Infinite series of $3$-designs in the extended quadratic residue code
Madoka Awada
TL;DR
The paper addresses the problem of generating $3$-designs from extended quadratic residue codes over ${\mathbb F}_{r^2}$ beyond those implied by transitivity or the Assmus–Mattson theorem. It develops a harmonic-analytic framework using Jacobi polynomials and harmonic weight enumerators, along with duadic-code duality, to prove a main result: if $p$ is an odd prime with $p \equiv 1 \pmod{4}$ and $r$ is a nonresidue modulo $p$, then every nonempty shell $(\widetilde{Q}_{r^2,p+1})_\ell$ is a $3$-design; special cases $r=2$ or $3$ (with $p$ satisfying congruence conditions) are also established. The paper then provides explicit examples and an extensive infinite series of $(r,p)$ pairs yielding $3$-designs, showing these designs often do not follow from transitivity or AM theorems. This work expands the known sources of $3$-designs in QR-code-related structures and demonstrates a robust method to obtain large families of designs in algebraic coding-theoretic objects with limited symmetry. The results have potential implications for combinatorial design theory and its applications in coding theory and related areas.
Abstract
In this paper, we show infinite series of $3$-designs in the extended quadratic residue codes over $\mathbb {F}_{r^2}$ for a prime $r$.
