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RANDOM

**Published in:** LIPIcs, Volume 275, Approximation, Randomization, and Combinatorial Optimization. Algorithms and Techniques (APPROX/RANDOM 2023)

Good codes over an alphabet of constant size q can approach but not surpass distance 1-1/q. This makes the use of q-ary codes a necessity in some applications, and much work has been devoted to the case of constant alphabet q. In the large distance regime, namely, distance 1-1/q-ε for small ε > 0, the Gilbert-Varshamov (GV) bound asserts that rate Ω_q(ε²) is achievable whereas the q-ary MRRW bound gives a rate upper bound of O_q(ε²log(1/ε)). In this sense, the GV bound is almost optimal in this regime. Prior to this work there was no known explicit and efficiently decodable q-ary codes near the GV bound, in this large distance regime, for any constant q ≥ 3.
We design an Õ_{ε,q}(N) time decoder for explicit (expander based) families of linear codes C_{N,q,ε} ⊆ F_q^N of distance (1-1/q)(1-ε) and rate Ω_q(ε^{2+o(1)}), for any desired ε > 0 and any constant prime q, namely, almost optimal in this regime. These codes are ε-balanced,i.e., for every non-zero codeword, the frequency of each symbol lies in the interval [1/q - ε, 1/q + ε]. A key ingredient of the q-ary decoder is a new near-linear time approximation algorithm for linear equations (k-LIN) over ℤ_q on expanding hypergraphs, in particular, those naturally arising in the decoding of these codes.
We also investigate k-CSPs on expanding hypergraphs in more generality. We show that special trade-offs available for k-LIN over ℤ_q hold for linear equations over a finite group. To handle general finite groups, we design a new matrix version of weak regularity for expanding hypergraphs. We also obtain a near-linear time approximation algorithm for general expanding k-CSPs over q-ary alphabet. This later algorithm runs in time Õ_{k,q}(m + n), where m is the number of constraints and n is the number of variables. This improves the previous best running time of O(n^{Θ_{k,q}(1)}) by a Sum-of-Squares based algorithm of [AJT, 2019] (in the expanding regular case).
We obtain our results by generalizing the framework of [JST, 2021] based on weak regularity decomposition for expanding hypergraphs. This framework was originally designed for binary k-XOR with the goal of providing near-linear time decoder for explicit binary codes, near the GV bound, from the breakthrough work of Ta-Shma [STOC, 2017]. The explicit families of codes over prime F_q are based on suitable instatiations of the Jalan-Moshkovitz (Abelian) generalization of Ta-Shma’s distance amplification procedure.

Fernando Granha Jeronimo. Fast Decoding of Explicit Almost Optimal ε-Balanced q-Ary Codes And Fast Approximation of Expanding k-CSPs. In Approximation, Randomization, and Combinatorial Optimization. Algorithms and Techniques (APPROX/RANDOM 2023). Leibniz International Proceedings in Informatics (LIPIcs), Volume 275, pp. 60:1-60:16, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2023)

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@InProceedings{jeronimo:LIPIcs.APPROX/RANDOM.2023.60, author = {Jeronimo, Fernando Granha}, title = {{Fast Decoding of Explicit Almost Optimal \epsilon-Balanced q-Ary Codes And Fast Approximation of Expanding k-CSPs}}, booktitle = {Approximation, Randomization, and Combinatorial Optimization. Algorithms and Techniques (APPROX/RANDOM 2023)}, pages = {60:1--60:16}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-296-9}, ISSN = {1868-8969}, year = {2023}, volume = {275}, editor = {Megow, Nicole and Smith, Adam}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.APPROX/RANDOM.2023.60}, URN = {urn:nbn:de:0030-drops-188858}, doi = {10.4230/LIPIcs.APPROX/RANDOM.2023.60}, annote = {Keywords: Decoding, Approximation, GV bound, CSPs, HDXs, Regularity} }

