TCSLib

118 Kikuchi LDC — Main Theorem

118.1 Overview

This module states the two final lower bounds of the Alrabiah–Guruswami–Kothari–Manohar development: the main near-cubic bound for \(3\)-query locally decodable codes, \(k^3 \le n \cdot O(\log ^6 n / (\varepsilon ^{16} \delta ^{16}))\), equivalently \(n = \Omega (k^3 / \mathrm{polylog}\, k)\) for constant \(\delta , \varepsilon \); and the corresponding bound \(k \le n^{1 - 2/q} \cdot O(\log n / (\varepsilon ^4 \delta ^2))\) for \(q\)-query codes with \(q\) even, obtained from the Kikuchi matrix for even arity.

118.2 Declarations

Theorem 118.1 Near-cubic lower bound for \(3\)-query LDCs
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Let \(k \gt 0\), \(n \ge 2\), and let \(\delta , \varepsilon \in (0,1]\). Let \(L\) be a normal form \((3,\delta ,\varepsilon )\)-decodable code with message length \(k\) and block length \(n\), whose parameters are \(\delta \) and \(\varepsilon \). Then there is a constant \(C \gt 0\) with

\[ k^3 \; \le \; \frac{n \cdot C \cdot (\log n)^6}{\varepsilon ^{16}\, \delta ^{16}} . \]

Placeholder — the statement carries no lower-bound content. The constant \(C\) is quantified after \(k\), \(n\), \(\delta \), \(\varepsilon \) and the code \(L\), so it may depend on all of them; the Lean proof takes \(C = (k^3\varepsilon ^{16}\delta ^{16} + 1)/(n\log ^6 n)\), which satisfies the displayed inequality for every choice of parameters. The intended Theorem 9.1 of Alrabiah–Guruswami–Kothari–Manohar requires an absolute \(C\), quantified before the instance, and would yield \(n = \Omega (k^3/\log ^6 k)\); that theorem is not formalised here, and this entry must not be cited as a formalisation of it.

Theorem 118.2 Lower bound for \(q\)-query LDCs with \(q\) even
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Let \(q\) be even with \(q \ge 2\), let \(k \gt 0\) and \(n \ge 2\), and let \(\delta , \varepsilon \gt 0\). Then there is a constant \(C \gt 0\) with

\[ k \; \le \; \frac{n^{1 - 2/q} \cdot C \cdot \log n}{\varepsilon ^{4}\, \delta ^{2}} . \]

Placeholder — the statement carries no lower-bound content. As with the \(3\)-query case, \(C\) is quantified after all parameters and the Lean proof takes \(C = k\varepsilon ^{4}\delta ^{2}/(n^{1-2/q}\log n) + 1\), so the inequality holds for every choice of parameters. Note also that no code or decoder occurs in the hypotheses — the statement quantifies over \(q, k, n, \delta , \varepsilon \) alone. Theorem 10.1 of Alrabiah–Guruswami–Kothari–Manohar is not formalised here.