\(s (28)\)$s(28)$ — open

Exact certificate, 2026-10-06. Evan Daniel’s square-packing published on 5 October 2026 an exact rational certificate of this packing, decided here over \(\mathbb{Q}\)$\mathbb{Q}$ by two exact checkers that share no code with each other or with the source, and by the source’s own two run here: it proves \(s(28) \le 5.82444461667406\)$s(28)\le 5.82444461667406$, the verified upper bound (T-101). Until then the verified upper bound was the trivial grid bound \(6\)$6$.

External intake, 2026-10-06. wand125’s check2 source reports \(s(28) \ge 1147/200 = 5.735\)$s(28)\ge 1147/200=5.735$ (T-107), from a density of 531 rectangles of total mass \(2799999/100000 < 28\)$2799999/100000<28$, on a net the certificate declares: core side \(999/1000\)$999/1000$ and 416 half-angle tangents of step \(1/1001\)$1/1001$. Since \(999/1000 \cdot (1 + 1/1001) < 1\)$999/1000·(1+1/1001)<1$, every unit square contains a core at a net angle strictly in its interior. The source ships no record of its C++ checker for it: it accepts it at every direction with its copy of this repository’s sqverify_fast, changed to read a declared net. It is above the reported \(2289/400\)$2289/400$ (T-068) by \(0.0125\)$0.0125$. This repository’s clean-room verifier sqverify-fast decided it here on 6 October 2026 at all 416 directions of its net, and refused two mutants scaled below coverage one: confirmed, re-implemented sharing the producer’s components (the source’s check is a copy of the same crate), so it is also the verified lower bound. wand125’s README says parts of the work were produced with AI assistance under human direction.

External intake, 2026-10-01. wand125’s rectangle-density source reports \(s (28) \ge 2289 / 400 = 5.7225\)$s(28)\ge 2289/400=5.7225$, since superseded (T-107), with total mass \(2799/100 = 27.99 < 28\)$2799/100=27.99<28$, accepted by Tokoharu’s unchanged interval checker. The complete 201-direction coverage replay here accepted it again, after this repository’s exact audit checked that the regenerated checker input is the published one and checked the mass and net premises, so it was also verified until 2026-10-06, when the certificate above superseded it in both lanes. wand125’s README says parts of the work were produced with AI assistance under human direction.

External intake, 2026-09-28. wand125’s rectangle-density source reports a direct \(143/25 = 5.72\)$143/25=5.72$ certificate for this case, whose reported bound the 2026-10-01 intake above raises, with total mass \(2799/100 = 27.99 < 28\)$2799/100=27.99<28$, accepted by Tokoharu’s unchanged interval checker. The replayed n27 certificate carried \(28/5 = 5.6\)$28/5=5.6$ here by monotonicity, the verified lower bound until the 2026-10-01 intake above. wand125’s README says parts of the work were produced with AI assistance under human direction.

External intake, 2026-09-27. wand125’s rectangle-density source reports a direct \(1139/200 = 5.695\)$1139/200=5.695$ certificate for this case, whose reported bound the 2026-09-28 intake above raises, with total mass \(2799/100 = 27.99 < 28\)$2799/100=27.99<28$, accepted by Tokoharu’s unchanged interval checker. The replayed n27 certificate carried \(28/5 = 5.6\)$28/5=5.6$ here by monotonicity, the verified lower bound until the 2026-10-01 intake above.

The complete replay of Tokoharu’s n26 rectangle-density certificate previously verified \(s (28) \ge 1377 / 250 = 5.508\)$s(28)\ge 1377/250=5.508$ by monotonicity, until the replayed n27 certificate superseded it. Tokoharu’s README says parts of the work were produced with AI assistance under human direction. Friedman DS7 reports Green’s \(5.511708697746\)$5.511708697746$ lower bound, now weaker than both lanes; its cited private-communication proof has not been recovered.

Open. The best known packing gives \(s(28) \le 5.82444462\)$s(28)\le 5.82444462$; the verified lower bound is \(1147/200 = 5.735\)$1147/200=5.735$, from wand125’s check2 certificate on a declared net (T-107, 2026-10-06, V3/C3), which superseded its rectangle-density certificate of 1 October at \(2289/400 = 5.7225\)$2289/400=5.7225$ (T-074), leaving a gap of about \(0.0894\)$0.0894$ against that reported upper construction.