Document

**Published in:** LIPIcs, Volume 251, 14th Innovations in Theoretical Computer Science Conference (ITCS 2023)

Determining the maximum size A₂(n,d) of a binary code of blocklength n and distance d remains an elusive open question even when restricted to the important class of linear codes. Recently, two linear programming hierarchies extending Delsarte’s LP were independently proposed to upper bound A₂^{Lin}(n,d) (the analogue of A₂(n,d) for linear codes). One of these hierarchies, by the authors, was shown to be approximately complete in the sense that the hierarchy converges to A₂^{Lin}(n,d) as the level grows beyond n². Despite some structural similarities, not even approximate completeness was known for the other hierarchy by Loyfer and Linial.
In this work, we prove that both hierarchies recover the exact value of A₂^{Lin}(n,d) at level n. We also prove that at this level the polytope of Loyfer and Linial is integral. Even though these hierarchies seem less powerful than general hierarchies such as Sum-of-Squares, we show that they have enough structure to yield exact completeness via pseudoprobabilities.

Leonardo Nagami Coregliano, Fernando Granha Jeronimo, and Chris Jones. Exact Completeness of LP Hierarchies for Linear Codes. In 14th Innovations in Theoretical Computer Science Conference (ITCS 2023). Leibniz International Proceedings in Informatics (LIPIcs), Volume 251, pp. 40:1-40:18, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2023)

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@InProceedings{coregliano_et_al:LIPIcs.ITCS.2023.40, author = {Coregliano, Leonardo Nagami and Jeronimo, Fernando Granha and Jones, Chris}, title = {{Exact Completeness of LP Hierarchies for Linear Codes}}, booktitle = {14th Innovations in Theoretical Computer Science Conference (ITCS 2023)}, pages = {40:1--40:18}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-263-1}, ISSN = {1868-8969}, year = {2023}, volume = {251}, editor = {Tauman Kalai, Yael}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ITCS.2023.40}, URN = {urn:nbn:de:0030-drops-175433}, doi = {10.4230/LIPIcs.ITCS.2023.40}, annote = {Keywords: LP bound, linear codes, Delsarte’s LP, combinatorial polytopes, pseudoexpectation} }

Document

**Published in:** LIPIcs, Volume 215, 13th Innovations in Theoretical Computer Science Conference (ITCS 2022)

A longstanding open problem in coding theory is to determine the best (asymptotic) rate R₂(δ) of binary codes with minimum constant (relative) distance δ. An existential lower bound was given by Gilbert and Varshamov in the 1950s. On the impossibility side, in the 1970s McEliece, Rodemich, Rumsey and Welch (MRRW) proved an upper bound by analyzing Delsarte’s linear programs. To date these results remain the best known lower and upper bounds on R₂(δ) with no improvement even for the important class of linear codes. Asymptotically, these bounds differ by an exponential factor in the blocklength.
In this work, we introduce a new hierarchy of linear programs (LPs) that converges to the true size A^{Lin}₂(n,d) of an optimum linear binary code (in fact, over any finite field) of a given blocklength n and distance d. This hierarchy has several notable features:
1) It is a natural generalization of the Delsarte LPs used in the first MRRW bound.
2) It is a hierarchy of linear programs rather than semi-definite programs potentially making it more amenable to theoretical analysis.
3) It is complete in the sense that the optimum code size can be retrieved from level O(n²).
4) It provides an answer in the form of a hierarchy (in larger dimensional spaces) to the question of how to cut Delsarte’s LP polytopes to approximate the true size of linear codes.
We obtain our hierarchy by generalizing the Krawtchouk polynomials and MacWilliams inequalities to a suitable "higher-order" version taking into account interactions of 𝓁 words. Our method also generalizes to translation schemes under mild assumptions.

Leonardo Nagami Coregliano, Fernando Granha Jeronimo, and Chris Jones. A Complete Linear Programming Hierarchy for Linear Codes. In 13th Innovations in Theoretical Computer Science Conference (ITCS 2022). Leibniz International Proceedings in Informatics (LIPIcs), Volume 215, pp. 51:1-51:22, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2022)

