n = 84 open ★=

94111000≤s(84)≤9+122

The best packing known for 84 squares, side 9+122, Evert Stenlund 1980
9.4119.707
91011
9.16510.165
nn+1

Proven

9.411000≤s(84)≤9.707107

  • new result
  • exact

Citation record n-084

lowerwand125 after Tokoharu, Levy et al. 2026, GitHub (confirmed T-094)

upperStenlund 1980, Squares in Squares

Open

  • optimality

Bounds

Best known packing

9+122

9.70710678118654
Found by
Evert Stenlund 1980
Construction
strip
Source
[Kingbird]
Evidence
E-kingbird-upper-register, E-gobel-strip-upper
Verified upper bound

9+122

9.70710678118654752440084436210485

The reported value, verified here.

Evidence
E-gobel-strip-upper
Reported lower bound

94111000

Proved by
wand125 2026
Kind
counting
Scope
Unrestricted unit-square packing with independent rotations and disjoint interiors.
Note
wand125's square-packing-bounds (5 October 2026) reports s(84)≥9411/1000 from a density of 686 uniform rectangles of total mass 8399999/100000, at core side 9977/10000 on 201 net half-angles, accepted there at every angle by its research copy of Tokoharu's verify.cpp at threshold one. It is above the 3763/400 of the source's mixed_n84_L94075 (T-090), which it supersedes. sqverify-fast decided it here on 5 October 2026 at all 201 net directions.
Source
[wand125 mixed bounds 2026-10-05]
Evidence
E-n084-wand125-mixed-9411-report
Verified lower bound

94111000

The reported value, verified here.

Evidence
E-n084-wand125-mixed-9411-sqverify-fast-replay
Gap

22−4111000≈ 0.29610678…

Verified upper minus verified lower.

Results in the register

Verification

upper: replayed here; lower: replayed here

—

Rigidity

not rigid, numerically checked, numerical multiprecision

Evidence: E-translation-escape-not-rigid

Scope

Square 36 of the retained witness (witness id 37) translates 0.449747 along (-0.707107, -0.707107) with the packing still valid, so the configuration admits a non-trivial feasible motion; 6 of its 84 squares do. Every constraint is exactly affine in the slide parameter, so the arithmetic carries no linearization error, but the coordinates are the witness's own finite-precision transcription: this settles the retained configuration, not the true optimum. Rigidity and optimality are independent, and this bears only on the former.

Open questions
  • Blocker (source evidence): Green's reported lower-bound proof, cited as private communication by Friedman, has not been recovered or independently replayed. E-green-ds7-theorem9-reported-lower

s(84) — open

External intake, 2026-10-05 (certificate of 5 October). wand125’s mixed-certificate source reports s(84)≥9411/1000=9.411 (T-094), from a density of 686 rectangles of total mass 8399999/100000<84, accepted there at every net angle by its research copy of Tokoharu’s checker at threshold one. It is above the source’s 3763/400 of the day before, by 0.0035, and the verified 47/5 by 0.011. It supersedes the source’s mixed_n84_L94075 (T-090). sqverify-fast, this repository’s clean-room verifier, decided it here on 5 October 2026 at all 201 net directions, so it is also the verified lower bound: confirmed, independently re-implemented. wand125’s README says parts of the work were produced with AI assistance under human direction.

External intake, 2026-10-05 (certificate of 4 October). wand125’s mixed-certificate source reports the mixed certificate mixed_n84_L94075 at side 3763/400=9.4075 (T-090), since superseded (T-094), from a density of 569 rectangles of total mass 8399999/100000<84, accepted there at every net angle by its research copy of Tokoharu’s checker at threshold one. It is above the 47/5 this case reported, by 0.0075. It supersedes the source’s mixed_n84_L940. sqverify-fast, this repository’s clean-room verifier, decided it here on 6 October 2026 at all 201 net directions: confirmed, independently re-implemented. The verified lower bound is unchanged, the certificate above being higher. wand125’s README says parts of the work were produced with AI assistance under human direction.

External intake, 2026-10-02. wand125’s mixed-certificate source reports the mixed certificate mixed_n84_L940 at side 47/5=9.4 (T-071), since superseded (T-090), from a density of 661 rectangles of total mass 8399999/100000<84, accepted there at every net angle by its research copy of Tokoharu’s checker at threshold one. It is above Green’s reported 9.2667335…, which this case held by monotonicity, by more than 0.1332. Its complete replay here on 2 October 2026 returned the certificate’s own record at all 201 directions, so it was the verified lower bound until 5 October. wand125’s README says parts of the work were produced with AI assistance under human direction.

Open. The best known packing gives s(84)≤9.70710679, and the verified lower bound is s(84)≥9411/1000=9.411, from wand125’s mixed rectangle-density certificate of 5 October, leaving a gap of 0.2961.

The packing

Found by Frits Göbel in 1979, via a diagonal-strip construction — the strip family at a=8, whose side is 9+2/2 exactly. cases/gobel_strip builds it and verifies it exactly over Q(sqrt 2), which is what moved verified_upper_bound from the grid ceiling onto the exact side. The retained witness declares that side rounded down at its own digits — about 4.85×10−30 below the exact value — and a different slack choice for the strip, so the certificate carries the construction’s coordinates rather than the witness’s (D-398).

The lower bound

Before 2 October the selected external report was [Friedman DS7], which gives the lower-bound expression 942/41+247/41 for s(84) (approximately 9.266733533246). Friedman’s DS7 survey, Theorem 9, k=9, reports this bound at n=82; reference [8] is Green’s private communication (2000). The source proof has not been recovered. The unavoidable-set argument DS7’s Figure 34 illustrates does not prove it: at k=9 that point pattern leaves a unit square empty (review). Inherited at n=84 by monotonicity. That changed the reported source field only; the verified field now holds wand125’s replayed mixed certificate. The source audit compares the exact theorem expressions separately from opaque table decimals.

The verified field rests on a complete decision here (E-n084-wand125-mixed-9411-sqverify-fast-replay, V3/C3) of wand125’s mixed certificate of 5 October by sqverify-fast, this repository’s clean-room measure verifier: all 201 net directions of the retained candidate on 5 October 2026, with two mutants scaled below coverage one refused. It shares no code with the source’s checker, so it decides coverage a second way with the same method; the source’s own checker was not run here.

From 2 October 2026 until this decision the verified field rested on a complete replay here (E-n084-wand125-mixed-940-source-replay) of the source’s certificate mixed_n84_L940, at 47/5: the source’s unchanged checker and per-angle functions on the pinned bundle, all 201 directions, each returning the certificate’s own record.

Until 2 October 2026 the verified lower bound was Nagamochi’s general closed form, now a reported bound (see the correction below). Karakuş’s general bound, independently verified here and below this certificate at this n, applies to every nonsquare N≥8:

s(N)≥12+N−⌊N⌋+14

Correction, 2 October 2026. Until that date the verified lower bound here was Nagamochi’s general closed form, s(N)≥min(⌈N⌉,N−2⌊N⌋+1+1), which is stronger at this n and is now recorded as a reported bound. Its published proof rests on Nagamochi’s Lemma 1, which Karakuş showed false; nothing is disproved, and no packing beating it is known (review of 2 October 2026). This register had recorded that proof as verified, its own error, logged as defect D-516.

See the Frontier corpus summary for the current aggregate count; source-reported bounds are recorded separately.

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.

bound evidence run code programs
verified lower E-n084-wand125-mixed-9411-sqverify-fast-replay replayed here independent V-sqverify-fast (first-party)
verified upper E-gobel-strip-upper replayed here independent V-sqpack-verify (first-party)