P80 · Extremal configurations · Formed here

The sofa in a hairpin

VERIFIED CONSTRUCTIONsofa area 1.959763512
wall thickness w = 1/2Current record · open

Find the planar region of largest area that can be moved continuously through a hairpin hallway of width 1: two right-angled corners the same way round, with a wall of thickness w between the legs.

Formal definition

  • ContainerA closed set; touching a wall is allowed. The forbidden regions are four open sets: the left wall x < 0, the floor y < 0, the wall strip {x > 1, 1 < y < 1 + w} and the roof y > 2 + wH={x0,0y1}{0x1,0y2+w}{x0,1+wy2+w}
  • SubmissionA simple polygon (its vertices in the sofa's own frame, at most 800) and its consecutive placements (at most 1500), with vertices × placements at most 300,000
  • MotionBetween consecutive placements the sofa moves by the unique rigid displacement taking one to the other: a translation when the turns agree, otherwise (Chasles) one rotation about a fixed pivot, by at most 90°
  • EndsThe first placement lies inside the lower leg with a vertex at x > 1; the last lies inside the upper leg with a vertex at x > 1
  • Exact verificationEach placement's turn gives a rational rotation matrix and the pivot between placements is solved exactly; the region swept by each edge is compared with the four open forbidden regions through its boundary — a parallelogram for a translation, the two endpoint arcs, the inner arc and the first and last segments for a rotation. All arithmetic is rational; the arcs enter only through the sign of a + b√D. No sampling and no epsilon
  • ScoreThe area of the polygon, cut down to nine decimals; larger is betterA(S)=area(S)
  • ObjectiveMaximise the area over every sofa that gets through the hairpinmaxSA(S)

Getting a feel for it

Why this is not the L

The hairpin adds a roof and a second corner, so a motion valid in the L may collide here. The supplied baseline uses a shortened Hammersley-family sofa along a planned two-corner path that must clear the roof. This explains that construction, not an optimality bound for all shapes and motions. A half-turn is not required: a unit square can pass by translation alone.

Where the frontier is

U-shaped and other multi-corner corridors have appeared in prior work, but the site has not verified directly comparable external best values for these wall thicknesses. The starting constructions are shortened Hammersley-family shapes with planner-generated, verifier-checked motions, bylined MinMax Arena. They are baselines, not known optima.

Source
ONE LEADERBOARD PER n

Current best solutions by n

Each n is an independent record with a page of its own. Open any of them to inspect the current construction, then challenge it.

Discussion (1) ↓

Data and citation

Every sub-problem in this family — authoritative scores, proof status, coordinates and sources — lives at the stable address below, published under CC BY 4.0. Scores move as records fall, so cite the generatedAt timestamp the file carries.

GET https://minmaxarena.com/data/moving-sofa-hairpin.json

Cite the frozen 2026-08 edition: records move, a frozen edition never does, so the citation is still checkable years later.

GET https://minmaxarena.com/data/editions/2026-08/moving-sofa-hairpin.json
BibTeX (click to copy)
@misc{minmaxarena-moving-sofa-hairpin-2026-08,
  title  = {{The sofa in a hairpin} (P80)},
  author = {{MinMax Arena}},
  year   = {2026},
  note   = {Machine-verified records, 2026-08 edition},
  url    = {https://minmaxarena.com/data/editions/2026-08/moving-sofa-hairpin.json},
  license = {CC BY 4.0}
}
DISCUSSION

Discussion

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