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@InProceedings{coregliano_et_al:LIPIcs.ITCS.2022.51, author = {Coregliano, Leonardo Nagami and Jeronimo, Fernando Granha and Jones, Chris}, title = {{A Complete Linear Programming Hierarchy for Linear Codes}}, booktitle = {13th Innovations in Theoretical Computer Science Conference (ITCS 2022)}, pages = {51:1--51:22}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-217-4}, ISSN = {1868-8969}, year = {2022}, volume = {215}, editor = {Braverman, Mark}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ITCS.2022.51}, URN = {urn:nbn:de:0030-drops-156474}, doi = {10.4230/LIPIcs.ITCS.2022.51}, annote = {Keywords: Coding theory, code bounds, convex programming, linear programming hierarchy} }

Document

**Published in:** LIPIcs, Volume 215, 13th Innovations in Theoretical Computer Science Conference (ITCS 2022)

For an abelian group H acting on the set [𝓁], an (H,𝓁)-lift of a graph G₀ is a graph obtained by replacing each vertex by 𝓁 copies, and each edge by a matching corresponding to the action of an element of H.
Expanding graphs obtained via abelian lifts, form a key ingredient in the recent breakthrough constructions of quantum LDPC codes, (implicitly) in the fiber bundle codes by Hastings, Haah and O'Donnell [STOC 2021] achieving distance Ω̃(N^{3/5}), and in those by Panteleev and Kalachev [IEEE Trans. Inf. Theory 2021] of distance Ω(N/log(N)). However, both these constructions are non-explicit. In particular, the latter relies on a randomized construction of expander graphs via abelian lifts by Agarwal et al. [SIAM J. Discrete Math 2019].
In this work, we show the following explicit constructions of expanders obtained via abelian lifts. For every (transitive) abelian group H ⩽ Sym(𝓁), constant degree d ≥ 3 and ε > 0, we construct explicit d-regular expander graphs G obtained from an (H,𝓁)-lift of a (suitable) base n-vertex expander G₀ with the following parameters:
ii) λ(G) ≤ 2√{d-1} + ε, for any lift size 𝓁 ≤ 2^{n^{δ}} where δ = δ(d,ε),
iii) λ(G) ≤ ε ⋅ d, for any lift size 𝓁 ≤ 2^{n^{δ₀}} for a fixed δ₀ > 0, when d ≥ d₀(ε), or
iv) λ(G) ≤ Õ(√d), for lift size "exactly" 𝓁 = 2^{Θ(n)}. As corollaries, we obtain explicit quantum lifted product codes of Panteleev and Kalachev of almost linear distance (and also in a wide range of parameters) and explicit classical quasi-cyclic LDPC codes with wide range of circulant sizes.
Items (i) and (ii) above are obtained by extending the techniques of Mohanty, O'Donnell and Paredes [STOC 2020] for 2-lifts to much larger abelian lift sizes (as a byproduct simplifying their construction). This is done by providing a new encoding of special walks arising in the trace power method, carefully "compressing" depth-first search traversals. Result (iii) is via a simpler proof of Agarwal et al. [SIAM J. Discrete Math 2019] at the expense of polylog factors in the expansion.

Fernando Granha Jeronimo, Tushant Mittal, Ryan O'Donnell, Pedro Paredes, and Madhur Tulsiani. Explicit Abelian Lifts and Quantum LDPC Codes. In 13th Innovations in Theoretical Computer Science Conference (ITCS 2022). Leibniz International Proceedings in Informatics (LIPIcs), Volume 215, pp. 88:1-88:21, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2022)

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@InProceedings{jeronimo_et_al:LIPIcs.ITCS.2022.88, author = {Jeronimo, Fernando Granha and Mittal, Tushant and O'Donnell, Ryan and Paredes, Pedro and Tulsiani, Madhur}, title = {{Explicit Abelian Lifts and Quantum LDPC Codes}}, booktitle = {13th Innovations in Theoretical Computer Science Conference (ITCS 2022)}, pages = {88:1--88:21}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-217-4}, ISSN = {1868-8969}, year = {2022}, volume = {215}, editor = {Braverman, Mark}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ITCS.2022.88}, URN = {urn:nbn:de:0030-drops-156846}, doi = {10.4230/LIPIcs.ITCS.2022.88}, annote = {Keywords: Graph lifts, expander graphs, quasi-cyclic LDPC codes, quantum LDPC codes} }

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