ε) vertices. Furthermore, this single exponential bound is optimal. We construct examples with no K_r-free homomorphic image of size smaller than 2^Ω_r(1/ε).

Cite as

Hong Liu, Chong Shangguan, Jozef Skokan, and Zixiang Xu. Beyond Chromatic Threshold via (p,q)-Theorem, and Blow-Up Phenomenon. In 40th International Symposium on Computational Geometry (SoCG 2024). Leibniz International Proceedings in Informatics (LIPIcs), Volume 293, pp. 71:1-71:15, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2024)

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@InProceedings{liu_et_al:LIPIcs.SoCG.2024.71,
  author =	{Liu, Hong and Shangguan, Chong and Skokan, Jozef and Xu, Zixiang},
  title =	{{Beyond Chromatic Threshold via (p,q)-Theorem, and Blow-Up Phenomenon}},
  booktitle =	{40th International Symposium on Computational Geometry (SoCG 2024)},
  pages =	{71:1--71:15},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-316-4},
  ISSN =	{1868-8969},
  year =	{2024},
  volume =	{293},
  editor =	{Mulzer, Wolfgang and Phillips, Jeff M.},
  publisher =	{Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.SoCG.2024.71},
  URN =		{urn:nbn:de:0030-drops-200162},
  doi =		{10.4230/LIPIcs.SoCG.2024.71},
  annote =	{Keywords: (p,q)-theorem, fractional Helly number, weak \epsilon-net, chromatic threshold, VC dimension}
}

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Documents authored by Shangguan, Chong

When Can an Expander Code Correct Ω(n) Errors in O(n) Time?

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Beyond Chromatic Threshold via (p,q)-Theorem, and Blow-Up Phenomenon

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