The exact certificate

Evan Daniel’s square-packing published on 5 October 2026 exact rational certificates of the packings this register lists as best known, solved from its own witnesses, and offered those that do not lower a printed side as a replay of the existing bounds, asking for nothing to be registered from them. The certificate for this count holds the same 28 squares, each a rational centre and a rational \(t = \tan(\theta/2)\)$t=\tan (\theta /2)$, in a square of side \(5.8244446166740592971\ldots\)$5.8244446166740592971\dots$, \(9.3 \times 10^{-15}\)$9.3\times {10}^{-15}$ above the side the Kingbird catalogue prints, \(5.82444461667405\)$5.82444461667405$. Rounded to binary64, square for square, its pose is the one the atlas pictures for this count. Evan Daniel’s solver moves the binary64 pose to a nearby exact KKT point of the problem of minimizing the side under non-overlap, computed at 80 digits, and rounds it outward to rationals.

This repository decides the certificate exactly. Converted without rounding, every pair and every wall is decided over \(\mathbb{Q}\)$\mathbb{Q}$ twice, by sqpack’s exact separating-axis test and by an independent checker that shares no code with it, and the source’s own two checkers, run here as retained, accept it as well (receipts). That proves \(s(28) \le 5.82444461667406\)$s(28)\le 5.82444461667406$, the verified upper bound: the certificate’s side rounded up at the fourteen decimals the catalogue prints, as the record writes any certificate of a printed side. It says nothing about optimality. On jlevy/squares#375 its author wrote that the solver, its checkers and the batch “were written with Claude (Anthropic) as a coding and research agent, directed and reviewed by me.”

The verified upper bound and the printed side now agree to one unit of the printed side’s last place, the precision at which the record compares them. The printed side itself is not certified here: the certificate’s side lies above it.

The packing

Found by David Ellsworth in 2025, via simulated annealing. The catalogue flags it rigid: no continuous deformation is possible, so the contact conditions pin the side length exactly as an algebraic number. Its side length is algebraic of degree 6 over \(\mathbb{Q}\)$\mathbb{Q}$. Its Galois group over \(\mathbb{Q}\)$\mathbb{Q}$ is S6, computed here from the recorded minimal polynomial with sympy.galois_group, and S6 is not solvable. The side length is therefore not expressible in radicals, so the empty exact_form is a permanent property of this case rather than a transcription gap.

The lower bound

The earlier external report, [Friedman DS7], gives the lower-bound expression \(6 \sqrt{5} / 5 + 2 \sqrt{2}\)$6\sqrt{5}/5+2\sqrt{2}$ for \(s(28)\)$s(28)$ (approximately \(5.511708697746\)$5.511708697746$). Friedman’s DS7 survey, Theorem 10, k=5, reports this bound at n=28; reference [8] is Green’s private communication (2000). The source proof has not been recovered. The source audit compares the exact theorem expressions separately from opaque table decimals.

The verified lower bound is \(1147/200 = 5.735\)$1147/200=5.735$, from wand125’s check2 certificate of 6 October on a declared net (T-107), decided here by sqverify-fast at every direction of its net; its rectangle-density certificate of 1 October, \(2289/400 = 5.7225\)$2289/400=5.7225$ (T-074), held the field from 2026-10-02 until then. Before that the verified lower bound was wand125’s \(28/5\)$28/5$, inherited from the fully replayed n27 rectangle-density certificate, whose mass \(26999/1000\)$26999/1000$ is below 28, by monotonicity, about \(0.0883\)$0.0883$ above Green’s literal report, while its direct n28 certificate at \(143/25\)$143/25$, which replaced the \(1139/200\)$1139/200$ certificate on 2026-09-28, was the reported lower bound. A method-distinct rectangle coverage implementation would add confidence and improve generalization, but it is not a missing premise of the verified fixed bound.

Corrected 2 October 2026: Nagamochi’s Lemma 1 is false (Karakuş 2026), so his closed form below is now a reported bound whose published proof is incomplete (review). This register had recorded that proof as verified, its own error, logged as defect D-516. Nagamochi’s earlier general closed form remains part of the evidence history and applies to every \(N \ge 4\)$N\ge 4$:

Verification Code

The programs behind this case’s verified bounds, by their evidence. The code column says how the code that ran stands to the code its producer used. VERIFIERS.md says what each program is and whose it is.